1libpng-manual.txt - A description on how to use and modify libpng 2 3 libpng version 1.6.20 - December 3, 2015 4 Updated and distributed by Glenn Randers-Pehrson 5 <glennrp at users.sourceforge.net> 6 Copyright (c) 1998-2015 Glenn Randers-Pehrson 7 8 This document is released under the libpng license. 9 For conditions of distribution and use, see the disclaimer 10 and license in png.h 11 12 Based on: 13 14 libpng versions 0.97, January 1998, through 1.6.20 - December 3, 2015 15 Updated and distributed by Glenn Randers-Pehrson 16 Copyright (c) 1998-2015 Glenn Randers-Pehrson 17 18 libpng 1.0 beta 6 - version 0.96 - May 28, 1997 19 Updated and distributed by Andreas Dilger 20 Copyright (c) 1996, 1997 Andreas Dilger 21 22 libpng 1.0 beta 2 - version 0.88 - January 26, 1996 23 For conditions of distribution and use, see copyright 24 notice in png.h. Copyright (c) 1995, 1996 Guy Eric 25 Schalnat, Group 42, Inc. 26 27 Updated/rewritten per request in the libpng FAQ 28 Copyright (c) 1995, 1996 Frank J. T. Wojcik 29 December 18, 1995 & January 20, 1996 30 31 TABLE OF CONTENTS 32 33 I. Introduction 34 II. Structures 35 III. Reading 36 IV. Writing 37 V. Simplified API 38 VI. Modifying/Customizing libpng 39 VII. MNG support 40 VIII. Changes to Libpng from version 0.88 41 IX. Changes to Libpng from version 1.0.x to 1.2.x 42 X. Changes to Libpng from version 1.0.x/1.2.x to 1.4.x 43 XI. Changes to Libpng from version 1.4.x to 1.5.x 44 XII. Changes to Libpng from version 1.5.x to 1.6.x 45 XIII. Detecting libpng 46 XIV. Source code repository 47 XV. Coding style 48 XVI. Y2K Compliance in libpng 49 50I. Introduction 51 52This file describes how to use and modify the PNG reference library 53(known as libpng) for your own use. In addition to this 54file, example.c is a good starting point for using the library, as 55it is heavily commented and should include everything most people 56will need. We assume that libpng is already installed; see the 57INSTALL file for instructions on how to configure and install libpng. 58 59For examples of libpng usage, see the files "example.c", "pngtest.c", 60and the files in the "contrib" directory, all of which are included in 61the libpng distribution. 62 63Libpng was written as a companion to the PNG specification, as a way 64of reducing the amount of time and effort it takes to support the PNG 65file format in application programs. 66 67The PNG specification (second edition), November 2003, is available as 68a W3C Recommendation and as an ISO Standard (ISO/IEC 15948:2004 (E)) at 69<http://www.w3.org/TR/2003/REC-PNG-20031110/ 70The W3C and ISO documents have identical technical content. 71 72The PNG-1.2 specification is available at 73<http://png-mng.sourceforge.net/pub/png/spec/1.2/>. 74It is technically equivalent 75to the PNG specification (second edition) but has some additional material. 76 77The PNG-1.0 specification is available as RFC 2083 78<http://png-mng.sourceforge.net/pub/png/spec/1.0/> and as a 79W3C Recommendation <http://www.w3.org/TR/REC-png-961001>. 80 81Some additional chunks are described in the special-purpose public chunks 82documents at <http://www.libpng.org/pub/png/spec/register/> 83 84Other information 85about PNG, and the latest version of libpng, can be found at the PNG home 86page, <http://www.libpng.org/pub/png/>. 87 88Most users will not have to modify the library significantly; advanced 89users may want to modify it more. All attempts were made to make it as 90complete as possible, while keeping the code easy to understand. 91Currently, this library only supports C. Support for other languages 92is being considered. 93 94Libpng has been designed to handle multiple sessions at one time, 95to be easily modifiable, to be portable to the vast majority of 96machines (ANSI, K&R, 16-, 32-, and 64-bit) available, and to be easy 97to use. The ultimate goal of libpng is to promote the acceptance of 98the PNG file format in whatever way possible. While there is still 99work to be done (see the TODO file), libpng should cover the 100majority of the needs of its users. 101 102Libpng uses zlib for its compression and decompression of PNG files. 103Further information about zlib, and the latest version of zlib, can 104be found at the zlib home page, <http://zlib.net/>. 105The zlib compression utility is a general purpose utility that is 106useful for more than PNG files, and can be used without libpng. 107See the documentation delivered with zlib for more details. 108You can usually find the source files for the zlib utility wherever you 109find the libpng source files. 110 111Libpng is thread safe, provided the threads are using different 112instances of the structures. Each thread should have its own 113png_struct and png_info instances, and thus its own image. 114Libpng does not protect itself against two threads using the 115same instance of a structure. 116 117II. Structures 118 119There are two main structures that are important to libpng, png_struct 120and png_info. Both are internal structures that are no longer exposed 121in the libpng interface (as of libpng 1.5.0). 122 123The png_info structure is designed to provide information about the 124PNG file. At one time, the fields of png_info were intended to be 125directly accessible to the user. However, this tended to cause problems 126with applications using dynamically loaded libraries, and as a result 127a set of interface functions for png_info (the png_get_*() and png_set_*() 128functions) was developed, and direct access to the png_info fields was 129deprecated.. 130 131The png_struct structure is the object used by the library to decode a 132single image. As of 1.5.0 this structure is also not exposed. 133 134Almost all libpng APIs require a pointer to a png_struct as the first argument. 135Many (in particular the png_set and png_get APIs) also require a pointer 136to png_info as the second argument. Some application visible macros 137defined in png.h designed for basic data access (reading and writing 138integers in the PNG format) don't take a png_info pointer, but it's almost 139always safe to assume that a (png_struct*) has to be passed to call an API 140function. 141 142You can have more than one png_info structure associated with an image, 143as illustrated in pngtest.c, one for information valid prior to the 144IDAT chunks and another (called "end_info" below) for things after them. 145 146The png.h header file is an invaluable reference for programming with libpng. 147And while I'm on the topic, make sure you include the libpng header file: 148 149#include <png.h> 150 151and also (as of libpng-1.5.0) the zlib header file, if you need it: 152 153#include <zlib.h> 154 155Types 156 157The png.h header file defines a number of integral types used by the 158APIs. Most of these are fairly obvious; for example types corresponding 159to integers of particular sizes and types for passing color values. 160 161One exception is how non-integral numbers are handled. For application 162convenience most APIs that take such numbers have C (double) arguments; 163however, internally PNG, and libpng, use 32 bit signed integers and encode 164the value by multiplying by 100,000. As of libpng 1.5.0 a convenience 165macro PNG_FP_1 is defined in png.h along with a type (png_fixed_point) 166which is simply (png_int_32). 167 168All APIs that take (double) arguments also have a matching API that 169takes the corresponding fixed point integer arguments. The fixed point 170API has the same name as the floating point one with "_fixed" appended. 171The actual range of values permitted in the APIs is frequently less than 172the full range of (png_fixed_point) (-21474 to +21474). When APIs require 173a non-negative argument the type is recorded as png_uint_32 above. Consult 174the header file and the text below for more information. 175 176Special care must be take with sCAL chunk handling because the chunk itself 177uses non-integral values encoded as strings containing decimal floating point 178numbers. See the comments in the header file. 179 180Configuration 181 182The main header file function declarations are frequently protected by C 183preprocessing directives of the form: 184 185 #ifdef PNG_feature_SUPPORTED 186 declare-function 187 #endif 188 ... 189 #ifdef PNG_feature_SUPPORTED 190 use-function 191 #endif 192 193The library can be built without support for these APIs, although a 194standard build will have all implemented APIs. Application programs 195should check the feature macros before using an API for maximum 196portability. From libpng 1.5.0 the feature macros set during the build 197of libpng are recorded in the header file "pnglibconf.h" and this file 198is always included by png.h. 199 200If you don't need to change the library configuration from the default, skip to 201the next section ("Reading"). 202 203Notice that some of the makefiles in the 'scripts' directory and (in 1.5.0) all 204of the build project files in the 'projects' directory simply copy 205scripts/pnglibconf.h.prebuilt to pnglibconf.h. This means that these build 206systems do not permit easy auto-configuration of the library - they only 207support the default configuration. 208 209The easiest way to make minor changes to the libpng configuration when 210auto-configuration is supported is to add definitions to the command line 211using (typically) CPPFLAGS. For example: 212 213CPPFLAGS=-DPNG_NO_FLOATING_ARITHMETIC 214 215will change the internal libpng math implementation for gamma correction and 216other arithmetic calculations to fixed point, avoiding the need for fast 217floating point support. The result can be seen in the generated pnglibconf.h - 218make sure it contains the changed feature macro setting. 219 220If you need to make more extensive configuration changes - more than one or two 221feature macro settings - you can either add -DPNG_USER_CONFIG to the build 222command line and put a list of feature macro settings in pngusr.h or you can set 223DFA_XTRA (a makefile variable) to a file containing the same information in the 224form of 'option' settings. 225 226A. Changing pnglibconf.h 227 228A variety of methods exist to build libpng. Not all of these support 229reconfiguration of pnglibconf.h. To reconfigure pnglibconf.h it must either be 230rebuilt from scripts/pnglibconf.dfa using awk or it must be edited by hand. 231 232Hand editing is achieved by copying scripts/pnglibconf.h.prebuilt to 233pnglibconf.h and changing the lines defining the supported features, paying 234very close attention to the 'option' information in scripts/pnglibconf.dfa 235that describes those features and their requirements. This is easy to get 236wrong. 237 238B. Configuration using DFA_XTRA 239 240Rebuilding from pnglibconf.dfa is easy if a functioning 'awk', or a later 241variant such as 'nawk' or 'gawk', is available. The configure build will 242automatically find an appropriate awk and build pnglibconf.h. 243The scripts/pnglibconf.mak file contains a set of make rules for doing the 244same thing if configure is not used, and many of the makefiles in the scripts 245directory use this approach. 246 247When rebuilding simply write a new file containing changed options and set 248DFA_XTRA to the name of this file. This causes the build to append the new file 249to the end of scripts/pnglibconf.dfa. The pngusr.dfa file should contain lines 250of the following forms: 251 252everything = off 253 254This turns all optional features off. Include it at the start of pngusr.dfa to 255make it easier to build a minimal configuration. You will need to turn at least 256some features on afterward to enable either reading or writing code, or both. 257 258option feature on 259option feature off 260 261Enable or disable a single feature. This will automatically enable other 262features required by a feature that is turned on or disable other features that 263require a feature which is turned off. Conflicting settings will cause an error 264message to be emitted by awk. 265 266setting feature default value 267 268Changes the default value of setting 'feature' to 'value'. There are a small 269number of settings listed at the top of pnglibconf.h, they are documented in the 270source code. Most of these values have performance implications for the library 271but most of them have no visible effect on the API. Some can also be overridden 272from the API. 273 274This method of building a customized pnglibconf.h is illustrated in 275contrib/pngminim/*. See the "$(PNGCONF):" target in the makefile and 276pngusr.dfa in these directories. 277 278C. Configuration using PNG_USER_CONFIG 279 280If -DPNG_USER_CONFIG is added to the CPPFLAGS when pnglibconf.h is built, 281the file pngusr.h will automatically be included before the options in 282scripts/pnglibconf.dfa are processed. Your pngusr.h file should contain only 283macro definitions turning features on or off or setting settings. 284 285Apart from the global setting "everything = off" all the options listed above 286can be set using macros in pngusr.h: 287 288#define PNG_feature_SUPPORTED 289 290is equivalent to: 291 292option feature on 293 294#define PNG_NO_feature 295 296is equivalent to: 297 298option feature off 299 300#define PNG_feature value 301 302is equivalent to: 303 304setting feature default value 305 306Notice that in both cases, pngusr.dfa and pngusr.h, the contents of the 307pngusr file you supply override the contents of scripts/pnglibconf.dfa 308 309If confusing or incomprehensible behavior results it is possible to 310examine the intermediate file pnglibconf.dfn to find the full set of 311dependency information for each setting and option. Simply locate the 312feature in the file and read the C comments that precede it. 313 314This method is also illustrated in the contrib/pngminim/* makefiles and 315pngusr.h. 316 317III. Reading 318 319We'll now walk you through the possible functions to call when reading 320in a PNG file sequentially, briefly explaining the syntax and purpose 321of each one. See example.c and png.h for more detail. While 322progressive reading is covered in the next section, you will still 323need some of the functions discussed in this section to read a PNG 324file. 325 326Setup 327 328You will want to do the I/O initialization(*) before you get into libpng, 329so if it doesn't work, you don't have much to undo. Of course, you 330will also want to insure that you are, in fact, dealing with a PNG 331file. Libpng provides a simple check to see if a file is a PNG file. 332To use it, pass in the first 1 to 8 bytes of the file to the function 333png_sig_cmp(), and it will return 0 (false) if the bytes match the 334corresponding bytes of the PNG signature, or nonzero (true) otherwise. 335Of course, the more bytes you pass in, the greater the accuracy of the 336prediction. 337 338If you are intending to keep the file pointer open for use in libpng, 339you must ensure you don't read more than 8 bytes from the beginning 340of the file, and you also have to make a call to png_set_sig_bytes() 341with the number of bytes you read from the beginning. Libpng will 342then only check the bytes (if any) that your program didn't read. 343 344(*): If you are not using the standard I/O functions, you will need 345to replace them with custom functions. See the discussion under 346Customizing libpng. 347 348 FILE *fp = fopen(file_name, "rb"); 349 if (!fp) 350 { 351 return (ERROR); 352 } 353 354 if (fread(header, 1, number, fp) != number) 355 { 356 return (ERROR); 357 } 358 359 is_png = !png_sig_cmp(header, 0, number); 360 if (!is_png) 361 { 362 return (NOT_PNG); 363 } 364 365Next, png_struct and png_info need to be allocated and initialized. In 366order to ensure that the size of these structures is correct even with a 367dynamically linked libpng, there are functions to initialize and 368allocate the structures. We also pass the library version, optional 369pointers to error handling functions, and a pointer to a data struct for 370use by the error functions, if necessary (the pointer and functions can 371be NULL if the default error handlers are to be used). See the section 372on Changes to Libpng below regarding the old initialization functions. 373The structure allocation functions quietly return NULL if they fail to 374create the structure, so your application should check for that. 375 376 png_structp png_ptr = png_create_read_struct 377 (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr, 378 user_error_fn, user_warning_fn); 379 380 if (!png_ptr) 381 return (ERROR); 382 383 png_infop info_ptr = png_create_info_struct(png_ptr); 384 385 if (!info_ptr) 386 { 387 png_destroy_read_struct(&png_ptr, 388 (png_infopp)NULL, (png_infopp)NULL); 389 return (ERROR); 390 } 391 392If you want to use your own memory allocation routines, 393use a libpng that was built with PNG_USER_MEM_SUPPORTED defined, and use 394png_create_read_struct_2() instead of png_create_read_struct(): 395 396 png_structp png_ptr = png_create_read_struct_2 397 (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr, 398 user_error_fn, user_warning_fn, (png_voidp) 399 user_mem_ptr, user_malloc_fn, user_free_fn); 400 401The error handling routines passed to png_create_read_struct() 402and the memory alloc/free routines passed to png_create_struct_2() 403are only necessary if you are not using the libpng supplied error 404handling and memory alloc/free functions. 405 406When libpng encounters an error, it expects to longjmp back 407to your routine. Therefore, you will need to call setjmp and pass 408your png_jmpbuf(png_ptr). If you read the file from different 409routines, you will need to update the longjmp buffer every time you enter 410a new routine that will call a png_*() function. 411 412See your documentation of setjmp/longjmp for your compiler for more 413information on setjmp/longjmp. See the discussion on libpng error 414handling in the Customizing Libpng section below for more information 415on the libpng error handling. If an error occurs, and libpng longjmp's 416back to your setjmp, you will want to call png_destroy_read_struct() to 417free any memory. 418 419 if (setjmp(png_jmpbuf(png_ptr))) 420 { 421 png_destroy_read_struct(&png_ptr, &info_ptr, 422 &end_info); 423 fclose(fp); 424 return (ERROR); 425 } 426 427Pass (png_infopp)NULL instead of &end_info if you didn't create 428an end_info structure. 429 430If you would rather avoid the complexity of setjmp/longjmp issues, 431you can compile libpng with PNG_NO_SETJMP, in which case 432errors will result in a call to PNG_ABORT() which defaults to abort(). 433 434You can #define PNG_ABORT() to a function that does something 435more useful than abort(), as long as your function does not 436return. 437 438Now you need to set up the input code. The default for libpng is to 439use the C function fread(). If you use this, you will need to pass a 440valid FILE * in the function png_init_io(). Be sure that the file is 441opened in binary mode. If you wish to handle reading data in another 442way, you need not call the png_init_io() function, but you must then 443implement the libpng I/O methods discussed in the Customizing Libpng 444section below. 445 446 png_init_io(png_ptr, fp); 447 448If you had previously opened the file and read any of the signature from 449the beginning in order to see if this was a PNG file, you need to let 450libpng know that there are some bytes missing from the start of the file. 451 452 png_set_sig_bytes(png_ptr, number); 453 454You can change the zlib compression buffer size to be used while 455reading compressed data with 456 457 png_set_compression_buffer_size(png_ptr, buffer_size); 458 459where the default size is 8192 bytes. Note that the buffer size 460is changed immediately and the buffer is reallocated immediately, 461instead of setting a flag to be acted upon later. 462 463If you want CRC errors to be handled in a different manner than 464the default, use 465 466 png_set_crc_action(png_ptr, crit_action, ancil_action); 467 468The values for png_set_crc_action() say how libpng is to handle CRC errors in 469ancillary and critical chunks, and whether to use the data contained 470therein. Note that it is impossible to "discard" data in a critical 471chunk. 472 473Choices for (int) crit_action are 474 PNG_CRC_DEFAULT 0 error/quit 475 PNG_CRC_ERROR_QUIT 1 error/quit 476 PNG_CRC_WARN_USE 3 warn/use data 477 PNG_CRC_QUIET_USE 4 quiet/use data 478 PNG_CRC_NO_CHANGE 5 use the current value 479 480Choices for (int) ancil_action are 481 PNG_CRC_DEFAULT 0 error/quit 482 PNG_CRC_ERROR_QUIT 1 error/quit 483 PNG_CRC_WARN_DISCARD 2 warn/discard data 484 PNG_CRC_WARN_USE 3 warn/use data 485 PNG_CRC_QUIET_USE 4 quiet/use data 486 PNG_CRC_NO_CHANGE 5 use the current value 487 488Setting up callback code 489 490You can set up a callback function to handle any unknown chunks in the 491input stream. You must supply the function 492 493 read_chunk_callback(png_structp png_ptr, 494 png_unknown_chunkp chunk); 495 { 496 /* The unknown chunk structure contains your 497 chunk data, along with similar data for any other 498 unknown chunks: */ 499 500 png_byte name[5]; 501 png_byte *data; 502 png_size_t size; 503 504 /* Note that libpng has already taken care of 505 the CRC handling */ 506 507 /* put your code here. Search for your chunk in the 508 unknown chunk structure, process it, and return one 509 of the following: */ 510 511 return (-n); /* chunk had an error */ 512 return (0); /* did not recognize */ 513 return (n); /* success */ 514 } 515 516(You can give your function another name that you like instead of 517"read_chunk_callback") 518 519To inform libpng about your function, use 520 521 png_set_read_user_chunk_fn(png_ptr, user_chunk_ptr, 522 read_chunk_callback); 523 524This names not only the callback function, but also a user pointer that 525you can retrieve with 526 527 png_get_user_chunk_ptr(png_ptr); 528 529If you call the png_set_read_user_chunk_fn() function, then all unknown 530chunks which the callback does not handle will be saved when read. You can 531cause them to be discarded by returning '1' ("handled") instead of '0'. This 532behavior will change in libpng 1.7 and the default handling set by the 533png_set_keep_unknown_chunks() function, described below, will be used when the 534callback returns 0. If you want the existing behavior you should set the global 535default to PNG_HANDLE_CHUNK_IF_SAFE now; this is compatible with all current 536versions of libpng and with 1.7. Libpng 1.6 issues a warning if you keep the 537default, or PNG_HANDLE_CHUNK_NEVER, and the callback returns 0. 538 539At this point, you can set up a callback function that will be 540called after each row has been read, which you can use to control 541a progress meter or the like. It's demonstrated in pngtest.c. 542You must supply a function 543 544 void read_row_callback(png_structp png_ptr, 545 png_uint_32 row, int pass); 546 { 547 /* put your code here */ 548 } 549 550(You can give it another name that you like instead of "read_row_callback") 551 552To inform libpng about your function, use 553 554 png_set_read_status_fn(png_ptr, read_row_callback); 555 556When this function is called the row has already been completely processed and 557the 'row' and 'pass' refer to the next row to be handled. For the 558non-interlaced case the row that was just handled is simply one less than the 559passed in row number, and pass will always be 0. For the interlaced case the 560same applies unless the row value is 0, in which case the row just handled was 561the last one from one of the preceding passes. Because interlacing may skip a 562pass you cannot be sure that the preceding pass is just 'pass-1', if you really 563need to know what the last pass is record (row,pass) from the callback and use 564the last recorded value each time. 565 566As with the user transform you can find the output row using the 567PNG_ROW_FROM_PASS_ROW macro. 568 569Unknown-chunk handling 570 571Now you get to set the way the library processes unknown chunks in the 572input PNG stream. Both known and unknown chunks will be read. Normal 573behavior is that known chunks will be parsed into information in 574various info_ptr members while unknown chunks will be discarded. This 575behavior can be wasteful if your application will never use some known 576chunk types. To change this, you can call: 577 578 png_set_keep_unknown_chunks(png_ptr, keep, 579 chunk_list, num_chunks); 580 581 keep - 0: default unknown chunk handling 582 1: ignore; do not keep 583 2: keep only if safe-to-copy 584 3: keep even if unsafe-to-copy 585 586 You can use these definitions: 587 PNG_HANDLE_CHUNK_AS_DEFAULT 0 588 PNG_HANDLE_CHUNK_NEVER 1 589 PNG_HANDLE_CHUNK_IF_SAFE 2 590 PNG_HANDLE_CHUNK_ALWAYS 3 591 592 chunk_list - list of chunks affected (a byte string, 593 five bytes per chunk, NULL or '\0' if 594 num_chunks is positive; ignored if 595 numchunks <= 0). 596 597 num_chunks - number of chunks affected; if 0, all 598 unknown chunks are affected. If positive, 599 only the chunks in the list are affected, 600 and if negative all unknown chunks and 601 all known chunks except for the IHDR, 602 PLTE, tRNS, IDAT, and IEND chunks are 603 affected. 604 605Unknown chunks declared in this way will be saved as raw data onto a 606list of png_unknown_chunk structures. If a chunk that is normally 607known to libpng is named in the list, it will be handled as unknown, 608according to the "keep" directive. If a chunk is named in successive 609instances of png_set_keep_unknown_chunks(), the final instance will 610take precedence. The IHDR and IEND chunks should not be named in 611chunk_list; if they are, libpng will process them normally anyway. 612If you know that your application will never make use of some particular 613chunks, use PNG_HANDLE_CHUNK_NEVER (or 1) as demonstrated below. 614 615Here is an example of the usage of png_set_keep_unknown_chunks(), 616where the private "vpAg" chunk will later be processed by a user chunk 617callback function: 618 619 png_byte vpAg[5]={118, 112, 65, 103, (png_byte) '\0'}; 620 621 #if defined(PNG_UNKNOWN_CHUNKS_SUPPORTED) 622 png_byte unused_chunks[]= 623 { 624 104, 73, 83, 84, (png_byte) '\0', /* hIST */ 625 105, 84, 88, 116, (png_byte) '\0', /* iTXt */ 626 112, 67, 65, 76, (png_byte) '\0', /* pCAL */ 627 115, 67, 65, 76, (png_byte) '\0', /* sCAL */ 628 115, 80, 76, 84, (png_byte) '\0', /* sPLT */ 629 116, 73, 77, 69, (png_byte) '\0', /* tIME */ 630 }; 631 #endif 632 633 ... 634 635 #if defined(PNG_UNKNOWN_CHUNKS_SUPPORTED) 636 /* ignore all unknown chunks 637 * (use global setting "2" for libpng16 and earlier): 638 */ 639 png_set_keep_unknown_chunks(read_ptr, 2, NULL, 0); 640 641 /* except for vpAg: */ 642 png_set_keep_unknown_chunks(read_ptr, 2, vpAg, 1); 643 644 /* also ignore unused known chunks: */ 645 png_set_keep_unknown_chunks(read_ptr, 1, unused_chunks, 646 (int)(sizeof unused_chunks)/5); 647 #endif 648 649User limits 650 651The PNG specification allows the width and height of an image to be as 652large as 2^31-1 (0x7fffffff), or about 2.147 billion rows and columns. 653For safety, libpng imposes a default limit of 1 million rows and columns. 654Larger images will be rejected immediately with a png_error() call. If 655you wish to change these limits, you can use 656 657 png_set_user_limits(png_ptr, width_max, height_max); 658 659to set your own limits (libpng may reject some very wide images 660anyway because of potential buffer overflow conditions). 661 662You should put this statement after you create the PNG structure and 663before calling png_read_info(), png_read_png(), or png_process_data(). 664 665When writing a PNG datastream, put this statement before calling 666png_write_info() or png_write_png(). 667 668If you need to retrieve the limits that are being applied, use 669 670 width_max = png_get_user_width_max(png_ptr); 671 height_max = png_get_user_height_max(png_ptr); 672 673The PNG specification sets no limit on the number of ancillary chunks 674allowed in a PNG datastream. By default, libpng imposes a limit of 675a total of 1000 sPLT, tEXt, iTXt, zTXt, and unknown chunks to be stored. 676If you have set up both info_ptr and end_info_ptr, the limit applies 677separately to each. You can change the limit on the total number of such 678chunks that will be stored, with 679 680 png_set_chunk_cache_max(png_ptr, user_chunk_cache_max); 681 682where 0x7fffffffL means unlimited. You can retrieve this limit with 683 684 chunk_cache_max = png_get_chunk_cache_max(png_ptr); 685 686Libpng imposes a limit of 8 Megabytes (8,000,000 bytes) on the amount of 687memory that a compressed chunk other than IDAT can occupy, when decompressed. 688You can change this limit with 689 690 png_set_chunk_malloc_max(png_ptr, user_chunk_malloc_max); 691 692and you can retrieve the limit with 693 694 chunk_malloc_max = png_get_chunk_malloc_max(png_ptr); 695 696Any chunks that would cause either of these limits to be exceeded will 697be ignored. 698 699Information about your system 700 701If you intend to display the PNG or to incorporate it in other image data you 702need to tell libpng information about your display or drawing surface so that 703libpng can convert the values in the image to match the display. 704 705From libpng-1.5.4 this information can be set before reading the PNG file 706header. In earlier versions png_set_gamma() existed but behaved incorrectly if 707called before the PNG file header had been read and png_set_alpha_mode() did not 708exist. 709 710If you need to support versions prior to libpng-1.5.4 test the version number 711as illustrated below using "PNG_LIBPNG_VER >= 10504" and follow the procedures 712described in the appropriate manual page. 713 714You give libpng the encoding expected by your system expressed as a 'gamma' 715value. You can also specify a default encoding for the PNG file in 716case the required information is missing from the file. By default libpng 717assumes that the PNG data matches your system, to keep this default call: 718 719 png_set_gamma(png_ptr, screen_gamma, output_gamma); 720 721or you can use the fixed point equivalent: 722 723 png_set_gamma_fixed(png_ptr, PNG_FP_1*screen_gamma, 724 PNG_FP_1*output_gamma); 725 726If you don't know the gamma for your system it is probably 2.2 - a good 727approximation to the IEC standard for display systems (sRGB). If images are 728too contrasty or washed out you got the value wrong - check your system 729documentation! 730 731Many systems permit the system gamma to be changed via a lookup table in the 732display driver, a few systems, including older Macs, change the response by 733default. As of 1.5.4 three special values are available to handle common 734situations: 735 736 PNG_DEFAULT_sRGB: Indicates that the system conforms to the 737 IEC 61966-2-1 standard. This matches almost 738 all systems. 739 PNG_GAMMA_MAC_18: Indicates that the system is an older 740 (pre Mac OS 10.6) Apple Macintosh system with 741 the default settings. 742 PNG_GAMMA_LINEAR: Just the fixed point value for 1.0 - indicates 743 that the system expects data with no gamma 744 encoding. 745 746You would use the linear (unencoded) value if you need to process the pixel 747values further because this avoids the need to decode and re-encode each 748component value whenever arithmetic is performed. A lot of graphics software 749uses linear values for this reason, often with higher precision component values 750to preserve overall accuracy. 751 752 753The output_gamma value expresses how to decode the output values, not how 754they are encoded. The values used correspond to the normal numbers used to 755describe the overall gamma of a computer display system; for example 2.2 for 756an sRGB conformant system. The values are scaled by 100000 in the _fixed 757version of the API (so 220000 for sRGB.) 758 759The inverse of the value is always used to provide a default for the PNG file 760encoding if it has no gAMA chunk and if png_set_gamma() has not been called 761to override the PNG gamma information. 762 763When the ALPHA_OPTIMIZED mode is selected the output gamma is used to encode 764opaque pixels however pixels with lower alpha values are not encoded, 765regardless of the output gamma setting. 766 767When the standard Porter Duff handling is requested with mode 1 the output 768encoding is set to be linear and the output_gamma value is only relevant 769as a default for input data that has no gamma information. The linear output 770encoding will be overridden if png_set_gamma() is called - the results may be 771highly unexpected! 772 773The following numbers are derived from the sRGB standard and the research 774behind it. sRGB is defined to be approximated by a PNG gAMA chunk value of 7750.45455 (1/2.2) for PNG. The value implicitly includes any viewing 776correction required to take account of any differences in the color 777environment of the original scene and the intended display environment; the 778value expresses how to *decode* the image for display, not how the original 779data was *encoded*. 780 781sRGB provides a peg for the PNG standard by defining a viewing environment. 782sRGB itself, and earlier TV standards, actually use a more complex transform 783(a linear portion then a gamma 2.4 power law) than PNG can express. (PNG is 784limited to simple power laws.) By saying that an image for direct display on 785an sRGB conformant system should be stored with a gAMA chunk value of 45455 786(11.3.3.2 and 11.3.3.5 of the ISO PNG specification) the PNG specification 787makes it possible to derive values for other display systems and 788environments. 789 790The Mac value is deduced from the sRGB based on an assumption that the actual 791extra viewing correction used in early Mac display systems was implemented as 792a power 1.45 lookup table. 793 794Any system where a programmable lookup table is used or where the behavior of 795the final display device characteristics can be changed requires system 796specific code to obtain the current characteristic. However this can be 797difficult and most PNG gamma correction only requires an approximate value. 798 799By default, if png_set_alpha_mode() is not called, libpng assumes that all 800values are unencoded, linear, values and that the output device also has a 801linear characteristic. This is only very rarely correct - it is invariably 802better to call png_set_alpha_mode() with PNG_DEFAULT_sRGB than rely on the 803default if you don't know what the right answer is! 804 805The special value PNG_GAMMA_MAC_18 indicates an older Mac system (pre Mac OS 80610.6) which used a correction table to implement a somewhat lower gamma on an 807otherwise sRGB system. 808 809Both these values are reserved (not simple gamma values) in order to allow 810more precise correction internally in the future. 811 812NOTE: the values can be passed to either the fixed or floating 813point APIs, but the floating point API will also accept floating point 814values. 815 816The second thing you may need to tell libpng about is how your system handles 817alpha channel information. Some, but not all, PNG files contain an alpha 818channel. To display these files correctly you need to compose the data onto a 819suitable background, as described in the PNG specification. 820 821Libpng only supports composing onto a single color (using png_set_background; 822see below). Otherwise you must do the composition yourself and, in this case, 823you may need to call png_set_alpha_mode: 824 825 #if PNG_LIBPNG_VER >= 10504 826 png_set_alpha_mode(png_ptr, mode, screen_gamma); 827 #else 828 png_set_gamma(png_ptr, screen_gamma, 1.0/screen_gamma); 829 #endif 830 831The screen_gamma value is the same as the argument to png_set_gamma; however, 832how it affects the output depends on the mode. png_set_alpha_mode() sets the 833file gamma default to 1/screen_gamma, so normally you don't need to call 834png_set_gamma. If you need different defaults call png_set_gamma() before 835png_set_alpha_mode() - if you call it after it will override the settings made 836by png_set_alpha_mode(). 837 838The mode is as follows: 839 840 PNG_ALPHA_PNG: The data is encoded according to the PNG 841specification. Red, green and blue, or gray, components are 842gamma encoded color values and are not premultiplied by the 843alpha value. The alpha value is a linear measure of the 844contribution of the pixel to the corresponding final output pixel. 845 846You should normally use this format if you intend to perform 847color correction on the color values; most, maybe all, color 848correction software has no handling for the alpha channel and, 849anyway, the math to handle pre-multiplied component values is 850unnecessarily complex. 851 852Before you do any arithmetic on the component values you need 853to remove the gamma encoding and multiply out the alpha 854channel. See the PNG specification for more detail. It is 855important to note that when an image with an alpha channel is 856scaled, linear encoded, pre-multiplied component values must 857be used! 858 859The remaining modes assume you don't need to do any further color correction or 860that if you do, your color correction software knows all about alpha (it 861probably doesn't!). They 'associate' the alpha with the color information by 862storing color channel values that have been scaled by the alpha. The 863advantage is that the color channels can be resampled (the image can be 864scaled) in this form. The disadvantage is that normal practice is to store 865linear, not (gamma) encoded, values and this requires 16-bit channels for 866still images rather than the 8-bit channels that are just about sufficient if 867gamma encoding is used. In addition all non-transparent pixel values, 868including completely opaque ones, must be gamma encoded to produce the final 869image. These are the 'STANDARD', 'ASSOCIATED' or 'PREMULTIPLIED' modes 870described below (the latter being the two common names for associated alpha 871color channels). Note that PNG files always contain non-associated color 872channels; png_set_alpha_mode() with one of the modes causes the decoder to 873convert the pixels to an associated form before returning them to your 874application. 875 876Since it is not necessary to perform arithmetic on opaque color values so 877long as they are not to be resampled and are in the final color space it is 878possible to optimize the handling of alpha by storing the opaque pixels in 879the PNG format (adjusted for the output color space) while storing partially 880opaque pixels in the standard, linear, format. The accuracy required for 881standard alpha composition is relatively low, because the pixels are 882isolated, therefore typically the accuracy loss in storing 8-bit linear 883values is acceptable. (This is not true if the alpha channel is used to 884simulate transparency over large areas - use 16 bits or the PNG mode in 885this case!) This is the 'OPTIMIZED' mode. For this mode a pixel is 886treated as opaque only if the alpha value is equal to the maximum value. 887 888 PNG_ALPHA_STANDARD: The data libpng produces is encoded in the 889standard way assumed by most correctly written graphics software. 890The gamma encoding will be removed by libpng and the 891linear component values will be pre-multiplied by the 892alpha channel. 893 894With this format the final image must be re-encoded to 895match the display gamma before the image is displayed. 896If your system doesn't do that, yet still seems to 897perform arithmetic on the pixels without decoding them, 898it is broken - check out the modes below. 899 900With PNG_ALPHA_STANDARD libpng always produces linear 901component values, whatever screen_gamma you supply. The 902screen_gamma value is, however, used as a default for 903the file gamma if the PNG file has no gamma information. 904 905If you call png_set_gamma() after png_set_alpha_mode() you 906will override the linear encoding. Instead the 907pre-multiplied pixel values will be gamma encoded but 908the alpha channel will still be linear. This may 909actually match the requirements of some broken software, 910but it is unlikely. 911 912While linear 8-bit data is often used it has 913insufficient precision for any image with a reasonable 914dynamic range. To avoid problems, and if your software 915supports it, use png_set_expand_16() to force all 916components to 16 bits. 917 918 PNG_ALPHA_OPTIMIZED: This mode is the same as PNG_ALPHA_STANDARD 919except that completely opaque pixels are gamma encoded according to 920the screen_gamma value. Pixels with alpha less than 1.0 921will still have linear components. 922 923Use this format if you have control over your 924compositing software and so don't do other arithmetic 925(such as scaling) on the data you get from libpng. Your 926compositing software can simply copy opaque pixels to 927the output but still has linear values for the 928non-opaque pixels. 929 930In normal compositing, where the alpha channel encodes 931partial pixel coverage (as opposed to broad area 932translucency), the inaccuracies of the 8-bit 933representation of non-opaque pixels are irrelevant. 934 935You can also try this format if your software is broken; 936it might look better. 937 938 PNG_ALPHA_BROKEN: This is PNG_ALPHA_STANDARD; however, all component 939values, including the alpha channel are gamma encoded. This is 940broken because, in practice, no implementation that uses this choice 941correctly undoes the encoding before handling alpha composition. Use this 942choice only if other serious errors in the software or hardware you use 943mandate it. In most cases of broken software or hardware the bug in the 944final display manifests as a subtle halo around composited parts of the 945image. You may not even perceive this as a halo; the composited part of 946the image may simply appear separate from the background, as though it had 947been cut out of paper and pasted on afterward. 948 949If you don't have to deal with bugs in software or hardware, or if you can fix 950them, there are three recommended ways of using png_set_alpha_mode(): 951 952 png_set_alpha_mode(png_ptr, PNG_ALPHA_PNG, 953 screen_gamma); 954 955You can do color correction on the result (libpng does not currently 956support color correction internally). When you handle the alpha channel 957you need to undo the gamma encoding and multiply out the alpha. 958 959 png_set_alpha_mode(png_ptr, PNG_ALPHA_STANDARD, 960 screen_gamma); 961 png_set_expand_16(png_ptr); 962 963If you are using the high level interface, don't call png_set_expand_16(); 964instead pass PNG_TRANSFORM_EXPAND_16 to the interface. 965 966With this mode you can't do color correction, but you can do arithmetic, 967including composition and scaling, on the data without further processing. 968 969 png_set_alpha_mode(png_ptr, PNG_ALPHA_OPTIMIZED, 970 screen_gamma); 971 972You can avoid the expansion to 16-bit components with this mode, but you 973lose the ability to scale the image or perform other linear arithmetic. 974All you can do is compose the result onto a matching output. Since this 975mode is libpng-specific you also need to write your own composition 976software. 977 978The following are examples of calls to png_set_alpha_mode to achieve the 979required overall gamma correction and, where necessary, alpha 980premultiplication. 981 982 png_set_alpha_mode(pp, PNG_ALPHA_PNG, PNG_DEFAULT_sRGB); 983 984This is the default libpng handling of the alpha channel - it is not 985pre-multiplied into the color components. In addition the call states 986that the output is for a sRGB system and causes all PNG files without gAMA 987chunks to be assumed to be encoded using sRGB. 988 989 png_set_alpha_mode(pp, PNG_ALPHA_PNG, PNG_GAMMA_MAC); 990 991In this case the output is assumed to be something like an sRGB conformant 992display preceeded by a power-law lookup table of power 1.45. This is how 993early Mac systems behaved. 994 995 png_set_alpha_mode(pp, PNG_ALPHA_STANDARD, PNG_GAMMA_LINEAR); 996 997This is the classic Jim Blinn approach and will work in academic 998environments where everything is done by the book. It has the shortcoming 999of assuming that input PNG data with no gamma information is linear - this 1000is unlikely to be correct unless the PNG files where generated locally. 1001Most of the time the output precision will be so low as to show 1002significant banding in dark areas of the image. 1003 1004 png_set_expand_16(pp); 1005 png_set_alpha_mode(pp, PNG_ALPHA_STANDARD, PNG_DEFAULT_sRGB); 1006 1007This is a somewhat more realistic Jim Blinn inspired approach. PNG files 1008are assumed to have the sRGB encoding if not marked with a gamma value and 1009the output is always 16 bits per component. This permits accurate scaling 1010and processing of the data. If you know that your input PNG files were 1011generated locally you might need to replace PNG_DEFAULT_sRGB with the 1012correct value for your system. 1013 1014 png_set_alpha_mode(pp, PNG_ALPHA_OPTIMIZED, PNG_DEFAULT_sRGB); 1015 1016If you just need to composite the PNG image onto an existing background 1017and if you control the code that does this you can use the optimization 1018setting. In this case you just copy completely opaque pixels to the 1019output. For pixels that are not completely transparent (you just skip 1020those) you do the composition math using png_composite or png_composite_16 1021below then encode the resultant 8-bit or 16-bit values to match the output 1022encoding. 1023 1024 Other cases 1025 1026If neither the PNG nor the standard linear encoding work for you because 1027of the software or hardware you use then you have a big problem. The PNG 1028case will probably result in halos around the image. The linear encoding 1029will probably result in a washed out, too bright, image (it's actually too 1030contrasty.) Try the ALPHA_OPTIMIZED mode above - this will probably 1031substantially reduce the halos. Alternatively try: 1032 1033 png_set_alpha_mode(pp, PNG_ALPHA_BROKEN, PNG_DEFAULT_sRGB); 1034 1035This option will also reduce the halos, but there will be slight dark 1036halos round the opaque parts of the image where the background is light. 1037In the OPTIMIZED mode the halos will be light halos where the background 1038is dark. Take your pick - the halos are unavoidable unless you can get 1039your hardware/software fixed! (The OPTIMIZED approach is slightly 1040faster.) 1041 1042When the default gamma of PNG files doesn't match the output gamma. 1043If you have PNG files with no gamma information png_set_alpha_mode allows 1044you to provide a default gamma, but it also sets the ouput gamma to the 1045matching value. If you know your PNG files have a gamma that doesn't 1046match the output you can take advantage of the fact that 1047png_set_alpha_mode always sets the output gamma but only sets the PNG 1048default if it is not already set: 1049 1050 png_set_alpha_mode(pp, PNG_ALPHA_PNG, PNG_DEFAULT_sRGB); 1051 png_set_alpha_mode(pp, PNG_ALPHA_PNG, PNG_GAMMA_MAC); 1052 1053The first call sets both the default and the output gamma values, the 1054second call overrides the output gamma without changing the default. This 1055is easier than achieving the same effect with png_set_gamma. You must use 1056PNG_ALPHA_PNG for the first call - internal checking in png_set_alpha will 1057fire if more than one call to png_set_alpha_mode and png_set_background is 1058made in the same read operation, however multiple calls with PNG_ALPHA_PNG 1059are ignored. 1060 1061If you don't need, or can't handle, the alpha channel you can call 1062png_set_background() to remove it by compositing against a fixed color. Don't 1063call png_set_strip_alpha() to do this - it will leave spurious pixel values in 1064transparent parts of this image. 1065 1066 png_set_background(png_ptr, &background_color, 1067 PNG_BACKGROUND_GAMMA_SCREEN, 0, 1); 1068 1069The background_color is an RGB or grayscale value according to the data format 1070libpng will produce for you. Because you don't yet know the format of the PNG 1071file, if you call png_set_background at this point you must arrange for the 1072format produced by libpng to always have 8-bit or 16-bit components and then 1073store the color as an 8-bit or 16-bit color as appropriate. The color contains 1074separate gray and RGB component values, so you can let libpng produce gray or 1075RGB output according to the input format, but low bit depth grayscale images 1076must always be converted to at least 8-bit format. (Even though low bit depth 1077grayscale images can't have an alpha channel they can have a transparent 1078color!) 1079 1080You set the transforms you need later, either as flags to the high level 1081interface or libpng API calls for the low level interface. For reference the 1082settings and API calls required are: 1083 10848-bit values: 1085 PNG_TRANSFORM_SCALE_16 | PNG_EXPAND 1086 png_set_expand(png_ptr); png_set_scale_16(png_ptr); 1087 1088 If you must get exactly the same inaccurate results 1089 produced by default in versions prior to libpng-1.5.4, 1090 use PNG_TRANSFORM_STRIP_16 and png_set_strip_16(png_ptr) 1091 instead. 1092 109316-bit values: 1094 PNG_TRANSFORM_EXPAND_16 1095 png_set_expand_16(png_ptr); 1096 1097In either case palette image data will be expanded to RGB. If you just want 1098color data you can add PNG_TRANSFORM_GRAY_TO_RGB or png_set_gray_to_rgb(png_ptr) 1099to the list. 1100 1101Calling png_set_background before the PNG file header is read will not work 1102prior to libpng-1.5.4. Because the failure may result in unexpected warnings or 1103errors it is therefore much safer to call png_set_background after the head has 1104been read. Unfortunately this means that prior to libpng-1.5.4 it cannot be 1105used with the high level interface. 1106 1107The high-level read interface 1108 1109At this point there are two ways to proceed; through the high-level 1110read interface, or through a sequence of low-level read operations. 1111You can use the high-level interface if (a) you are willing to read 1112the entire image into memory, and (b) the input transformations 1113you want to do are limited to the following set: 1114 1115 PNG_TRANSFORM_IDENTITY No transformation 1116 PNG_TRANSFORM_SCALE_16 Strip 16-bit samples to 1117 8-bit accurately 1118 PNG_TRANSFORM_STRIP_16 Chop 16-bit samples to 1119 8-bit less accurately 1120 PNG_TRANSFORM_STRIP_ALPHA Discard the alpha channel 1121 PNG_TRANSFORM_PACKING Expand 1, 2 and 4-bit 1122 samples to bytes 1123 PNG_TRANSFORM_PACKSWAP Change order of packed 1124 pixels to LSB first 1125 PNG_TRANSFORM_EXPAND Perform set_expand() 1126 PNG_TRANSFORM_INVERT_MONO Invert monochrome images 1127 PNG_TRANSFORM_SHIFT Normalize pixels to the 1128 sBIT depth 1129 PNG_TRANSFORM_BGR Flip RGB to BGR, RGBA 1130 to BGRA 1131 PNG_TRANSFORM_SWAP_ALPHA Flip RGBA to ARGB or GA 1132 to AG 1133 PNG_TRANSFORM_INVERT_ALPHA Change alpha from opacity 1134 to transparency 1135 PNG_TRANSFORM_SWAP_ENDIAN Byte-swap 16-bit samples 1136 PNG_TRANSFORM_GRAY_TO_RGB Expand grayscale samples 1137 to RGB (or GA to RGBA) 1138 PNG_TRANSFORM_EXPAND_16 Expand samples to 16 bits 1139 1140(This excludes setting a background color, doing gamma transformation, 1141quantizing, and setting filler.) If this is the case, simply do this: 1142 1143 png_read_png(png_ptr, info_ptr, png_transforms, NULL) 1144 1145where png_transforms is an integer containing the bitwise OR of some 1146set of transformation flags. This call is equivalent to png_read_info(), 1147followed the set of transformations indicated by the transform mask, 1148then png_read_image(), and finally png_read_end(). 1149 1150(The final parameter of this call is not yet used. Someday it might point 1151to transformation parameters required by some future input transform.) 1152 1153You must use png_transforms and not call any png_set_transform() functions 1154when you use png_read_png(). 1155 1156After you have called png_read_png(), you can retrieve the image data 1157with 1158 1159 row_pointers = png_get_rows(png_ptr, info_ptr); 1160 1161where row_pointers is an array of pointers to the pixel data for each row: 1162 1163 png_bytep row_pointers[height]; 1164 1165If you know your image size and pixel size ahead of time, you can allocate 1166row_pointers prior to calling png_read_png() with 1167 1168 if (height > PNG_UINT_32_MAX/(sizeof (png_byte))) 1169 png_error (png_ptr, 1170 "Image is too tall to process in memory"); 1171 1172 if (width > PNG_UINT_32_MAX/pixel_size) 1173 png_error (png_ptr, 1174 "Image is too wide to process in memory"); 1175 1176 row_pointers = png_malloc(png_ptr, 1177 height*(sizeof (png_bytep))); 1178 1179 for (int i=0; i<height, i++) 1180 row_pointers[i]=NULL; /* security precaution */ 1181 1182 for (int i=0; i<height, i++) 1183 row_pointers[i]=png_malloc(png_ptr, 1184 width*pixel_size); 1185 1186 png_set_rows(png_ptr, info_ptr, &row_pointers); 1187 1188Alternatively you could allocate your image in one big block and define 1189row_pointers[i] to point into the proper places in your block. 1190 1191If you use png_set_rows(), the application is responsible for freeing 1192row_pointers (and row_pointers[i], if they were separately allocated). 1193 1194If you don't allocate row_pointers ahead of time, png_read_png() will 1195do it, and it'll be free'ed by libpng when you call png_destroy_*(). 1196 1197The low-level read interface 1198 1199If you are going the low-level route, you are now ready to read all 1200the file information up to the actual image data. You do this with a 1201call to png_read_info(). 1202 1203 png_read_info(png_ptr, info_ptr); 1204 1205This will process all chunks up to but not including the image data. 1206 1207This also copies some of the data from the PNG file into the decode structure 1208for use in later transformations. Important information copied in is: 1209 12101) The PNG file gamma from the gAMA chunk. This overwrites the default value 1211provided by an earlier call to png_set_gamma or png_set_alpha_mode. 1212 12132) Prior to libpng-1.5.4 the background color from a bKGd chunk. This 1214damages the information provided by an earlier call to png_set_background 1215resulting in unexpected behavior. Libpng-1.5.4 no longer does this. 1216 12173) The number of significant bits in each component value. Libpng uses this to 1218optimize gamma handling by reducing the internal lookup table sizes. 1219 12204) The transparent color information from a tRNS chunk. This can be modified by 1221a later call to png_set_tRNS. 1222 1223Querying the info structure 1224 1225Functions are used to get the information from the info_ptr once it 1226has been read. Note that these fields may not be completely filled 1227in until png_read_end() has read the chunk data following the image. 1228 1229 png_get_IHDR(png_ptr, info_ptr, &width, &height, 1230 &bit_depth, &color_type, &interlace_type, 1231 &compression_type, &filter_method); 1232 1233 width - holds the width of the image 1234 in pixels (up to 2^31). 1235 1236 height - holds the height of the image 1237 in pixels (up to 2^31). 1238 1239 bit_depth - holds the bit depth of one of the 1240 image channels. (valid values are 1241 1, 2, 4, 8, 16 and depend also on 1242 the color_type. See also 1243 significant bits (sBIT) below). 1244 1245 color_type - describes which color/alpha channels 1246 are present. 1247 PNG_COLOR_TYPE_GRAY 1248 (bit depths 1, 2, 4, 8, 16) 1249 PNG_COLOR_TYPE_GRAY_ALPHA 1250 (bit depths 8, 16) 1251 PNG_COLOR_TYPE_PALETTE 1252 (bit depths 1, 2, 4, 8) 1253 PNG_COLOR_TYPE_RGB 1254 (bit_depths 8, 16) 1255 PNG_COLOR_TYPE_RGB_ALPHA 1256 (bit_depths 8, 16) 1257 1258 PNG_COLOR_MASK_PALETTE 1259 PNG_COLOR_MASK_COLOR 1260 PNG_COLOR_MASK_ALPHA 1261 1262 interlace_type - (PNG_INTERLACE_NONE or 1263 PNG_INTERLACE_ADAM7) 1264 1265 compression_type - (must be PNG_COMPRESSION_TYPE_BASE 1266 for PNG 1.0) 1267 1268 filter_method - (must be PNG_FILTER_TYPE_BASE 1269 for PNG 1.0, and can also be 1270 PNG_INTRAPIXEL_DIFFERENCING if 1271 the PNG datastream is embedded in 1272 a MNG-1.0 datastream) 1273 1274 Any of width, height, color_type, bit_depth, 1275 interlace_type, compression_type, or filter_method can 1276 be NULL if you are not interested in their values. 1277 1278 Note that png_get_IHDR() returns 32-bit data into 1279 the application's width and height variables. 1280 This is an unsafe situation if these are not png_uint_32 1281 variables. In such situations, the 1282 png_get_image_width() and png_get_image_height() 1283 functions described below are safer. 1284 1285 width = png_get_image_width(png_ptr, 1286 info_ptr); 1287 1288 height = png_get_image_height(png_ptr, 1289 info_ptr); 1290 1291 bit_depth = png_get_bit_depth(png_ptr, 1292 info_ptr); 1293 1294 color_type = png_get_color_type(png_ptr, 1295 info_ptr); 1296 1297 interlace_type = png_get_interlace_type(png_ptr, 1298 info_ptr); 1299 1300 compression_type = png_get_compression_type(png_ptr, 1301 info_ptr); 1302 1303 filter_method = png_get_filter_type(png_ptr, 1304 info_ptr); 1305 1306 channels = png_get_channels(png_ptr, info_ptr); 1307 1308 channels - number of channels of info for the 1309 color type (valid values are 1 (GRAY, 1310 PALETTE), 2 (GRAY_ALPHA), 3 (RGB), 1311 4 (RGB_ALPHA or RGB + filler byte)) 1312 1313 rowbytes = png_get_rowbytes(png_ptr, info_ptr); 1314 1315 rowbytes - number of bytes needed to hold a row 1316 1317 signature = png_get_signature(png_ptr, info_ptr); 1318 1319 signature - holds the signature read from the 1320 file (if any). The data is kept in 1321 the same offset it would be if the 1322 whole signature were read (i.e. if an 1323 application had already read in 4 1324 bytes of signature before starting 1325 libpng, the remaining 4 bytes would 1326 be in signature[4] through signature[7] 1327 (see png_set_sig_bytes())). 1328 1329These are also important, but their validity depends on whether the chunk 1330has been read. The png_get_valid(png_ptr, info_ptr, PNG_INFO_<chunk>) and 1331png_get_<chunk>(png_ptr, info_ptr, ...) functions return non-zero if the 1332data has been read, or zero if it is missing. The parameters to the 1333png_get_<chunk> are set directly if they are simple data types, or a 1334pointer into the info_ptr is returned for any complex types. 1335 1336The colorspace data from gAMA, cHRM, sRGB, iCCP, and sBIT chunks 1337is simply returned to give the application information about how the 1338image was encoded. Libpng itself only does transformations using the file 1339gamma when combining semitransparent pixels with the background color, and, 1340since libpng-1.6.0, when converting between 8-bit sRGB and 16-bit linear pixels 1341within the simplified API. Libpng also uses the file gamma when converting 1342RGB to gray, beginning with libpng-1.0.5, if the application calls 1343png_set_rgb_to_gray()). 1344 1345 png_get_PLTE(png_ptr, info_ptr, &palette, 1346 &num_palette); 1347 1348 palette - the palette for the file 1349 (array of png_color) 1350 1351 num_palette - number of entries in the palette 1352 1353 png_get_gAMA(png_ptr, info_ptr, &file_gamma); 1354 png_get_gAMA_fixed(png_ptr, info_ptr, &int_file_gamma); 1355 1356 file_gamma - the gamma at which the file is 1357 written (PNG_INFO_gAMA) 1358 1359 int_file_gamma - 100,000 times the gamma at which the 1360 file is written 1361 1362 png_get_cHRM(png_ptr, info_ptr, &white_x, &white_y, &red_x, 1363 &red_y, &green_x, &green_y, &blue_x, &blue_y) 1364 png_get_cHRM_XYZ(png_ptr, info_ptr, &red_X, &red_Y, &red_Z, 1365 &green_X, &green_Y, &green_Z, &blue_X, &blue_Y, 1366 &blue_Z) 1367 png_get_cHRM_fixed(png_ptr, info_ptr, &int_white_x, 1368 &int_white_y, &int_red_x, &int_red_y, 1369 &int_green_x, &int_green_y, &int_blue_x, 1370 &int_blue_y) 1371 png_get_cHRM_XYZ_fixed(png_ptr, info_ptr, &int_red_X, &int_red_Y, 1372 &int_red_Z, &int_green_X, &int_green_Y, 1373 &int_green_Z, &int_blue_X, &int_blue_Y, 1374 &int_blue_Z) 1375 1376 {white,red,green,blue}_{x,y} 1377 A color space encoding specified using the 1378 chromaticities of the end points and the 1379 white point. (PNG_INFO_cHRM) 1380 1381 {red,green,blue}_{X,Y,Z} 1382 A color space encoding specified using the 1383 encoding end points - the CIE tristimulus 1384 specification of the intended color of the red, 1385 green and blue channels in the PNG RGB data. 1386 The white point is simply the sum of the three 1387 end points. (PNG_INFO_cHRM) 1388 1389 png_get_sRGB(png_ptr, info_ptr, &srgb_intent); 1390 1391 srgb_intent - the rendering intent (PNG_INFO_sRGB) 1392 The presence of the sRGB chunk 1393 means that the pixel data is in the 1394 sRGB color space. This chunk also 1395 implies specific values of gAMA and 1396 cHRM. 1397 1398 png_get_iCCP(png_ptr, info_ptr, &name, 1399 &compression_type, &profile, &proflen); 1400 1401 name - The profile name. 1402 1403 compression_type - The compression type; always 1404 PNG_COMPRESSION_TYPE_BASE for PNG 1.0. 1405 You may give NULL to this argument to 1406 ignore it. 1407 1408 profile - International Color Consortium color 1409 profile data. May contain NULs. 1410 1411 proflen - length of profile data in bytes. 1412 1413 png_get_sBIT(png_ptr, info_ptr, &sig_bit); 1414 1415 sig_bit - the number of significant bits for 1416 (PNG_INFO_sBIT) each of the gray, 1417 red, green, and blue channels, 1418 whichever are appropriate for the 1419 given color type (png_color_16) 1420 1421 png_get_tRNS(png_ptr, info_ptr, &trans_alpha, 1422 &num_trans, &trans_color); 1423 1424 trans_alpha - array of alpha (transparency) 1425 entries for palette (PNG_INFO_tRNS) 1426 1427 num_trans - number of transparent entries 1428 (PNG_INFO_tRNS) 1429 1430 trans_color - graylevel or color sample values of 1431 the single transparent color for 1432 non-paletted images (PNG_INFO_tRNS) 1433 1434 png_get_hIST(png_ptr, info_ptr, &hist); 1435 (PNG_INFO_hIST) 1436 1437 hist - histogram of palette (array of 1438 png_uint_16) 1439 1440 png_get_tIME(png_ptr, info_ptr, &mod_time); 1441 1442 mod_time - time image was last modified 1443 (PNG_VALID_tIME) 1444 1445 png_get_bKGD(png_ptr, info_ptr, &background); 1446 1447 background - background color (of type 1448 png_color_16p) (PNG_VALID_bKGD) 1449 valid 16-bit red, green and blue 1450 values, regardless of color_type 1451 1452 num_comments = png_get_text(png_ptr, info_ptr, 1453 &text_ptr, &num_text); 1454 1455 num_comments - number of comments 1456 1457 text_ptr - array of png_text holding image 1458 comments 1459 1460 text_ptr[i].compression - type of compression used 1461 on "text" PNG_TEXT_COMPRESSION_NONE 1462 PNG_TEXT_COMPRESSION_zTXt 1463 PNG_ITXT_COMPRESSION_NONE 1464 PNG_ITXT_COMPRESSION_zTXt 1465 1466 text_ptr[i].key - keyword for comment. Must contain 1467 1-79 characters. 1468 1469 text_ptr[i].text - text comments for current 1470 keyword. Can be empty. 1471 1472 text_ptr[i].text_length - length of text string, 1473 after decompression, 0 for iTXt 1474 1475 text_ptr[i].itxt_length - length of itxt string, 1476 after decompression, 0 for tEXt/zTXt 1477 1478 text_ptr[i].lang - language of comment (empty 1479 string for unknown). 1480 1481 text_ptr[i].lang_key - keyword in UTF-8 1482 (empty string for unknown). 1483 1484 Note that the itxt_length, lang, and lang_key 1485 members of the text_ptr structure only exist when the 1486 library is built with iTXt chunk support. Prior to 1487 libpng-1.4.0 the library was built by default without 1488 iTXt support. Also note that when iTXt is supported, 1489 they contain NULL pointers when the "compression" 1490 field contains PNG_TEXT_COMPRESSION_NONE or 1491 PNG_TEXT_COMPRESSION_zTXt. 1492 1493 num_text - number of comments (same as 1494 num_comments; you can put NULL here 1495 to avoid the duplication) 1496 1497 Note while png_set_text() will accept text, language, 1498 and translated keywords that can be NULL pointers, the 1499 structure returned by png_get_text will always contain 1500 regular zero-terminated C strings. They might be 1501 empty strings but they will never be NULL pointers. 1502 1503 num_spalettes = png_get_sPLT(png_ptr, info_ptr, 1504 &palette_ptr); 1505 1506 num_spalettes - number of sPLT chunks read. 1507 1508 palette_ptr - array of palette structures holding 1509 contents of one or more sPLT chunks 1510 read. 1511 1512 png_get_oFFs(png_ptr, info_ptr, &offset_x, &offset_y, 1513 &unit_type); 1514 1515 offset_x - positive offset from the left edge 1516 of the screen (can be negative) 1517 1518 offset_y - positive offset from the top edge 1519 of the screen (can be negative) 1520 1521 unit_type - PNG_OFFSET_PIXEL, PNG_OFFSET_MICROMETER 1522 1523 png_get_pHYs(png_ptr, info_ptr, &res_x, &res_y, 1524 &unit_type); 1525 1526 res_x - pixels/unit physical resolution in 1527 x direction 1528 1529 res_y - pixels/unit physical resolution in 1530 x direction 1531 1532 unit_type - PNG_RESOLUTION_UNKNOWN, 1533 PNG_RESOLUTION_METER 1534 1535 png_get_sCAL(png_ptr, info_ptr, &unit, &width, 1536 &height) 1537 1538 unit - physical scale units (an integer) 1539 1540 width - width of a pixel in physical scale units 1541 1542 height - height of a pixel in physical scale units 1543 (width and height are doubles) 1544 1545 png_get_sCAL_s(png_ptr, info_ptr, &unit, &width, 1546 &height) 1547 1548 unit - physical scale units (an integer) 1549 1550 width - width of a pixel in physical scale units 1551 (expressed as a string) 1552 1553 height - height of a pixel in physical scale units 1554 (width and height are strings like "2.54") 1555 1556 num_unknown_chunks = png_get_unknown_chunks(png_ptr, 1557 info_ptr, &unknowns) 1558 1559 unknowns - array of png_unknown_chunk 1560 structures holding unknown chunks 1561 1562 unknowns[i].name - name of unknown chunk 1563 1564 unknowns[i].data - data of unknown chunk 1565 1566 unknowns[i].size - size of unknown chunk's data 1567 1568 unknowns[i].location - position of chunk in file 1569 1570 The value of "i" corresponds to the order in which the 1571 chunks were read from the PNG file or inserted with the 1572 png_set_unknown_chunks() function. 1573 1574 The value of "location" is a bitwise "or" of 1575 1576 PNG_HAVE_IHDR (0x01) 1577 PNG_HAVE_PLTE (0x02) 1578 PNG_AFTER_IDAT (0x08) 1579 1580The data from the pHYs chunk can be retrieved in several convenient 1581forms: 1582 1583 res_x = png_get_x_pixels_per_meter(png_ptr, 1584 info_ptr) 1585 1586 res_y = png_get_y_pixels_per_meter(png_ptr, 1587 info_ptr) 1588 1589 res_x_and_y = png_get_pixels_per_meter(png_ptr, 1590 info_ptr) 1591 1592 res_x = png_get_x_pixels_per_inch(png_ptr, 1593 info_ptr) 1594 1595 res_y = png_get_y_pixels_per_inch(png_ptr, 1596 info_ptr) 1597 1598 res_x_and_y = png_get_pixels_per_inch(png_ptr, 1599 info_ptr) 1600 1601 aspect_ratio = png_get_pixel_aspect_ratio(png_ptr, 1602 info_ptr) 1603 1604 Each of these returns 0 [signifying "unknown"] if 1605 the data is not present or if res_x is 0; 1606 res_x_and_y is 0 if res_x != res_y 1607 1608 Note that because of the way the resolutions are 1609 stored internally, the inch conversions won't 1610 come out to exactly even number. For example, 1611 72 dpi is stored as 0.28346 pixels/meter, and 1612 when this is retrieved it is 71.9988 dpi, so 1613 be sure to round the returned value appropriately 1614 if you want to display a reasonable-looking result. 1615 1616The data from the oFFs chunk can be retrieved in several convenient 1617forms: 1618 1619 x_offset = png_get_x_offset_microns(png_ptr, info_ptr); 1620 1621 y_offset = png_get_y_offset_microns(png_ptr, info_ptr); 1622 1623 x_offset = png_get_x_offset_inches(png_ptr, info_ptr); 1624 1625 y_offset = png_get_y_offset_inches(png_ptr, info_ptr); 1626 1627 Each of these returns 0 [signifying "unknown" if both 1628 x and y are 0] if the data is not present or if the 1629 chunk is present but the unit is the pixel. The 1630 remark about inexact inch conversions applies here 1631 as well, because a value in inches can't always be 1632 converted to microns and back without some loss 1633 of precision. 1634 1635For more information, see the 1636PNG specification for chunk contents. Be careful with trusting 1637rowbytes, as some of the transformations could increase the space 1638needed to hold a row (expand, filler, gray_to_rgb, etc.). 1639See png_read_update_info(), below. 1640 1641A quick word about text_ptr and num_text. PNG stores comments in 1642keyword/text pairs, one pair per chunk, with no limit on the number 1643of text chunks, and a 2^31 byte limit on their size. While there are 1644suggested keywords, there is no requirement to restrict the use to these 1645strings. It is strongly suggested that keywords and text be sensible 1646to humans (that's the point), so don't use abbreviations. Non-printing 1647symbols are not allowed. See the PNG specification for more details. 1648There is also no requirement to have text after the keyword. 1649 1650Keywords should be limited to 79 Latin-1 characters without leading or 1651trailing spaces, but non-consecutive spaces are allowed within the 1652keyword. It is possible to have the same keyword any number of times. 1653The text_ptr is an array of png_text structures, each holding a 1654pointer to a language string, a pointer to a keyword and a pointer to 1655a text string. The text string, language code, and translated 1656keyword may be empty or NULL pointers. The keyword/text 1657pairs are put into the array in the order that they are received. 1658However, some or all of the text chunks may be after the image, so, to 1659make sure you have read all the text chunks, don't mess with these 1660until after you read the stuff after the image. This will be 1661mentioned again below in the discussion that goes with png_read_end(). 1662 1663Input transformations 1664 1665After you've read the header information, you can set up the library 1666to handle any special transformations of the image data. The various 1667ways to transform the data will be described in the order that they 1668should occur. This is important, as some of these change the color 1669type and/or bit depth of the data, and some others only work on 1670certain color types and bit depths. 1671 1672Transformations you request are ignored if they don't have any meaning for a 1673particular input data format. However some transformations can have an effect 1674as a result of a previous transformation. If you specify a contradictory set of 1675transformations, for example both adding and removing the alpha channel, you 1676cannot predict the final result. 1677 1678The color used for the transparency values should be supplied in the same 1679format/depth as the current image data. It is stored in the same format/depth 1680as the image data in a tRNS chunk, so this is what libpng expects for this data. 1681 1682The color used for the background value depends on the need_expand argument as 1683described below. 1684 1685Data will be decoded into the supplied row buffers packed into bytes 1686unless the library has been told to transform it into another format. 1687For example, 4 bit/pixel paletted or grayscale data will be returned 16882 pixels/byte with the leftmost pixel in the high-order bits of the byte, 1689unless png_set_packing() is called. 8-bit RGB data will be stored 1690in RGB RGB RGB format unless png_set_filler() or png_set_add_alpha() 1691is called to insert filler bytes, either before or after each RGB triplet. 1692 169316-bit RGB data will be returned RRGGBB RRGGBB, with the most significant 1694byte of the color value first, unless png_set_scale_16() is called to 1695transform it to regular RGB RGB triplets, or png_set_filler() or 1696png_set_add alpha() is called to insert two filler bytes, either before 1697or after each RRGGBB triplet. Similarly, 8-bit or 16-bit grayscale data can 1698be modified with png_set_filler(), png_set_add_alpha(), png_set_strip_16(), 1699or png_set_scale_16(). 1700 1701The following code transforms grayscale images of less than 8 to 8 bits, 1702changes paletted images to RGB, and adds a full alpha channel if there is 1703transparency information in a tRNS chunk. This is most useful on 1704grayscale images with bit depths of 2 or 4 or if there is a multiple-image 1705viewing application that wishes to treat all images in the same way. 1706 1707 if (color_type == PNG_COLOR_TYPE_PALETTE) 1708 png_set_palette_to_rgb(png_ptr); 1709 1710 if (png_get_valid(png_ptr, info_ptr, 1711 PNG_INFO_tRNS)) png_set_tRNS_to_alpha(png_ptr); 1712 1713 if (color_type == PNG_COLOR_TYPE_GRAY && 1714 bit_depth < 8) png_set_expand_gray_1_2_4_to_8(png_ptr); 1715 1716The first two functions are actually aliases for png_set_expand(), added 1717in libpng version 1.0.4, with the function names expanded to improve code 1718readability. In some future version they may actually do different 1719things. 1720 1721As of libpng version 1.2.9, png_set_expand_gray_1_2_4_to_8() was 1722added. It expands the sample depth without changing tRNS to alpha. 1723 1724As of libpng version 1.5.2, png_set_expand_16() was added. It behaves as 1725png_set_expand(); however, the resultant channels have 16 bits rather than 8. 1726Use this when the output color or gray channels are made linear to avoid fairly 1727severe accuracy loss. 1728 1729 if (bit_depth < 16) 1730 png_set_expand_16(png_ptr); 1731 1732PNG can have files with 16 bits per channel. If you only can handle 17338 bits per channel, this will strip the pixels down to 8-bit. 1734 1735 if (bit_depth == 16) 1736#if PNG_LIBPNG_VER >= 10504 1737 png_set_scale_16(png_ptr); 1738#else 1739 png_set_strip_16(png_ptr); 1740#endif 1741 1742(The more accurate "png_set_scale_16()" API became available in libpng version 17431.5.4). 1744 1745If you need to process the alpha channel on the image separately from the image 1746data (for example if you convert it to a bitmap mask) it is possible to have 1747libpng strip the channel leaving just RGB or gray data: 1748 1749 if (color_type & PNG_COLOR_MASK_ALPHA) 1750 png_set_strip_alpha(png_ptr); 1751 1752If you strip the alpha channel you need to find some other way of dealing with 1753the information. If, instead, you want to convert the image to an opaque 1754version with no alpha channel use png_set_background; see below. 1755 1756As of libpng version 1.5.2, almost all useful expansions are supported, the 1757major ommissions are conversion of grayscale to indexed images (which can be 1758done trivially in the application) and conversion of indexed to grayscale (which 1759can be done by a trivial manipulation of the palette.) 1760 1761In the following table, the 01 means grayscale with depth<8, 31 means 1762indexed with depth<8, other numerals represent the color type, "T" means 1763the tRNS chunk is present, A means an alpha channel is present, and O 1764means tRNS or alpha is present but all pixels in the image are opaque. 1765 1766 FROM 01 31 0 0T 0O 2 2T 2O 3 3T 3O 4A 4O 6A 6O 1767 TO 1768 01 - [G] - - - - - - - - - - - - - 1769 31 [Q] Q [Q] [Q] [Q] Q Q Q Q Q Q [Q] [Q] Q Q 1770 0 1 G + . . G G G G G G B B GB GB 1771 0T lt Gt t + . Gt G G Gt G G Bt Bt GBt GBt 1772 0O lt Gt t . + Gt Gt G Gt Gt G Bt Bt GBt GBt 1773 2 C P C C C + . . C - - CB CB B B 1774 2T Ct - Ct C C t + t - - - CBt CBt Bt Bt 1775 2O Ct - Ct C C t t + - - - CBt CBt Bt Bt 1776 3 [Q] p [Q] [Q] [Q] Q Q Q + . . [Q] [Q] Q Q 1777 3T [Qt] p [Qt][Q] [Q] Qt Qt Qt t + t [Qt][Qt] Qt Qt 1778 3O [Qt] p [Qt][Q] [Q] Qt Qt Qt t t + [Qt][Qt] Qt Qt 1779 4A lA G A T T GA GT GT GA GT GT + BA G GBA 1780 4O lA GBA A T T GA GT GT GA GT GT BA + GBA G 1781 6A CA PA CA C C A T tT PA P P C CBA + BA 1782 6O CA PBA CA C C A tT T PA P P CBA C BA + 1783 1784Within the matrix, 1785 "+" identifies entries where 'from' and 'to' are the same. 1786 "-" means the transformation is not supported. 1787 "." means nothing is necessary (a tRNS chunk can just be ignored). 1788 "t" means the transformation is obtained by png_set_tRNS. 1789 "A" means the transformation is obtained by png_set_add_alpha(). 1790 "X" means the transformation is obtained by png_set_expand(). 1791 "1" means the transformation is obtained by 1792 png_set_expand_gray_1_2_4_to_8() (and by png_set_expand() 1793 if there is no transparency in the original or the final 1794 format). 1795 "C" means the transformation is obtained by png_set_gray_to_rgb(). 1796 "G" means the transformation is obtained by png_set_rgb_to_gray(). 1797 "P" means the transformation is obtained by 1798 png_set_expand_palette_to_rgb(). 1799 "p" means the transformation is obtained by png_set_packing(). 1800 "Q" means the transformation is obtained by png_set_quantize(). 1801 "T" means the transformation is obtained by 1802 png_set_tRNS_to_alpha(). 1803 "B" means the transformation is obtained by 1804 png_set_background(), or png_strip_alpha(). 1805 1806When an entry has multiple transforms listed all are required to cause the 1807right overall transformation. When two transforms are separated by a comma 1808either will do the job. When transforms are enclosed in [] the transform should 1809do the job but this is currently unimplemented - a different format will result 1810if the suggested transformations are used. 1811 1812In PNG files, the alpha channel in an image 1813is the level of opacity. If you need the alpha channel in an image to 1814be the level of transparency instead of opacity, you can invert the 1815alpha channel (or the tRNS chunk data) after it's read, so that 0 is 1816fully opaque and 255 (in 8-bit or paletted images) or 65535 (in 16-bit 1817images) is fully transparent, with 1818 1819 png_set_invert_alpha(png_ptr); 1820 1821PNG files pack pixels of bit depths 1, 2, and 4 into bytes as small as 1822they can, resulting in, for example, 8 pixels per byte for 1 bit 1823files. This code expands to 1 pixel per byte without changing the 1824values of the pixels: 1825 1826 if (bit_depth < 8) 1827 png_set_packing(png_ptr); 1828 1829PNG files have possible bit depths of 1, 2, 4, 8, and 16. All pixels 1830stored in a PNG image have been "scaled" or "shifted" up to the next 1831higher possible bit depth (e.g. from 5 bits/sample in the range [0,31] 1832to 8 bits/sample in the range [0, 255]). However, it is also possible 1833to convert the PNG pixel data back to the original bit depth of the 1834image. This call reduces the pixels back down to the original bit depth: 1835 1836 png_color_8p sig_bit; 1837 1838 if (png_get_sBIT(png_ptr, info_ptr, &sig_bit)) 1839 png_set_shift(png_ptr, sig_bit); 1840 1841PNG files store 3-color pixels in red, green, blue order. This code 1842changes the storage of the pixels to blue, green, red: 1843 1844 if (color_type == PNG_COLOR_TYPE_RGB || 1845 color_type == PNG_COLOR_TYPE_RGB_ALPHA) 1846 png_set_bgr(png_ptr); 1847 1848PNG files store RGB pixels packed into 3 or 6 bytes. This code expands them 1849into 4 or 8 bytes for windowing systems that need them in this format: 1850 1851 if (color_type == PNG_COLOR_TYPE_RGB) 1852 png_set_filler(png_ptr, filler, PNG_FILLER_BEFORE); 1853 1854where "filler" is the 8-bit or 16-bit number to fill with, and the location 1855is either PNG_FILLER_BEFORE or PNG_FILLER_AFTER, depending upon whether 1856you want the filler before the RGB or after. When filling an 8-bit pixel, 1857the least significant 8 bits of the number are used, if a 16-bit number is 1858supplied. This transformation does not affect images that already have full 1859alpha channels. To add an opaque alpha channel, use filler=0xffff and 1860PNG_FILLER_AFTER which will generate RGBA pixels. 1861 1862Note that png_set_filler() does not change the color type. If you want 1863to do that, you can add a true alpha channel with 1864 1865 if (color_type == PNG_COLOR_TYPE_RGB || 1866 color_type == PNG_COLOR_TYPE_GRAY) 1867 png_set_add_alpha(png_ptr, filler, PNG_FILLER_AFTER); 1868 1869where "filler" contains the alpha value to assign to each pixel. 1870The png_set_add_alpha() function was added in libpng-1.2.7. 1871 1872If you are reading an image with an alpha channel, and you need the 1873data as ARGB instead of the normal PNG format RGBA: 1874 1875 if (color_type == PNG_COLOR_TYPE_RGB_ALPHA) 1876 png_set_swap_alpha(png_ptr); 1877 1878For some uses, you may want a grayscale image to be represented as 1879RGB. This code will do that conversion: 1880 1881 if (color_type == PNG_COLOR_TYPE_GRAY || 1882 color_type == PNG_COLOR_TYPE_GRAY_ALPHA) 1883 png_set_gray_to_rgb(png_ptr); 1884 1885Conversely, you can convert an RGB or RGBA image to grayscale or grayscale 1886with alpha. 1887 1888 if (color_type == PNG_COLOR_TYPE_RGB || 1889 color_type == PNG_COLOR_TYPE_RGB_ALPHA) 1890 png_set_rgb_to_gray(png_ptr, error_action, 1891 double red_weight, double green_weight); 1892 1893 error_action = 1: silently do the conversion 1894 1895 error_action = 2: issue a warning if the original 1896 image has any pixel where 1897 red != green or red != blue 1898 1899 error_action = 3: issue an error and abort the 1900 conversion if the original 1901 image has any pixel where 1902 red != green or red != blue 1903 1904 red_weight: weight of red component 1905 1906 green_weight: weight of green component 1907 If either weight is negative, default 1908 weights are used. 1909 1910In the corresponding fixed point API the red_weight and green_weight values are 1911simply scaled by 100,000: 1912 1913 png_set_rgb_to_gray(png_ptr, error_action, 1914 png_fixed_point red_weight, 1915 png_fixed_point green_weight); 1916 1917If you have set error_action = 1 or 2, you can 1918later check whether the image really was gray, after processing 1919the image rows, with the png_get_rgb_to_gray_status(png_ptr) function. 1920It will return a png_byte that is zero if the image was gray or 19211 if there were any non-gray pixels. Background and sBIT data 1922will be silently converted to grayscale, using the green channel 1923data for sBIT, regardless of the error_action setting. 1924 1925The default values come from the PNG file cHRM chunk if present; otherwise, the 1926defaults correspond to the ITU-R recommendation 709, and also the sRGB color 1927space, as recommended in the Charles Poynton's Colour FAQ, 1928Copyright (c) 2006-11-28 Charles Poynton, in section 9: 1929 1930<http://www.poynton.com/notes/colour_and_gamma/ColorFAQ.html#RTFToC9> 1931 1932 Y = 0.2126 * R + 0.7152 * G + 0.0722 * B 1933 1934Previous versions of this document, 1998 through 2002, recommended a slightly 1935different formula: 1936 1937 Y = 0.212671 * R + 0.715160 * G + 0.072169 * B 1938 1939Libpng uses an integer approximation: 1940 1941 Y = (6968 * R + 23434 * G + 2366 * B)/32768 1942 1943The calculation is done in a linear colorspace, if the image gamma 1944can be determined. 1945 1946The png_set_background() function has been described already; it tells libpng to 1947composite images with alpha or simple transparency against the supplied 1948background color. For compatibility with versions of libpng earlier than 1949libpng-1.5.4 it is recommended that you call the function after reading the file 1950header, even if you don't want to use the color in a bKGD chunk, if one exists. 1951 1952If the PNG file contains a bKGD chunk (PNG_INFO_bKGD valid), 1953you may use this color, or supply another color more suitable for 1954the current display (e.g., the background color from a web page). You 1955need to tell libpng how the color is represented, both the format of the 1956component values in the color (the number of bits) and the gamma encoding of the 1957color. The function takes two arguments, background_gamma_mode and need_expand 1958to convey this information; however, only two combinations are likely to be 1959useful: 1960 1961 png_color_16 my_background; 1962 png_color_16p image_background; 1963 1964 if (png_get_bKGD(png_ptr, info_ptr, &image_background)) 1965 png_set_background(png_ptr, image_background, 1966 PNG_BACKGROUND_GAMMA_FILE, 1/*needs to be expanded*/, 1); 1967 else 1968 png_set_background(png_ptr, &my_background, 1969 PNG_BACKGROUND_GAMMA_SCREEN, 0/*do not expand*/, 1); 1970 1971The second call was described above - my_background is in the format of the 1972final, display, output produced by libpng. Because you now know the format of 1973the PNG it is possible to avoid the need to choose either 8-bit or 16-bit 1974output and to retain palette images (the palette colors will be modified 1975appropriately and the tRNS chunk removed.) However, if you are doing this, 1976take great care not to ask for transformations without checking first that 1977they apply! 1978 1979In the first call the background color has the original bit depth and color type 1980of the PNG file. So, for palette images the color is supplied as a palette 1981index and for low bit greyscale images the color is a reduced bit value in 1982image_background->gray. 1983 1984If you didn't call png_set_gamma() before reading the file header, for example 1985if you need your code to remain compatible with older versions of libpng prior 1986to libpng-1.5.4, this is the place to call it. 1987 1988Do not call it if you called png_set_alpha_mode(); doing so will damage the 1989settings put in place by png_set_alpha_mode(). (If png_set_alpha_mode() is 1990supported then you can certainly do png_set_gamma() before reading the PNG 1991header.) 1992 1993This API unconditionally sets the screen and file gamma values, so it will 1994override the value in the PNG file unless it is called before the PNG file 1995reading starts. For this reason you must always call it with the PNG file 1996value when you call it in this position: 1997 1998 if (png_get_gAMA(png_ptr, info_ptr, &file_gamma)) 1999 png_set_gamma(png_ptr, screen_gamma, file_gamma); 2000 2001 else 2002 png_set_gamma(png_ptr, screen_gamma, 0.45455); 2003 2004If you need to reduce an RGB file to a paletted file, or if a paletted 2005file has more entries than will fit on your screen, png_set_quantize() 2006will do that. Note that this is a simple match quantization that merely 2007finds the closest color available. This should work fairly well with 2008optimized palettes, but fairly badly with linear color cubes. If you 2009pass a palette that is larger than maximum_colors, the file will 2010reduce the number of colors in the palette so it will fit into 2011maximum_colors. If there is a histogram, libpng will use it to make 2012more intelligent choices when reducing the palette. If there is no 2013histogram, it may not do as good a job. 2014 2015 if (color_type & PNG_COLOR_MASK_COLOR) 2016 { 2017 if (png_get_valid(png_ptr, info_ptr, 2018 PNG_INFO_PLTE)) 2019 { 2020 png_uint_16p histogram = NULL; 2021 2022 png_get_hIST(png_ptr, info_ptr, 2023 &histogram); 2024 png_set_quantize(png_ptr, palette, num_palette, 2025 max_screen_colors, histogram, 1); 2026 } 2027 2028 else 2029 { 2030 png_color std_color_cube[MAX_SCREEN_COLORS] = 2031 { ... colors ... }; 2032 2033 png_set_quantize(png_ptr, std_color_cube, 2034 MAX_SCREEN_COLORS, MAX_SCREEN_COLORS, 2035 NULL,0); 2036 } 2037 } 2038 2039PNG files describe monochrome as black being zero and white being one. 2040The following code will reverse this (make black be one and white be 2041zero): 2042 2043 if (bit_depth == 1 && color_type == PNG_COLOR_TYPE_GRAY) 2044 png_set_invert_mono(png_ptr); 2045 2046This function can also be used to invert grayscale and gray-alpha images: 2047 2048 if (color_type == PNG_COLOR_TYPE_GRAY || 2049 color_type == PNG_COLOR_TYPE_GRAY_ALPHA) 2050 png_set_invert_mono(png_ptr); 2051 2052PNG files store 16-bit pixels in network byte order (big-endian, 2053ie. most significant bits first). This code changes the storage to the 2054other way (little-endian, i.e. least significant bits first, the 2055way PCs store them): 2056 2057 if (bit_depth == 16) 2058 png_set_swap(png_ptr); 2059 2060If you are using packed-pixel images (1, 2, or 4 bits/pixel), and you 2061need to change the order the pixels are packed into bytes, you can use: 2062 2063 if (bit_depth < 8) 2064 png_set_packswap(png_ptr); 2065 2066Finally, you can write your own transformation function if none of 2067the existing ones meets your needs. This is done by setting a callback 2068with 2069 2070 png_set_read_user_transform_fn(png_ptr, 2071 read_transform_fn); 2072 2073You must supply the function 2074 2075 void read_transform_fn(png_structp png_ptr, png_row_infop 2076 row_info, png_bytep data) 2077 2078See pngtest.c for a working example. Your function will be called 2079after all of the other transformations have been processed. Take care with 2080interlaced images if you do the interlace yourself - the width of the row is the 2081width in 'row_info', not the overall image width. 2082 2083If supported, libpng provides two information routines that you can use to find 2084where you are in processing the image: 2085 2086 png_get_current_pass_number(png_structp png_ptr); 2087 png_get_current_row_number(png_structp png_ptr); 2088 2089Don't try using these outside a transform callback - firstly they are only 2090supported if user transforms are supported, secondly they may well return 2091unexpected results unless the row is actually being processed at the moment they 2092are called. 2093 2094With interlaced 2095images the value returned is the row in the input sub-image image. Use 2096PNG_ROW_FROM_PASS_ROW(row, pass) and PNG_COL_FROM_PASS_COL(col, pass) to 2097find the output pixel (x,y) given an interlaced sub-image pixel (row,col,pass). 2098 2099The discussion of interlace handling above contains more information on how to 2100use these values. 2101 2102You can also set up a pointer to a user structure for use by your 2103callback function, and you can inform libpng that your transform 2104function will change the number of channels or bit depth with the 2105function 2106 2107 png_set_user_transform_info(png_ptr, user_ptr, 2108 user_depth, user_channels); 2109 2110The user's application, not libpng, is responsible for allocating and 2111freeing any memory required for the user structure. 2112 2113You can retrieve the pointer via the function 2114png_get_user_transform_ptr(). For example: 2115 2116 voidp read_user_transform_ptr = 2117 png_get_user_transform_ptr(png_ptr); 2118 2119The last thing to handle is interlacing; this is covered in detail below, 2120but you must call the function here if you want libpng to handle expansion 2121of the interlaced image. 2122 2123 number_of_passes = png_set_interlace_handling(png_ptr); 2124 2125After setting the transformations, libpng can update your png_info 2126structure to reflect any transformations you've requested with this 2127call. 2128 2129 png_read_update_info(png_ptr, info_ptr); 2130 2131This is most useful to update the info structure's rowbytes 2132field so you can use it to allocate your image memory. This function 2133will also update your palette with the correct screen_gamma and 2134background if these have been given with the calls above. You may 2135only call png_read_update_info() once with a particular info_ptr. 2136 2137After you call png_read_update_info(), you can allocate any 2138memory you need to hold the image. The row data is simply 2139raw byte data for all forms of images. As the actual allocation 2140varies among applications, no example will be given. If you 2141are allocating one large chunk, you will need to build an 2142array of pointers to each row, as it will be needed for some 2143of the functions below. 2144 2145Remember: Before you call png_read_update_info(), the png_get_*() 2146functions return the values corresponding to the original PNG image. 2147After you call png_read_update_info the values refer to the image 2148that libpng will output. Consequently you must call all the png_set_ 2149functions before you call png_read_update_info(). This is particularly 2150important for png_set_interlace_handling() - if you are going to call 2151png_read_update_info() you must call png_set_interlace_handling() before 2152it unless you want to receive interlaced output. 2153 2154Reading image data 2155 2156After you've allocated memory, you can read the image data. 2157The simplest way to do this is in one function call. If you are 2158allocating enough memory to hold the whole image, you can just 2159call png_read_image() and libpng will read in all the image data 2160and put it in the memory area supplied. You will need to pass in 2161an array of pointers to each row. 2162 2163This function automatically handles interlacing, so you don't 2164need to call png_set_interlace_handling() (unless you call 2165png_read_update_info()) or call this function multiple times, or any 2166of that other stuff necessary with png_read_rows(). 2167 2168 png_read_image(png_ptr, row_pointers); 2169 2170where row_pointers is: 2171 2172 png_bytep row_pointers[height]; 2173 2174You can point to void or char or whatever you use for pixels. 2175 2176If you don't want to read in the whole image at once, you can 2177use png_read_rows() instead. If there is no interlacing (check 2178interlace_type == PNG_INTERLACE_NONE), this is simple: 2179 2180 png_read_rows(png_ptr, row_pointers, NULL, 2181 number_of_rows); 2182 2183where row_pointers is the same as in the png_read_image() call. 2184 2185If you are doing this just one row at a time, you can do this with 2186a single row_pointer instead of an array of row_pointers: 2187 2188 png_bytep row_pointer = row; 2189 png_read_row(png_ptr, row_pointer, NULL); 2190 2191If the file is interlaced (interlace_type != 0 in the IHDR chunk), things 2192get somewhat harder. The only current (PNG Specification version 1.2) 2193interlacing type for PNG is (interlace_type == PNG_INTERLACE_ADAM7); 2194a somewhat complicated 2D interlace scheme, known as Adam7, that 2195breaks down an image into seven smaller images of varying size, based 2196on an 8x8 grid. This number is defined (from libpng 1.5) as 2197PNG_INTERLACE_ADAM7_PASSES in png.h 2198 2199libpng can fill out those images or it can give them to you "as is". 2200It is almost always better to have libpng handle the interlacing for you. 2201If you want the images filled out, there are two ways to do that. The one 2202mentioned in the PNG specification is to expand each pixel to cover 2203those pixels that have not been read yet (the "rectangle" method). 2204This results in a blocky image for the first pass, which gradually 2205smooths out as more pixels are read. The other method is the "sparkle" 2206method, where pixels are drawn only in their final locations, with the 2207rest of the image remaining whatever colors they were initialized to 2208before the start of the read. The first method usually looks better, 2209but tends to be slower, as there are more pixels to put in the rows. 2210 2211If, as is likely, you want libpng to expand the images, call this before 2212calling png_start_read_image() or png_read_update_info(): 2213 2214 if (interlace_type == PNG_INTERLACE_ADAM7) 2215 number_of_passes 2216 = png_set_interlace_handling(png_ptr); 2217 2218This will return the number of passes needed. Currently, this is seven, 2219but may change if another interlace type is added. This function can be 2220called even if the file is not interlaced, where it will return one pass. 2221You then need to read the whole image 'number_of_passes' times. Each time 2222will distribute the pixels from the current pass to the correct place in 2223the output image, so you need to supply the same rows to png_read_rows in 2224each pass. 2225 2226If you are not going to display the image after each pass, but are 2227going to wait until the entire image is read in, use the sparkle 2228effect. This effect is faster and the end result of either method 2229is exactly the same. If you are planning on displaying the image 2230after each pass, the "rectangle" effect is generally considered the 2231better looking one. 2232 2233If you only want the "sparkle" effect, just call png_read_rows() as 2234normal, with the third parameter NULL. Make sure you make pass over 2235the image number_of_passes times, and you don't change the data in the 2236rows between calls. You can change the locations of the data, just 2237not the data. Each pass only writes the pixels appropriate for that 2238pass, and assumes the data from previous passes is still valid. 2239 2240 png_read_rows(png_ptr, row_pointers, NULL, 2241 number_of_rows); 2242 2243If you only want the first effect (the rectangles), do the same as 2244before except pass the row buffer in the third parameter, and leave 2245the second parameter NULL. 2246 2247 png_read_rows(png_ptr, NULL, row_pointers, 2248 number_of_rows); 2249 2250If you don't want libpng to handle the interlacing details, just call 2251png_read_rows() PNG_INTERLACE_ADAM7_PASSES times to read in all the images. 2252Each of the images is a valid image by itself; however, you will almost 2253certainly need to distribute the pixels from each sub-image to the 2254correct place. This is where everything gets very tricky. 2255 2256If you want to retrieve the separate images you must pass the correct 2257number of rows to each successive call of png_read_rows(). The calculation 2258gets pretty complicated for small images, where some sub-images may 2259not even exist because either their width or height ends up zero. 2260libpng provides two macros to help you in 1.5 and later versions: 2261 2262 png_uint_32 width = PNG_PASS_COLS(image_width, pass_number); 2263 png_uint_32 height = PNG_PASS_ROWS(image_height, pass_number); 2264 2265Respectively these tell you the width and height of the sub-image 2266corresponding to the numbered pass. 'pass' is in in the range 0 to 6 - 2267this can be confusing because the specification refers to the same passes 2268as 1 to 7! Be careful, you must check both the width and height before 2269calling png_read_rows() and not call it for that pass if either is zero. 2270 2271You can, of course, read each sub-image row by row. If you want to 2272produce optimal code to make a pixel-by-pixel transformation of an 2273interlaced image this is the best approach; read each row of each pass, 2274transform it, and write it out to a new interlaced image. 2275 2276If you want to de-interlace the image yourself libpng provides further 2277macros to help that tell you where to place the pixels in the output image. 2278Because the interlacing scheme is rectangular - sub-image pixels are always 2279arranged on a rectangular grid - all you need to know for each pass is the 2280starting column and row in the output image of the first pixel plus the 2281spacing between each pixel. As of libpng 1.5 there are four macros to 2282retrieve this information: 2283 2284 png_uint_32 x = PNG_PASS_START_COL(pass); 2285 png_uint_32 y = PNG_PASS_START_ROW(pass); 2286 png_uint_32 xStep = 1U << PNG_PASS_COL_SHIFT(pass); 2287 png_uint_32 yStep = 1U << PNG_PASS_ROW_SHIFT(pass); 2288 2289These allow you to write the obvious loop: 2290 2291 png_uint_32 input_y = 0; 2292 png_uint_32 output_y = PNG_PASS_START_ROW(pass); 2293 2294 while (output_y < output_image_height) 2295 { 2296 png_uint_32 input_x = 0; 2297 png_uint_32 output_x = PNG_PASS_START_COL(pass); 2298 2299 while (output_x < output_image_width) 2300 { 2301 image[output_y][output_x] = 2302 subimage[pass][input_y][input_x++]; 2303 2304 output_x += xStep; 2305 } 2306 2307 ++input_y; 2308 output_y += yStep; 2309 } 2310 2311Notice that the steps between successive output rows and columns are 2312returned as shifts. This is possible because the pixels in the subimages 2313are always a power of 2 apart - 1, 2, 4 or 8 pixels - in the original 2314image. In practice you may need to directly calculate the output coordinate 2315given an input coordinate. libpng provides two further macros for this 2316purpose: 2317 2318 png_uint_32 output_x = PNG_COL_FROM_PASS_COL(input_x, pass); 2319 png_uint_32 output_y = PNG_ROW_FROM_PASS_ROW(input_y, pass); 2320 2321Finally a pair of macros are provided to tell you if a particular image 2322row or column appears in a given pass: 2323 2324 int col_in_pass = PNG_COL_IN_INTERLACE_PASS(output_x, pass); 2325 int row_in_pass = PNG_ROW_IN_INTERLACE_PASS(output_y, pass); 2326 2327Bear in mind that you will probably also need to check the width and height 2328of the pass in addition to the above to be sure the pass even exists! 2329 2330With any luck you are convinced by now that you don't want to do your own 2331interlace handling. In reality normally the only good reason for doing this 2332is if you are processing PNG files on a pixel-by-pixel basis and don't want 2333to load the whole file into memory when it is interlaced. 2334 2335libpng includes a test program, pngvalid, that illustrates reading and 2336writing of interlaced images. If you can't get interlacing to work in your 2337code and don't want to leave it to libpng (the recommended approach), see 2338how pngvalid.c does it. 2339 2340Finishing a sequential read 2341 2342After you are finished reading the image through the 2343low-level interface, you can finish reading the file. 2344 2345If you want to use a different crc action for handling CRC errors in 2346chunks after the image data, you can call png_set_crc_action() 2347again at this point. 2348 2349If you are interested in comments or time, which may be stored either 2350before or after the image data, you should pass the separate png_info 2351struct if you want to keep the comments from before and after the image 2352separate. 2353 2354 png_infop end_info = png_create_info_struct(png_ptr); 2355 2356 if (!end_info) 2357 { 2358 png_destroy_read_struct(&png_ptr, &info_ptr, 2359 (png_infopp)NULL); 2360 return (ERROR); 2361 } 2362 2363 png_read_end(png_ptr, end_info); 2364 2365If you are not interested, you should still call png_read_end() 2366but you can pass NULL, avoiding the need to create an end_info structure. 2367If you do this, libpng will not process any chunks after IDAT other than 2368skipping over them and perhaps (depending on whether you have called 2369png_set_crc_action) checking their CRCs while looking for the IEND chunk. 2370 2371 png_read_end(png_ptr, (png_infop)NULL); 2372 2373If you don't call png_read_end(), then your file pointer will be 2374left pointing to the first chunk after the last IDAT, which is probably 2375not what you want if you expect to read something beyond the end of 2376the PNG datastream. 2377 2378When you are done, you can free all memory allocated by libpng like this: 2379 2380 png_destroy_read_struct(&png_ptr, &info_ptr, 2381 &end_info); 2382 2383or, if you didn't create an end_info structure, 2384 2385 png_destroy_read_struct(&png_ptr, &info_ptr, 2386 (png_infopp)NULL); 2387 2388It is also possible to individually free the info_ptr members that 2389point to libpng-allocated storage with the following function: 2390 2391 png_free_data(png_ptr, info_ptr, mask, seq) 2392 2393 mask - identifies data to be freed, a mask 2394 containing the bitwise OR of one or 2395 more of 2396 PNG_FREE_PLTE, PNG_FREE_TRNS, 2397 PNG_FREE_HIST, PNG_FREE_ICCP, 2398 PNG_FREE_PCAL, PNG_FREE_ROWS, 2399 PNG_FREE_SCAL, PNG_FREE_SPLT, 2400 PNG_FREE_TEXT, PNG_FREE_UNKN, 2401 or simply PNG_FREE_ALL 2402 2403 seq - sequence number of item to be freed 2404 (-1 for all items) 2405 2406This function may be safely called when the relevant storage has 2407already been freed, or has not yet been allocated, or was allocated 2408by the user and not by libpng, and will in those cases do nothing. 2409The "seq" parameter is ignored if only one item of the selected data 2410type, such as PLTE, is allowed. If "seq" is not -1, and multiple items 2411are allowed for the data type identified in the mask, such as text or 2412sPLT, only the n'th item in the structure is freed, where n is "seq". 2413 2414The default behavior is only to free data that was allocated internally 2415by libpng. This can be changed, so that libpng will not free the data, 2416or so that it will free data that was allocated by the user with png_malloc() 2417or png_calloc() and passed in via a png_set_*() function, with 2418 2419 png_data_freer(png_ptr, info_ptr, freer, mask) 2420 2421 freer - one of 2422 PNG_DESTROY_WILL_FREE_DATA 2423 PNG_SET_WILL_FREE_DATA 2424 PNG_USER_WILL_FREE_DATA 2425 2426 mask - which data elements are affected 2427 same choices as in png_free_data() 2428 2429This function only affects data that has already been allocated. 2430You can call this function after reading the PNG data but before calling 2431any png_set_*() functions, to control whether the user or the png_set_*() 2432function is responsible for freeing any existing data that might be present, 2433and again after the png_set_*() functions to control whether the user 2434or png_destroy_*() is supposed to free the data. When the user assumes 2435responsibility for libpng-allocated data, the application must use 2436png_free() to free it, and when the user transfers responsibility to libpng 2437for data that the user has allocated, the user must have used png_malloc() 2438or png_calloc() to allocate it. 2439 2440If you allocated your row_pointers in a single block, as suggested above in 2441the description of the high level read interface, you must not transfer 2442responsibility for freeing it to the png_set_rows or png_read_destroy function, 2443because they would also try to free the individual row_pointers[i]. 2444 2445If you allocated text_ptr.text, text_ptr.lang, and text_ptr.translated_keyword 2446separately, do not transfer responsibility for freeing text_ptr to libpng, 2447because when libpng fills a png_text structure it combines these members with 2448the key member, and png_free_data() will free only text_ptr.key. Similarly, 2449if you transfer responsibility for free'ing text_ptr from libpng to your 2450application, your application must not separately free those members. 2451 2452The png_free_data() function will turn off the "valid" flag for anything 2453it frees. If you need to turn the flag off for a chunk that was freed by 2454your application instead of by libpng, you can use 2455 2456 png_set_invalid(png_ptr, info_ptr, mask); 2457 2458 mask - identifies the chunks to be made invalid, 2459 containing the bitwise OR of one or 2460 more of 2461 PNG_INFO_gAMA, PNG_INFO_sBIT, 2462 PNG_INFO_cHRM, PNG_INFO_PLTE, 2463 PNG_INFO_tRNS, PNG_INFO_bKGD, 2464 PNG_INFO_hIST, PNG_INFO_pHYs, 2465 PNG_INFO_oFFs, PNG_INFO_tIME, 2466 PNG_INFO_pCAL, PNG_INFO_sRGB, 2467 PNG_INFO_iCCP, PNG_INFO_sPLT, 2468 PNG_INFO_sCAL, PNG_INFO_IDAT 2469 2470For a more compact example of reading a PNG image, see the file example.c. 2471 2472Reading PNG files progressively 2473 2474The progressive reader is slightly different from the non-progressive 2475reader. Instead of calling png_read_info(), png_read_rows(), and 2476png_read_end(), you make one call to png_process_data(), which calls 2477callbacks when it has the info, a row, or the end of the image. You 2478set up these callbacks with png_set_progressive_read_fn(). You don't 2479have to worry about the input/output functions of libpng, as you are 2480giving the library the data directly in png_process_data(). I will 2481assume that you have read the section on reading PNG files above, 2482so I will only highlight the differences (although I will show 2483all of the code). 2484 2485png_structp png_ptr; 2486png_infop info_ptr; 2487 2488 /* An example code fragment of how you would 2489 initialize the progressive reader in your 2490 application. */ 2491 int 2492 initialize_png_reader() 2493 { 2494 png_ptr = png_create_read_struct 2495 (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr, 2496 user_error_fn, user_warning_fn); 2497 2498 if (!png_ptr) 2499 return (ERROR); 2500 2501 info_ptr = png_create_info_struct(png_ptr); 2502 2503 if (!info_ptr) 2504 { 2505 png_destroy_read_struct(&png_ptr, 2506 (png_infopp)NULL, (png_infopp)NULL); 2507 return (ERROR); 2508 } 2509 2510 if (setjmp(png_jmpbuf(png_ptr))) 2511 { 2512 png_destroy_read_struct(&png_ptr, &info_ptr, 2513 (png_infopp)NULL); 2514 return (ERROR); 2515 } 2516 2517 /* This one's new. You can provide functions 2518 to be called when the header info is valid, 2519 when each row is completed, and when the image 2520 is finished. If you aren't using all functions, 2521 you can specify NULL parameters. Even when all 2522 three functions are NULL, you need to call 2523 png_set_progressive_read_fn(). You can use 2524 any struct as the user_ptr (cast to a void pointer 2525 for the function call), and retrieve the pointer 2526 from inside the callbacks using the function 2527 2528 png_get_progressive_ptr(png_ptr); 2529 2530 which will return a void pointer, which you have 2531 to cast appropriately. 2532 */ 2533 png_set_progressive_read_fn(png_ptr, (void *)user_ptr, 2534 info_callback, row_callback, end_callback); 2535 2536 return 0; 2537 } 2538 2539 /* A code fragment that you call as you receive blocks 2540 of data */ 2541 int 2542 process_data(png_bytep buffer, png_uint_32 length) 2543 { 2544 if (setjmp(png_jmpbuf(png_ptr))) 2545 { 2546 png_destroy_read_struct(&png_ptr, &info_ptr, 2547 (png_infopp)NULL); 2548 return (ERROR); 2549 } 2550 2551 /* This one's new also. Simply give it a chunk 2552 of data from the file stream (in order, of 2553 course). On machines with segmented memory 2554 models machines, don't give it any more than 2555 64K. The library seems to run fine with sizes 2556 of 4K. Although you can give it much less if 2557 necessary (I assume you can give it chunks of 2558 1 byte, I haven't tried less than 256 bytes 2559 yet). When this function returns, you may 2560 want to display any rows that were generated 2561 in the row callback if you don't already do 2562 so there. 2563 */ 2564 png_process_data(png_ptr, info_ptr, buffer, length); 2565 2566 /* At this point you can call png_process_data_skip if 2567 you want to handle data the library will skip yourself; 2568 it simply returns the number of bytes to skip (and stops 2569 libpng skipping that number of bytes on the next 2570 png_process_data call). 2571 return 0; 2572 } 2573 2574 /* This function is called (as set by 2575 png_set_progressive_read_fn() above) when enough data 2576 has been supplied so all of the header has been 2577 read. 2578 */ 2579 void 2580 info_callback(png_structp png_ptr, png_infop info) 2581 { 2582 /* Do any setup here, including setting any of 2583 the transformations mentioned in the Reading 2584 PNG files section. For now, you _must_ call 2585 either png_start_read_image() or 2586 png_read_update_info() after all the 2587 transformations are set (even if you don't set 2588 any). You may start getting rows before 2589 png_process_data() returns, so this is your 2590 last chance to prepare for that. 2591 2592 This is where you turn on interlace handling, 2593 assuming you don't want to do it yourself. 2594 2595 If you need to you can stop the processing of 2596 your original input data at this point by calling 2597 png_process_data_pause. This returns the number 2598 of unprocessed bytes from the last png_process_data 2599 call - it is up to you to ensure that the next call 2600 sees these bytes again. If you don't want to bother 2601 with this you can get libpng to cache the unread 2602 bytes by setting the 'save' parameter (see png.h) but 2603 then libpng will have to copy the data internally. 2604 */ 2605 } 2606 2607 /* This function is called when each row of image 2608 data is complete */ 2609 void 2610 row_callback(png_structp png_ptr, png_bytep new_row, 2611 png_uint_32 row_num, int pass) 2612 { 2613 /* If the image is interlaced, and you turned 2614 on the interlace handler, this function will 2615 be called for every row in every pass. Some 2616 of these rows will not be changed from the 2617 previous pass. When the row is not changed, 2618 the new_row variable will be NULL. The rows 2619 and passes are called in order, so you don't 2620 really need the row_num and pass, but I'm 2621 supplying them because it may make your life 2622 easier. 2623 2624 If you did not turn on interlace handling then 2625 the callback is called for each row of each 2626 sub-image when the image is interlaced. In this 2627 case 'row_num' is the row in the sub-image, not 2628 the row in the output image as it is in all other 2629 cases. 2630 2631 For the non-NULL rows of interlaced images when 2632 you have switched on libpng interlace handling, 2633 you must call png_progressive_combine_row() 2634 passing in the row and the old row. You can 2635 call this function for NULL rows (it will just 2636 return) and for non-interlaced images (it just 2637 does the memcpy for you) if it will make the 2638 code easier. Thus, you can just do this for 2639 all cases if you switch on interlace handling; 2640 */ 2641 2642 png_progressive_combine_row(png_ptr, old_row, 2643 new_row); 2644 2645 /* where old_row is what was displayed 2646 previously for the row. Note that the first 2647 pass (pass == 0, really) will completely cover 2648 the old row, so the rows do not have to be 2649 initialized. After the first pass (and only 2650 for interlaced images), you will have to pass 2651 the current row, and the function will combine 2652 the old row and the new row. 2653 2654 You can also call png_process_data_pause in this 2655 callback - see above. 2656 */ 2657 } 2658 2659 void 2660 end_callback(png_structp png_ptr, png_infop info) 2661 { 2662 /* This function is called after the whole image 2663 has been read, including any chunks after the 2664 image (up to and including the IEND). You 2665 will usually have the same info chunk as you 2666 had in the header, although some data may have 2667 been added to the comments and time fields. 2668 2669 Most people won't do much here, perhaps setting 2670 a flag that marks the image as finished. 2671 */ 2672 } 2673 2674 2675 2676IV. Writing 2677 2678Much of this is very similar to reading. However, everything of 2679importance is repeated here, so you won't have to constantly look 2680back up in the reading section to understand writing. 2681 2682Setup 2683 2684You will want to do the I/O initialization before you get into libpng, 2685so if it doesn't work, you don't have anything to undo. If you are not 2686using the standard I/O functions, you will need to replace them with 2687custom writing functions. See the discussion under Customizing libpng. 2688 2689 FILE *fp = fopen(file_name, "wb"); 2690 2691 if (!fp) 2692 return (ERROR); 2693 2694Next, png_struct and png_info need to be allocated and initialized. 2695As these can be both relatively large, you may not want to store these 2696on the stack, unless you have stack space to spare. Of course, you 2697will want to check if they return NULL. If you are also reading, 2698you won't want to name your read structure and your write structure 2699both "png_ptr"; you can call them anything you like, such as 2700"read_ptr" and "write_ptr". Look at pngtest.c, for example. 2701 2702 png_structp png_ptr = png_create_write_struct 2703 (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr, 2704 user_error_fn, user_warning_fn); 2705 2706 if (!png_ptr) 2707 return (ERROR); 2708 2709 png_infop info_ptr = png_create_info_struct(png_ptr); 2710 if (!info_ptr) 2711 { 2712 png_destroy_write_struct(&png_ptr, 2713 (png_infopp)NULL); 2714 return (ERROR); 2715 } 2716 2717If you want to use your own memory allocation routines, 2718define PNG_USER_MEM_SUPPORTED and use 2719png_create_write_struct_2() instead of png_create_write_struct(): 2720 2721 png_structp png_ptr = png_create_write_struct_2 2722 (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr, 2723 user_error_fn, user_warning_fn, (png_voidp) 2724 user_mem_ptr, user_malloc_fn, user_free_fn); 2725 2726After you have these structures, you will need to set up the 2727error handling. When libpng encounters an error, it expects to 2728longjmp() back to your routine. Therefore, you will need to call 2729setjmp() and pass the png_jmpbuf(png_ptr). If you 2730write the file from different routines, you will need to update 2731the png_jmpbuf(png_ptr) every time you enter a new routine that will 2732call a png_*() function. See your documentation of setjmp/longjmp 2733for your compiler for more information on setjmp/longjmp. See 2734the discussion on libpng error handling in the Customizing Libpng 2735section below for more information on the libpng error handling. 2736 2737 if (setjmp(png_jmpbuf(png_ptr))) 2738 { 2739 png_destroy_write_struct(&png_ptr, &info_ptr); 2740 fclose(fp); 2741 return (ERROR); 2742 } 2743 ... 2744 return; 2745 2746If you would rather avoid the complexity of setjmp/longjmp issues, 2747you can compile libpng with PNG_NO_SETJMP, in which case 2748errors will result in a call to PNG_ABORT() which defaults to abort(). 2749 2750You can #define PNG_ABORT() to a function that does something 2751more useful than abort(), as long as your function does not 2752return. 2753 2754Checking for invalid palette index on write was added at libpng 27551.5.10. If a pixel contains an invalid (out-of-range) index libpng issues 2756a benign error. This is enabled by default because this condition is an 2757error according to the PNG specification, Clause 11.3.2, but the error can 2758be ignored in each png_ptr with 2759 2760 png_set_check_for_invalid_index(png_ptr, 0); 2761 2762If the error is ignored, or if png_benign_error() treats it as a warning, 2763any invalid pixels are written as-is by the encoder, resulting in an 2764invalid PNG datastream as output. In this case the application is 2765responsible for ensuring that the pixel indexes are in range when it writes 2766a PLTE chunk with fewer entries than the bit depth would allow. 2767 2768Now you need to set up the output code. The default for libpng is to 2769use the C function fwrite(). If you use this, you will need to pass a 2770valid FILE * in the function png_init_io(). Be sure that the file is 2771opened in binary mode. Again, if you wish to handle writing data in 2772another way, see the discussion on libpng I/O handling in the Customizing 2773Libpng section below. 2774 2775 png_init_io(png_ptr, fp); 2776 2777If you are embedding your PNG into a datastream such as MNG, and don't 2778want libpng to write the 8-byte signature, or if you have already 2779written the signature in your application, use 2780 2781 png_set_sig_bytes(png_ptr, 8); 2782 2783to inform libpng that it should not write a signature. 2784 2785Write callbacks 2786 2787At this point, you can set up a callback function that will be 2788called after each row has been written, which you can use to control 2789a progress meter or the like. It's demonstrated in pngtest.c. 2790You must supply a function 2791 2792 void write_row_callback(png_structp png_ptr, png_uint_32 row, 2793 int pass); 2794 { 2795 /* put your code here */ 2796 } 2797 2798(You can give it another name that you like instead of "write_row_callback") 2799 2800To inform libpng about your function, use 2801 2802 png_set_write_status_fn(png_ptr, write_row_callback); 2803 2804When this function is called the row has already been completely processed and 2805it has also been written out. The 'row' and 'pass' refer to the next row to be 2806handled. For the 2807non-interlaced case the row that was just handled is simply one less than the 2808passed in row number, and pass will always be 0. For the interlaced case the 2809same applies unless the row value is 0, in which case the row just handled was 2810the last one from one of the preceding passes. Because interlacing may skip a 2811pass you cannot be sure that the preceding pass is just 'pass-1', if you really 2812need to know what the last pass is record (row,pass) from the callback and use 2813the last recorded value each time. 2814 2815As with the user transform you can find the output row using the 2816PNG_ROW_FROM_PASS_ROW macro. 2817 2818You now have the option of modifying how the compression library will 2819run. The following functions are mainly for testing, but may be useful 2820in some cases, like if you need to write PNG files extremely fast and 2821are willing to give up some compression, or if you want to get the 2822maximum possible compression at the expense of slower writing. If you 2823have no special needs in this area, let the library do what it wants by 2824not calling this function at all, as it has been tuned to deliver a good 2825speed/compression ratio. The second parameter to png_set_filter() is 2826the filter method, for which the only valid values are 0 (as of the 2827July 1999 PNG specification, version 1.2) or 64 (if you are writing 2828a PNG datastream that is to be embedded in a MNG datastream). The third 2829parameter is a flag that indicates which filter type(s) are to be tested 2830for each scanline. See the PNG specification for details on the specific 2831filter types. 2832 2833 2834 /* turn on or off filtering, and/or choose 2835 specific filters. You can use either a single 2836 PNG_FILTER_VALUE_NAME or the bitwise OR of one 2837 or more PNG_FILTER_NAME masks. 2838 */ 2839 png_set_filter(png_ptr, 0, 2840 PNG_FILTER_NONE | PNG_FILTER_VALUE_NONE | 2841 PNG_FILTER_SUB | PNG_FILTER_VALUE_SUB | 2842 PNG_FILTER_UP | PNG_FILTER_VALUE_UP | 2843 PNG_FILTER_AVG | PNG_FILTER_VALUE_AVG | 2844 PNG_FILTER_PAETH | PNG_FILTER_VALUE_PAETH| 2845 PNG_ALL_FILTERS); 2846 2847If an application wants to start and stop using particular filters during 2848compression, it should start out with all of the filters (to ensure that 2849the previous row of pixels will be stored in case it's needed later), 2850and then add and remove them after the start of compression. 2851 2852If you are writing a PNG datastream that is to be embedded in a MNG 2853datastream, the second parameter can be either 0 or 64. 2854 2855The png_set_compression_*() functions interface to the zlib compression 2856library, and should mostly be ignored unless you really know what you are 2857doing. The only generally useful call is png_set_compression_level() 2858which changes how much time zlib spends on trying to compress the image 2859data. See the Compression Library (zlib.h and algorithm.txt, distributed 2860with zlib) for details on the compression levels. 2861 2862 #include zlib.h 2863 2864 /* Set the zlib compression level */ 2865 png_set_compression_level(png_ptr, 2866 Z_BEST_COMPRESSION); 2867 2868 /* Set other zlib parameters for compressing IDAT */ 2869 png_set_compression_mem_level(png_ptr, 8); 2870 png_set_compression_strategy(png_ptr, 2871 Z_DEFAULT_STRATEGY); 2872 png_set_compression_window_bits(png_ptr, 15); 2873 png_set_compression_method(png_ptr, 8); 2874 png_set_compression_buffer_size(png_ptr, 8192) 2875 2876 /* Set zlib parameters for text compression 2877 * If you don't call these, the parameters 2878 * fall back on those defined for IDAT chunks 2879 */ 2880 png_set_text_compression_mem_level(png_ptr, 8); 2881 png_set_text_compression_strategy(png_ptr, 2882 Z_DEFAULT_STRATEGY); 2883 png_set_text_compression_window_bits(png_ptr, 15); 2884 png_set_text_compression_method(png_ptr, 8); 2885 2886Setting the contents of info for output 2887 2888You now need to fill in the png_info structure with all the data you 2889wish to write before the actual image. Note that the only thing you 2890are allowed to write after the image is the text chunks and the time 2891chunk (as of PNG Specification 1.2, anyway). See png_write_end() and 2892the latest PNG specification for more information on that. If you 2893wish to write them before the image, fill them in now, and flag that 2894data as being valid. If you want to wait until after the data, don't 2895fill them until png_write_end(). For all the fields in png_info and 2896their data types, see png.h. For explanations of what the fields 2897contain, see the PNG specification. 2898 2899Some of the more important parts of the png_info are: 2900 2901 png_set_IHDR(png_ptr, info_ptr, width, height, 2902 bit_depth, color_type, interlace_type, 2903 compression_type, filter_method) 2904 2905 width - holds the width of the image 2906 in pixels (up to 2^31). 2907 2908 height - holds the height of the image 2909 in pixels (up to 2^31). 2910 2911 bit_depth - holds the bit depth of one of the 2912 image channels. 2913 (valid values are 1, 2, 4, 8, 16 2914 and depend also on the 2915 color_type. See also significant 2916 bits (sBIT) below). 2917 2918 color_type - describes which color/alpha 2919 channels are present. 2920 PNG_COLOR_TYPE_GRAY 2921 (bit depths 1, 2, 4, 8, 16) 2922 PNG_COLOR_TYPE_GRAY_ALPHA 2923 (bit depths 8, 16) 2924 PNG_COLOR_TYPE_PALETTE 2925 (bit depths 1, 2, 4, 8) 2926 PNG_COLOR_TYPE_RGB 2927 (bit_depths 8, 16) 2928 PNG_COLOR_TYPE_RGB_ALPHA 2929 (bit_depths 8, 16) 2930 2931 PNG_COLOR_MASK_PALETTE 2932 PNG_COLOR_MASK_COLOR 2933 PNG_COLOR_MASK_ALPHA 2934 2935 interlace_type - PNG_INTERLACE_NONE or 2936 PNG_INTERLACE_ADAM7 2937 2938 compression_type - (must be 2939 PNG_COMPRESSION_TYPE_DEFAULT) 2940 2941 filter_method - (must be PNG_FILTER_TYPE_DEFAULT 2942 or, if you are writing a PNG to 2943 be embedded in a MNG datastream, 2944 can also be 2945 PNG_INTRAPIXEL_DIFFERENCING) 2946 2947If you call png_set_IHDR(), the call must appear before any of the 2948other png_set_*() functions, because they might require access to some of 2949the IHDR settings. The remaining png_set_*() functions can be called 2950in any order. 2951 2952If you wish, you can reset the compression_type, interlace_type, or 2953filter_method later by calling png_set_IHDR() again; if you do this, the 2954width, height, bit_depth, and color_type must be the same in each call. 2955 2956 png_set_PLTE(png_ptr, info_ptr, palette, 2957 num_palette); 2958 2959 palette - the palette for the file 2960 (array of png_color) 2961 num_palette - number of entries in the palette 2962 2963 2964 png_set_gAMA(png_ptr, info_ptr, file_gamma); 2965 png_set_gAMA_fixed(png_ptr, info_ptr, int_file_gamma); 2966 2967 file_gamma - the gamma at which the image was 2968 created (PNG_INFO_gAMA) 2969 2970 int_file_gamma - 100,000 times the gamma at which 2971 the image was created 2972 2973 png_set_cHRM(png_ptr, info_ptr, white_x, white_y, red_x, red_y, 2974 green_x, green_y, blue_x, blue_y) 2975 png_set_cHRM_XYZ(png_ptr, info_ptr, red_X, red_Y, red_Z, green_X, 2976 green_Y, green_Z, blue_X, blue_Y, blue_Z) 2977 png_set_cHRM_fixed(png_ptr, info_ptr, int_white_x, int_white_y, 2978 int_red_x, int_red_y, int_green_x, int_green_y, 2979 int_blue_x, int_blue_y) 2980 png_set_cHRM_XYZ_fixed(png_ptr, info_ptr, int_red_X, int_red_Y, 2981 int_red_Z, int_green_X, int_green_Y, int_green_Z, 2982 int_blue_X, int_blue_Y, int_blue_Z) 2983 2984 {white,red,green,blue}_{x,y} 2985 A color space encoding specified using the chromaticities 2986 of the end points and the white point. 2987 2988 {red,green,blue}_{X,Y,Z} 2989 A color space encoding specified using the encoding end 2990 points - the CIE tristimulus specification of the intended 2991 color of the red, green and blue channels in the PNG RGB 2992 data. The white point is simply the sum of the three end 2993 points. 2994 2995 png_set_sRGB(png_ptr, info_ptr, srgb_intent); 2996 2997 srgb_intent - the rendering intent 2998 (PNG_INFO_sRGB) The presence of 2999 the sRGB chunk means that the pixel 3000 data is in the sRGB color space. 3001 This chunk also implies specific 3002 values of gAMA and cHRM. Rendering 3003 intent is the CSS-1 property that 3004 has been defined by the International 3005 Color Consortium 3006 (http://www.color.org). 3007 It can be one of 3008 PNG_sRGB_INTENT_SATURATION, 3009 PNG_sRGB_INTENT_PERCEPTUAL, 3010 PNG_sRGB_INTENT_ABSOLUTE, or 3011 PNG_sRGB_INTENT_RELATIVE. 3012 3013 3014 png_set_sRGB_gAMA_and_cHRM(png_ptr, info_ptr, 3015 srgb_intent); 3016 3017 srgb_intent - the rendering intent 3018 (PNG_INFO_sRGB) The presence of the 3019 sRGB chunk means that the pixel 3020 data is in the sRGB color space. 3021 This function also causes gAMA and 3022 cHRM chunks with the specific values 3023 that are consistent with sRGB to be 3024 written. 3025 3026 png_set_iCCP(png_ptr, info_ptr, name, compression_type, 3027 profile, proflen); 3028 3029 name - The profile name. 3030 3031 compression_type - The compression type; always 3032 PNG_COMPRESSION_TYPE_BASE for PNG 1.0. 3033 You may give NULL to this argument to 3034 ignore it. 3035 3036 profile - International Color Consortium color 3037 profile data. May contain NULs. 3038 3039 proflen - length of profile data in bytes. 3040 3041 png_set_sBIT(png_ptr, info_ptr, sig_bit); 3042 3043 sig_bit - the number of significant bits for 3044 (PNG_INFO_sBIT) each of the gray, red, 3045 green, and blue channels, whichever are 3046 appropriate for the given color type 3047 (png_color_16) 3048 3049 png_set_tRNS(png_ptr, info_ptr, trans_alpha, 3050 num_trans, trans_color); 3051 3052 trans_alpha - array of alpha (transparency) 3053 entries for palette (PNG_INFO_tRNS) 3054 3055 num_trans - number of transparent entries 3056 (PNG_INFO_tRNS) 3057 3058 trans_color - graylevel or color sample values 3059 (in order red, green, blue) of the 3060 single transparent color for 3061 non-paletted images (PNG_INFO_tRNS) 3062 3063 png_set_hIST(png_ptr, info_ptr, hist); 3064 3065 hist - histogram of palette (array of 3066 png_uint_16) (PNG_INFO_hIST) 3067 3068 png_set_tIME(png_ptr, info_ptr, mod_time); 3069 3070 mod_time - time image was last modified 3071 (PNG_VALID_tIME) 3072 3073 png_set_bKGD(png_ptr, info_ptr, background); 3074 3075 background - background color (of type 3076 png_color_16p) (PNG_VALID_bKGD) 3077 3078 png_set_text(png_ptr, info_ptr, text_ptr, num_text); 3079 3080 text_ptr - array of png_text holding image 3081 comments 3082 3083 text_ptr[i].compression - type of compression used 3084 on "text" PNG_TEXT_COMPRESSION_NONE 3085 PNG_TEXT_COMPRESSION_zTXt 3086 PNG_ITXT_COMPRESSION_NONE 3087 PNG_ITXT_COMPRESSION_zTXt 3088 text_ptr[i].key - keyword for comment. Must contain 3089 1-79 characters. 3090 text_ptr[i].text - text comments for current 3091 keyword. Can be NULL or empty. 3092 text_ptr[i].text_length - length of text string, 3093 after decompression, 0 for iTXt 3094 text_ptr[i].itxt_length - length of itxt string, 3095 after decompression, 0 for tEXt/zTXt 3096 text_ptr[i].lang - language of comment (NULL or 3097 empty for unknown). 3098 text_ptr[i].translated_keyword - keyword in UTF-8 (NULL 3099 or empty for unknown). 3100 3101 Note that the itxt_length, lang, and lang_key 3102 members of the text_ptr structure only exist when the 3103 library is built with iTXt chunk support. Prior to 3104 libpng-1.4.0 the library was built by default without 3105 iTXt support. Also note that when iTXt is supported, 3106 they contain NULL pointers when the "compression" 3107 field contains PNG_TEXT_COMPRESSION_NONE or 3108 PNG_TEXT_COMPRESSION_zTXt. 3109 3110 num_text - number of comments 3111 3112 png_set_sPLT(png_ptr, info_ptr, &palette_ptr, 3113 num_spalettes); 3114 3115 palette_ptr - array of png_sPLT_struct structures 3116 to be added to the list of palettes 3117 in the info structure. 3118 num_spalettes - number of palette structures to be 3119 added. 3120 3121 png_set_oFFs(png_ptr, info_ptr, offset_x, offset_y, 3122 unit_type); 3123 3124 offset_x - positive offset from the left 3125 edge of the screen 3126 3127 offset_y - positive offset from the top 3128 edge of the screen 3129 3130 unit_type - PNG_OFFSET_PIXEL, PNG_OFFSET_MICROMETER 3131 3132 png_set_pHYs(png_ptr, info_ptr, res_x, res_y, 3133 unit_type); 3134 3135 res_x - pixels/unit physical resolution 3136 in x direction 3137 3138 res_y - pixels/unit physical resolution 3139 in y direction 3140 3141 unit_type - PNG_RESOLUTION_UNKNOWN, 3142 PNG_RESOLUTION_METER 3143 3144 png_set_sCAL(png_ptr, info_ptr, unit, width, height) 3145 3146 unit - physical scale units (an integer) 3147 3148 width - width of a pixel in physical scale units 3149 3150 height - height of a pixel in physical scale units 3151 (width and height are doubles) 3152 3153 png_set_sCAL_s(png_ptr, info_ptr, unit, width, height) 3154 3155 unit - physical scale units (an integer) 3156 3157 width - width of a pixel in physical scale units 3158 expressed as a string 3159 3160 height - height of a pixel in physical scale units 3161 (width and height are strings like "2.54") 3162 3163 png_set_unknown_chunks(png_ptr, info_ptr, &unknowns, 3164 num_unknowns) 3165 3166 unknowns - array of png_unknown_chunk 3167 structures holding unknown chunks 3168 unknowns[i].name - name of unknown chunk 3169 unknowns[i].data - data of unknown chunk 3170 unknowns[i].size - size of unknown chunk's data 3171 unknowns[i].location - position to write chunk in file 3172 0: do not write chunk 3173 PNG_HAVE_IHDR: before PLTE 3174 PNG_HAVE_PLTE: before IDAT 3175 PNG_AFTER_IDAT: after IDAT 3176 3177The "location" member is set automatically according to 3178what part of the output file has already been written. 3179You can change its value after calling png_set_unknown_chunks() 3180as demonstrated in pngtest.c. Within each of the "locations", 3181the chunks are sequenced according to their position in the 3182structure (that is, the value of "i", which is the order in which 3183the chunk was either read from the input file or defined with 3184png_set_unknown_chunks). 3185 3186A quick word about text and num_text. text is an array of png_text 3187structures. num_text is the number of valid structures in the array. 3188Each png_text structure holds a language code, a keyword, a text value, 3189and a compression type. 3190 3191The compression types have the same valid numbers as the compression 3192types of the image data. Currently, the only valid number is zero. 3193However, you can store text either compressed or uncompressed, unlike 3194images, which always have to be compressed. So if you don't want the 3195text compressed, set the compression type to PNG_TEXT_COMPRESSION_NONE. 3196Because tEXt and zTXt chunks don't have a language field, if you 3197specify PNG_TEXT_COMPRESSION_NONE or PNG_TEXT_COMPRESSION_zTXt 3198any language code or translated keyword will not be written out. 3199 3200Until text gets around a few hundred bytes, it is not worth compressing it. 3201After the text has been written out to the file, the compression type 3202is set to PNG_TEXT_COMPRESSION_NONE_WR or PNG_TEXT_COMPRESSION_zTXt_WR, 3203so that it isn't written out again at the end (in case you are calling 3204png_write_end() with the same struct). 3205 3206The keywords that are given in the PNG Specification are: 3207 3208 Title Short (one line) title or 3209 caption for image 3210 3211 Author Name of image's creator 3212 3213 Description Description of image (possibly long) 3214 3215 Copyright Copyright notice 3216 3217 Creation Time Time of original image creation 3218 (usually RFC 1123 format, see below) 3219 3220 Software Software used to create the image 3221 3222 Disclaimer Legal disclaimer 3223 3224 Warning Warning of nature of content 3225 3226 Source Device used to create the image 3227 3228 Comment Miscellaneous comment; conversion 3229 from other image format 3230 3231The keyword-text pairs work like this. Keywords should be short 3232simple descriptions of what the comment is about. Some typical 3233keywords are found in the PNG specification, as is some recommendations 3234on keywords. You can repeat keywords in a file. You can even write 3235some text before the image and some after. For example, you may want 3236to put a description of the image before the image, but leave the 3237disclaimer until after, so viewers working over modem connections 3238don't have to wait for the disclaimer to go over the modem before 3239they start seeing the image. Finally, keywords should be full 3240words, not abbreviations. Keywords and text are in the ISO 8859-1 3241(Latin-1) character set (a superset of regular ASCII) and can not 3242contain NUL characters, and should not contain control or other 3243unprintable characters. To make the comments widely readable, stick 3244with basic ASCII, and avoid machine specific character set extensions 3245like the IBM-PC character set. The keyword must be present, but 3246you can leave off the text string on non-compressed pairs. 3247Compressed pairs must have a text string, as only the text string 3248is compressed anyway, so the compression would be meaningless. 3249 3250PNG supports modification time via the png_time structure. Two 3251conversion routines are provided, png_convert_from_time_t() for 3252time_t and png_convert_from_struct_tm() for struct tm. The 3253time_t routine uses gmtime(). You don't have to use either of 3254these, but if you wish to fill in the png_time structure directly, 3255you should provide the time in universal time (GMT) if possible 3256instead of your local time. Note that the year number is the full 3257year (e.g. 1998, rather than 98 - PNG is year 2000 compliant!), and 3258that months start with 1. 3259 3260If you want to store the time of the original image creation, you should 3261use a plain tEXt chunk with the "Creation Time" keyword. This is 3262necessary because the "creation time" of a PNG image is somewhat vague, 3263depending on whether you mean the PNG file, the time the image was 3264created in a non-PNG format, a still photo from which the image was 3265scanned, or possibly the subject matter itself. In order to facilitate 3266machine-readable dates, it is recommended that the "Creation Time" 3267tEXt chunk use RFC 1123 format dates (e.g. "22 May 1997 18:07:10 GMT"), 3268although this isn't a requirement. Unlike the tIME chunk, the 3269"Creation Time" tEXt chunk is not expected to be automatically changed 3270by the software. To facilitate the use of RFC 1123 dates, a function 3271png_convert_to_rfc1123_buffer(buffer, png_timep) is provided to 3272convert from PNG time to an RFC 1123 format string. The caller must provide 3273a writeable buffer of at least 29 bytes. 3274 3275Writing unknown chunks 3276 3277You can use the png_set_unknown_chunks function to queue up private chunks 3278for writing. You give it a chunk name, location, raw data, and a size. You 3279also must use png_set_keep_unknown_chunks() to ensure that libpng will 3280handle them. That's all there is to it. The chunks will be written by the 3281next following png_write_info_before_PLTE, png_write_info, or png_write_end 3282function, depending upon the specified location. Any chunks previously 3283read into the info structure's unknown-chunk list will also be written out 3284in a sequence that satisfies the PNG specification's ordering rules. 3285 3286Here is an example of writing two private chunks, prVt and miNE: 3287 3288 #ifdef PNG_WRITE_UNKNOWN_CHUNKS_SUPPORTED 3289 /* Set unknown chunk data */ 3290 png_unknown_chunk unk_chunk[2]; 3291 strcpy((char *) unk_chunk[0].name, "prVt"; 3292 unk_chunk[0].data = (unsigned char *) "PRIVATE DATA"; 3293 unk_chunk[0].size = strlen(unk_chunk[0].data)+1; 3294 unk_chunk[0].location = PNG_HAVE_IHDR; 3295 strcpy((char *) unk_chunk[1].name, "miNE"; 3296 unk_chunk[1].data = (unsigned char *) "MY CHUNK DATA"; 3297 unk_chunk[1].size = strlen(unk_chunk[0].data)+1; 3298 unk_chunk[1].location = PNG_AFTER_IDAT; 3299 png_set_unknown_chunks(write_ptr, write_info_ptr, 3300 unk_chunk, 2); 3301 /* Needed because miNE is not safe-to-copy */ 3302 png_set_keep_unknown_chunks(png, PNG_HANDLE_CHUNK_ALWAYS, 3303 (png_bytep) "miNE", 1); 3304 # if PNG_LIBPNG_VER < 10600 3305 /* Deal with unknown chunk location bug in 1.5.x and earlier */ 3306 png_set_unknown_chunk_location(png, info, 0, PNG_HAVE_IHDR); 3307 png_set_unknown_chunk_location(png, info, 1, PNG_AFTER_IDAT); 3308 # endif 3309 # if PNG_LIBPNG_VER < 10500 3310 /* PNG_AFTER_IDAT writes two copies of the chunk prior to libpng-1.5.0, 3311 * one before IDAT and another after IDAT, so don't use it; only use 3312 * PNG_HAVE_IHDR location. This call resets the location previously 3313 * set by assignment and png_set_unknown_chunk_location() for chunk 1. 3314 */ 3315 png_set_unknown_chunk_location(png, info, 1, PNG_HAVE_IHDR); 3316 # endif 3317 #endif 3318 3319The high-level write interface 3320 3321At this point there are two ways to proceed; through the high-level 3322write interface, or through a sequence of low-level write operations. 3323You can use the high-level interface if your image data is present 3324in the info structure. All defined output 3325transformations are permitted, enabled by the following masks. 3326 3327 PNG_TRANSFORM_IDENTITY No transformation 3328 PNG_TRANSFORM_PACKING Pack 1, 2 and 4-bit samples 3329 PNG_TRANSFORM_PACKSWAP Change order of packed 3330 pixels to LSB first 3331 PNG_TRANSFORM_INVERT_MONO Invert monochrome images 3332 PNG_TRANSFORM_SHIFT Normalize pixels to the 3333 sBIT depth 3334 PNG_TRANSFORM_BGR Flip RGB to BGR, RGBA 3335 to BGRA 3336 PNG_TRANSFORM_SWAP_ALPHA Flip RGBA to ARGB or GA 3337 to AG 3338 PNG_TRANSFORM_INVERT_ALPHA Change alpha from opacity 3339 to transparency 3340 PNG_TRANSFORM_SWAP_ENDIAN Byte-swap 16-bit samples 3341 PNG_TRANSFORM_STRIP_FILLER Strip out filler 3342 bytes (deprecated). 3343 PNG_TRANSFORM_STRIP_FILLER_BEFORE Strip out leading 3344 filler bytes 3345 PNG_TRANSFORM_STRIP_FILLER_AFTER Strip out trailing 3346 filler bytes 3347 3348If you have valid image data in the info structure (you can use 3349png_set_rows() to put image data in the info structure), simply do this: 3350 3351 png_write_png(png_ptr, info_ptr, png_transforms, NULL) 3352 3353where png_transforms is an integer containing the bitwise OR of some set of 3354transformation flags. This call is equivalent to png_write_info(), 3355followed the set of transformations indicated by the transform mask, 3356then png_write_image(), and finally png_write_end(). 3357 3358(The final parameter of this call is not yet used. Someday it might point 3359to transformation parameters required by some future output transform.) 3360 3361You must use png_transforms and not call any png_set_transform() functions 3362when you use png_write_png(). 3363 3364The low-level write interface 3365 3366If you are going the low-level route instead, you are now ready to 3367write all the file information up to the actual image data. You do 3368this with a call to png_write_info(). 3369 3370 png_write_info(png_ptr, info_ptr); 3371 3372Note that there is one transformation you may need to do before 3373png_write_info(). In PNG files, the alpha channel in an image is the 3374level of opacity. If your data is supplied as a level of transparency, 3375you can invert the alpha channel before you write it, so that 0 is 3376fully transparent and 255 (in 8-bit or paletted images) or 65535 3377(in 16-bit images) is fully opaque, with 3378 3379 png_set_invert_alpha(png_ptr); 3380 3381This must appear before png_write_info() instead of later with the 3382other transformations because in the case of paletted images the tRNS 3383chunk data has to be inverted before the tRNS chunk is written. If 3384your image is not a paletted image, the tRNS data (which in such cases 3385represents a single color to be rendered as transparent) won't need to 3386be changed, and you can safely do this transformation after your 3387png_write_info() call. 3388 3389If you need to write a private chunk that you want to appear before 3390the PLTE chunk when PLTE is present, you can write the PNG info in 3391two steps, and insert code to write your own chunk between them: 3392 3393 png_write_info_before_PLTE(png_ptr, info_ptr); 3394 png_set_unknown_chunks(png_ptr, info_ptr, ...); 3395 png_write_info(png_ptr, info_ptr); 3396 3397After you've written the file information, you can set up the library 3398to handle any special transformations of the image data. The various 3399ways to transform the data will be described in the order that they 3400should occur. This is important, as some of these change the color 3401type and/or bit depth of the data, and some others only work on 3402certain color types and bit depths. Even though each transformation 3403checks to see if it has data that it can do something with, you should 3404make sure to only enable a transformation if it will be valid for the 3405data. For example, don't swap red and blue on grayscale data. 3406 3407PNG files store RGB pixels packed into 3 or 6 bytes. This code tells 3408the library to strip input data that has 4 or 8 bytes per pixel down 3409to 3 or 6 bytes (or strip 2 or 4-byte grayscale+filler data to 1 or 2 3410bytes per pixel). 3411 3412 png_set_filler(png_ptr, 0, PNG_FILLER_BEFORE); 3413 3414where the 0 is unused, and the location is either PNG_FILLER_BEFORE or 3415PNG_FILLER_AFTER, depending upon whether the filler byte in the pixel 3416is stored XRGB or RGBX. 3417 3418PNG files pack pixels of bit depths 1, 2, and 4 into bytes as small as 3419they can, resulting in, for example, 8 pixels per byte for 1 bit files. 3420If the data is supplied at 1 pixel per byte, use this code, which will 3421correctly pack the pixels into a single byte: 3422 3423 png_set_packing(png_ptr); 3424 3425PNG files reduce possible bit depths to 1, 2, 4, 8, and 16. If your 3426data is of another bit depth, you can write an sBIT chunk into the 3427file so that decoders can recover the original data if desired. 3428 3429 /* Set the true bit depth of the image data */ 3430 if (color_type & PNG_COLOR_MASK_COLOR) 3431 { 3432 sig_bit.red = true_bit_depth; 3433 sig_bit.green = true_bit_depth; 3434 sig_bit.blue = true_bit_depth; 3435 } 3436 3437 else 3438 { 3439 sig_bit.gray = true_bit_depth; 3440 } 3441 3442 if (color_type & PNG_COLOR_MASK_ALPHA) 3443 { 3444 sig_bit.alpha = true_bit_depth; 3445 } 3446 3447 png_set_sBIT(png_ptr, info_ptr, &sig_bit); 3448 3449If the data is stored in the row buffer in a bit depth other than 3450one supported by PNG (e.g. 3 bit data in the range 0-7 for a 4-bit PNG), 3451this will scale the values to appear to be the correct bit depth as 3452is required by PNG. 3453 3454 png_set_shift(png_ptr, &sig_bit); 3455 3456PNG files store 16-bit pixels in network byte order (big-endian, 3457ie. most significant bits first). This code would be used if they are 3458supplied the other way (little-endian, i.e. least significant bits 3459first, the way PCs store them): 3460 3461 if (bit_depth > 8) 3462 png_set_swap(png_ptr); 3463 3464If you are using packed-pixel images (1, 2, or 4 bits/pixel), and you 3465need to change the order the pixels are packed into bytes, you can use: 3466 3467 if (bit_depth < 8) 3468 png_set_packswap(png_ptr); 3469 3470PNG files store 3 color pixels in red, green, blue order. This code 3471would be used if they are supplied as blue, green, red: 3472 3473 png_set_bgr(png_ptr); 3474 3475PNG files describe monochrome as black being zero and white being 3476one. This code would be used if the pixels are supplied with this reversed 3477(black being one and white being zero): 3478 3479 png_set_invert_mono(png_ptr); 3480 3481Finally, you can write your own transformation function if none of 3482the existing ones meets your needs. This is done by setting a callback 3483with 3484 3485 png_set_write_user_transform_fn(png_ptr, 3486 write_transform_fn); 3487 3488You must supply the function 3489 3490 void write_transform_fn(png_structp png_ptr, png_row_infop 3491 row_info, png_bytep data) 3492 3493See pngtest.c for a working example. Your function will be called 3494before any of the other transformations are processed. If supported 3495libpng also supplies an information routine that may be called from 3496your callback: 3497 3498 png_get_current_row_number(png_ptr); 3499 png_get_current_pass_number(png_ptr); 3500 3501This returns the current row passed to the transform. With interlaced 3502images the value returned is the row in the input sub-image image. Use 3503PNG_ROW_FROM_PASS_ROW(row, pass) and PNG_COL_FROM_PASS_COL(col, pass) to 3504find the output pixel (x,y) given an interlaced sub-image pixel (row,col,pass). 3505 3506The discussion of interlace handling above contains more information on how to 3507use these values. 3508 3509You can also set up a pointer to a user structure for use by your 3510callback function. 3511 3512 png_set_user_transform_info(png_ptr, user_ptr, 0, 0); 3513 3514The user_channels and user_depth parameters of this function are ignored 3515when writing; you can set them to zero as shown. 3516 3517You can retrieve the pointer via the function png_get_user_transform_ptr(). 3518For example: 3519 3520 voidp write_user_transform_ptr = 3521 png_get_user_transform_ptr(png_ptr); 3522 3523It is possible to have libpng flush any pending output, either manually, 3524or automatically after a certain number of lines have been written. To 3525flush the output stream a single time call: 3526 3527 png_write_flush(png_ptr); 3528 3529and to have libpng flush the output stream periodically after a certain 3530number of scanlines have been written, call: 3531 3532 png_set_flush(png_ptr, nrows); 3533 3534Note that the distance between rows is from the last time png_write_flush() 3535was called, or the first row of the image if it has never been called. 3536So if you write 50 lines, and then png_set_flush 25, it will flush the 3537output on the next scanline, and every 25 lines thereafter, unless 3538png_write_flush() is called before 25 more lines have been written. 3539If nrows is too small (less than about 10 lines for a 640 pixel wide 3540RGB image) the image compression may decrease noticeably (although this 3541may be acceptable for real-time applications). Infrequent flushing will 3542only degrade the compression performance by a few percent over images 3543that do not use flushing. 3544 3545Writing the image data 3546 3547That's it for the transformations. Now you can write the image data. 3548The simplest way to do this is in one function call. If you have the 3549whole image in memory, you can just call png_write_image() and libpng 3550will write the image. You will need to pass in an array of pointers to 3551each row. This function automatically handles interlacing, so you don't 3552need to call png_set_interlace_handling() or call this function multiple 3553times, or any of that other stuff necessary with png_write_rows(). 3554 3555 png_write_image(png_ptr, row_pointers); 3556 3557where row_pointers is: 3558 3559 png_byte *row_pointers[height]; 3560 3561You can point to void or char or whatever you use for pixels. 3562 3563If you don't want to write the whole image at once, you can 3564use png_write_rows() instead. If the file is not interlaced, 3565this is simple: 3566 3567 png_write_rows(png_ptr, row_pointers, 3568 number_of_rows); 3569 3570row_pointers is the same as in the png_write_image() call. 3571 3572If you are just writing one row at a time, you can do this with 3573a single row_pointer instead of an array of row_pointers: 3574 3575 png_bytep row_pointer = row; 3576 3577 png_write_row(png_ptr, row_pointer); 3578 3579When the file is interlaced, things can get a good deal more complicated. 3580The only currently (as of the PNG Specification version 1.2, dated July 35811999) defined interlacing scheme for PNG files is the "Adam7" interlace 3582scheme, that breaks down an image into seven smaller images of varying 3583size. libpng will build these images for you, or you can do them 3584yourself. If you want to build them yourself, see the PNG specification 3585for details of which pixels to write when. 3586 3587If you don't want libpng to handle the interlacing details, just 3588use png_set_interlace_handling() and call png_write_rows() the 3589correct number of times to write all the sub-images 3590(png_set_interlace_handling() returns the number of sub-images.) 3591 3592If you want libpng to build the sub-images, call this before you start 3593writing any rows: 3594 3595 number_of_passes = png_set_interlace_handling(png_ptr); 3596 3597This will return the number of passes needed. Currently, this is seven, 3598but may change if another interlace type is added. 3599 3600Then write the complete image number_of_passes times. 3601 3602 png_write_rows(png_ptr, row_pointers, number_of_rows); 3603 3604Think carefully before you write an interlaced image. Typically code that 3605reads such images reads all the image data into memory, uncompressed, before 3606doing any processing. Only code that can display an image on the fly can 3607take advantage of the interlacing and even then the image has to be exactly 3608the correct size for the output device, because scaling an image requires 3609adjacent pixels and these are not available until all the passes have been 3610read. 3611 3612If you do write an interlaced image you will hardly ever need to handle 3613the interlacing yourself. Call png_set_interlace_handling() and use the 3614approach described above. 3615 3616The only time it is conceivable that you will really need to write an 3617interlaced image pass-by-pass is when you have read one pass by pass and 3618made some pixel-by-pixel transformation to it, as described in the read 3619code above. In this case use the PNG_PASS_ROWS and PNG_PASS_COLS macros 3620to determine the size of each sub-image in turn and simply write the rows 3621you obtained from the read code. 3622 3623Finishing a sequential write 3624 3625After you are finished writing the image, you should finish writing 3626the file. If you are interested in writing comments or time, you should 3627pass an appropriately filled png_info pointer. If you are not interested, 3628you can pass NULL. 3629 3630 png_write_end(png_ptr, info_ptr); 3631 3632When you are done, you can free all memory used by libpng like this: 3633 3634 png_destroy_write_struct(&png_ptr, &info_ptr); 3635 3636It is also possible to individually free the info_ptr members that 3637point to libpng-allocated storage with the following function: 3638 3639 png_free_data(png_ptr, info_ptr, mask, seq) 3640 3641 mask - identifies data to be freed, a mask 3642 containing the bitwise OR of one or 3643 more of 3644 PNG_FREE_PLTE, PNG_FREE_TRNS, 3645 PNG_FREE_HIST, PNG_FREE_ICCP, 3646 PNG_FREE_PCAL, PNG_FREE_ROWS, 3647 PNG_FREE_SCAL, PNG_FREE_SPLT, 3648 PNG_FREE_TEXT, PNG_FREE_UNKN, 3649 or simply PNG_FREE_ALL 3650 3651 seq - sequence number of item to be freed 3652 (-1 for all items) 3653 3654This function may be safely called when the relevant storage has 3655already been freed, or has not yet been allocated, or was allocated 3656by the user and not by libpng, and will in those cases do nothing. 3657The "seq" parameter is ignored if only one item of the selected data 3658type, such as PLTE, is allowed. If "seq" is not -1, and multiple items 3659are allowed for the data type identified in the mask, such as text or 3660sPLT, only the n'th item in the structure is freed, where n is "seq". 3661 3662If you allocated data such as a palette that you passed in to libpng 3663with png_set_*, you must not free it until just before the call to 3664png_destroy_write_struct(). 3665 3666The default behavior is only to free data that was allocated internally 3667by libpng. This can be changed, so that libpng will not free the data, 3668or so that it will free data that was allocated by the user with png_malloc() 3669or png_calloc() and passed in via a png_set_*() function, with 3670 3671 png_data_freer(png_ptr, info_ptr, freer, mask) 3672 3673 freer - one of 3674 PNG_DESTROY_WILL_FREE_DATA 3675 PNG_SET_WILL_FREE_DATA 3676 PNG_USER_WILL_FREE_DATA 3677 3678 mask - which data elements are affected 3679 same choices as in png_free_data() 3680 3681For example, to transfer responsibility for some data from a read structure 3682to a write structure, you could use 3683 3684 png_data_freer(read_ptr, read_info_ptr, 3685 PNG_USER_WILL_FREE_DATA, 3686 PNG_FREE_PLTE|PNG_FREE_tRNS|PNG_FREE_hIST) 3687 3688 png_data_freer(write_ptr, write_info_ptr, 3689 PNG_DESTROY_WILL_FREE_DATA, 3690 PNG_FREE_PLTE|PNG_FREE_tRNS|PNG_FREE_hIST) 3691 3692thereby briefly reassigning responsibility for freeing to the user but 3693immediately afterwards reassigning it once more to the write_destroy 3694function. Having done this, it would then be safe to destroy the read 3695structure and continue to use the PLTE, tRNS, and hIST data in the write 3696structure. 3697 3698This function only affects data that has already been allocated. 3699You can call this function before calling after the png_set_*() functions 3700to control whether the user or png_destroy_*() is supposed to free the data. 3701When the user assumes responsibility for libpng-allocated data, the 3702application must use 3703png_free() to free it, and when the user transfers responsibility to libpng 3704for data that the user has allocated, the user must have used png_malloc() 3705or png_calloc() to allocate it. 3706 3707If you allocated text_ptr.text, text_ptr.lang, and text_ptr.translated_keyword 3708separately, do not transfer responsibility for freeing text_ptr to libpng, 3709because when libpng fills a png_text structure it combines these members with 3710the key member, and png_free_data() will free only text_ptr.key. Similarly, 3711if you transfer responsibility for free'ing text_ptr from libpng to your 3712application, your application must not separately free those members. 3713For a more compact example of writing a PNG image, see the file example.c. 3714 3715V. Simplified API 3716 3717The simplified API, which became available in libpng-1.6.0, hides the details 3718of both libpng and the PNG file format itself. 3719It allows PNG files to be read into a very limited number of 3720in-memory bitmap formats or to be written from the same formats. If these 3721formats do not accommodate your needs then you can, and should, use the more 3722sophisticated APIs above - these support a wide variety of in-memory formats 3723and a wide variety of sophisticated transformations to those formats as well 3724as a wide variety of APIs to manipulate ancilliary information. 3725 3726To read a PNG file using the simplified API: 3727 3728 1) Declare a 'png_image' structure (see below) on the stack, set the 3729 version field to PNG_IMAGE_VERSION and the 'opaque' pointer to NULL 3730 (this is REQUIRED, your program may crash if you don't do it.) 3731 3732 2) Call the appropriate png_image_begin_read... function. 3733 3734 3) Set the png_image 'format' member to the required sample format. 3735 3736 4) Allocate a buffer for the image and, if required, the color-map. 3737 3738 5) Call png_image_finish_read to read the image and, if required, the 3739 color-map into your buffers. 3740 3741There are no restrictions on the format of the PNG input itself; all valid 3742color types, bit depths, and interlace methods are acceptable, and the 3743input image is transformed as necessary to the requested in-memory format 3744during the png_image_finish_read() step. The only caveat is that if you 3745request a color-mapped image from a PNG that is full-color or makes 3746complex use of an alpha channel the transformation is extremely lossy and the 3747result may look terrible. 3748 3749To write a PNG file using the simplified API: 3750 3751 1) Declare a 'png_image' structure on the stack and memset() 3752 it to all zero. 3753 3754 2) Initialize the members of the structure that describe the 3755 image, setting the 'format' member to the format of the 3756 image samples. 3757 3758 3) Call the appropriate png_image_write... function with a 3759 pointer to the image and, if necessary, the color-map to write 3760 the PNG data. 3761 3762png_image is a structure that describes the in-memory format of an image 3763when it is being read or defines the in-memory format of an image that you 3764need to write. The "png_image" structure contains the following members: 3765 3766 png_controlp opaque Initialize to NULL, free with png_image_free 3767 png_uint_32 version Set to PNG_IMAGE_VERSION 3768 png_uint_32 width Image width in pixels (columns) 3769 png_uint_32 height Image height in pixels (rows) 3770 png_uint_32 format Image format as defined below 3771 png_uint_32 flags A bit mask containing informational flags 3772 png_uint_32 colormap_entries; Number of entries in the color-map 3773 png_uint_32 warning_or_error; 3774 char message[64]; 3775 3776In the event of an error or warning the "warning_or_error" 3777field will be set to a non-zero value and the 'message' field will contain 3778a '\0' terminated string with the libpng error or warning message. If both 3779warnings and an error were encountered, only the error is recorded. If there 3780are multiple warnings, only the first one is recorded. 3781 3782The upper 30 bits of the "warning_or_error" value are reserved; the low two 3783bits contain a two bit code such that a value more than 1 indicates a failure 3784in the API just called: 3785 3786 0 - no warning or error 3787 1 - warning 3788 2 - error 3789 3 - error preceded by warning 3790 3791The pixels (samples) of the image have one to four channels whose components 3792have original values in the range 0 to 1.0: 3793 3794 1: A single gray or luminance channel (G). 3795 2: A gray/luminance channel and an alpha channel (GA). 3796 3: Three red, green, blue color channels (RGB). 3797 4: Three color channels and an alpha channel (RGBA). 3798 3799The channels are encoded in one of two ways: 3800 3801 a) As a small integer, value 0..255, contained in a single byte. For the 3802alpha channel the original value is simply value/255. For the color or 3803luminance channels the value is encoded according to the sRGB specification 3804and matches the 8-bit format expected by typical display devices. 3805 3806The color/gray channels are not scaled (pre-multiplied) by the alpha 3807channel and are suitable for passing to color management software. 3808 3809 b) As a value in the range 0..65535, contained in a 2-byte integer, in 3810the native byte order of the platform on which the application is running. 3811All channels can be converted to the original value by dividing by 65535; all 3812channels are linear. Color channels use the RGB encoding (RGB end-points) of 3813the sRGB specification. This encoding is identified by the 3814PNG_FORMAT_FLAG_LINEAR flag below. 3815 3816When the simplified API needs to convert between sRGB and linear colorspaces, 3817the actual sRGB transfer curve defined in the sRGB specification (see the 3818article at http://en.wikipedia.org/wiki/SRGB) is used, not the gamma=1/2.2 3819approximation used elsewhere in libpng. 3820 3821When an alpha channel is present it is expected to denote pixel coverage 3822of the color or luminance channels and is returned as an associated alpha 3823channel: the color/gray channels are scaled (pre-multiplied) by the alpha 3824value. 3825 3826The samples are either contained directly in the image data, between 1 and 8 3827bytes per pixel according to the encoding, or are held in a color-map indexed 3828by bytes in the image data. In the case of a color-map the color-map entries 3829are individual samples, encoded as above, and the image data has one byte per 3830pixel to select the relevant sample from the color-map. 3831 3832PNG_FORMAT_* 3833 3834The #defines to be used in png_image::format. Each #define identifies a 3835particular layout of channel data and, if present, alpha values. There are 3836separate defines for each of the two component encodings. 3837 3838A format is built up using single bit flag values. All combinations are 3839valid. Formats can be built up from the flag values or you can use one of 3840the predefined values below. When testing formats always use the FORMAT_FLAG 3841macros to test for individual features - future versions of the library may 3842add new flags. 3843 3844When reading or writing color-mapped images the format should be set to the 3845format of the entries in the color-map then png_image_{read,write}_colormap 3846called to read or write the color-map and set the format correctly for the 3847image data. Do not set the PNG_FORMAT_FLAG_COLORMAP bit directly! 3848 3849NOTE: libpng can be built with particular features disabled. If you see 3850compiler errors because the definition of one of the following flags has been 3851compiled out it is because libpng does not have the required support. It is 3852possible, however, for the libpng configuration to enable the format on just 3853read or just write; in that case you may see an error at run time. 3854You can guard against this by checking for the definition of the 3855appropriate "_SUPPORTED" macro, one of: 3856 3857 PNG_SIMPLIFIED_{READ,WRITE}_{BGR,AFIRST}_SUPPORTED 3858 3859 PNG_FORMAT_FLAG_ALPHA format with an alpha channel 3860 PNG_FORMAT_FLAG_COLOR color format: otherwise grayscale 3861 PNG_FORMAT_FLAG_LINEAR 2-byte channels else 1-byte 3862 PNG_FORMAT_FLAG_COLORMAP image data is color-mapped 3863 PNG_FORMAT_FLAG_BGR BGR colors, else order is RGB 3864 PNG_FORMAT_FLAG_AFIRST alpha channel comes first 3865 3866Supported formats are as follows. Future versions of libpng may support more 3867formats; for compatibility with older versions simply check if the format 3868macro is defined using #ifdef. These defines describe the in-memory layout 3869of the components of the pixels of the image. 3870 3871First the single byte (sRGB) formats: 3872 3873 PNG_FORMAT_GRAY 3874 PNG_FORMAT_GA 3875 PNG_FORMAT_AG 3876 PNG_FORMAT_RGB 3877 PNG_FORMAT_BGR 3878 PNG_FORMAT_RGBA 3879 PNG_FORMAT_ARGB 3880 PNG_FORMAT_BGRA 3881 PNG_FORMAT_ABGR 3882 3883Then the linear 2-byte formats. When naming these "Y" is used to 3884indicate a luminance (gray) channel. The component order within the pixel 3885is always the same - there is no provision for swapping the order of the 3886components in the linear format. The components are 16-bit integers in 3887the native byte order for your platform, and there is no provision for 3888swapping the bytes to a different endian condition. 3889 3890 PNG_FORMAT_LINEAR_Y 3891 PNG_FORMAT_LINEAR_Y_ALPHA 3892 PNG_FORMAT_LINEAR_RGB 3893 PNG_FORMAT_LINEAR_RGB_ALPHA 3894 3895With color-mapped formats the image data is one byte for each pixel. The byte 3896is an index into the color-map which is formatted as above. To obtain a 3897color-mapped format it is sufficient just to add the PNG_FOMAT_FLAG_COLORMAP 3898to one of the above definitions, or you can use one of the definitions below. 3899 3900 PNG_FORMAT_RGB_COLORMAP 3901 PNG_FORMAT_BGR_COLORMAP 3902 PNG_FORMAT_RGBA_COLORMAP 3903 PNG_FORMAT_ARGB_COLORMAP 3904 PNG_FORMAT_BGRA_COLORMAP 3905 PNG_FORMAT_ABGR_COLORMAP 3906 3907PNG_IMAGE macros 3908 3909These are convenience macros to derive information from a png_image 3910structure. The PNG_IMAGE_SAMPLE_ macros return values appropriate to the 3911actual image sample values - either the entries in the color-map or the 3912pixels in the image. The PNG_IMAGE_PIXEL_ macros return corresponding values 3913for the pixels and will always return 1 for color-mapped formats. The 3914remaining macros return information about the rows in the image and the 3915complete image. 3916 3917NOTE: All the macros that take a png_image::format parameter are compile time 3918constants if the format parameter is, itself, a constant. Therefore these 3919macros can be used in array declarations and case labels where required. 3920Similarly the macros are also pre-processor constants (sizeof is not used) so 3921they can be used in #if tests. 3922 3923 PNG_IMAGE_SAMPLE_CHANNELS(fmt) 3924 Returns the total number of channels in a given format: 1..4 3925 3926 PNG_IMAGE_SAMPLE_COMPONENT_SIZE(fmt) 3927 Returns the size in bytes of a single component of a pixel or color-map 3928 entry (as appropriate) in the image: 1 or 2. 3929 3930 PNG_IMAGE_SAMPLE_SIZE(fmt) 3931 This is the size of the sample data for one sample. If the image is 3932 color-mapped it is the size of one color-map entry (and image pixels are 3933 one byte in size), otherwise it is the size of one image pixel. 3934 3935 PNG_IMAGE_MAXIMUM_COLORMAP_COMPONENTS(fmt) 3936 The maximum size of the color-map required by the format expressed in a 3937 count of components. This can be used to compile-time allocate a 3938 color-map: 3939 3940 png_uint_16 colormap[PNG_IMAGE_MAXIMUM_COLORMAP_COMPONENTS(linear_fmt)]; 3941 3942 png_byte colormap[PNG_IMAGE_MAXIMUM_COLORMAP_COMPONENTS(sRGB_fmt)]; 3943 3944 Alternatively use the PNG_IMAGE_COLORMAP_SIZE macro below to use the 3945 information from one of the png_image_begin_read_ APIs and dynamically 3946 allocate the required memory. 3947 3948 PNG_IMAGE_COLORMAP_SIZE(fmt) 3949 The size of the color-map required by the format; this is the size of the 3950 color-map buffer passed to the png_image_{read,write}_colormap APIs. It is 3951 a fixed number determined by the format so can easily be allocated on the 3952 stack if necessary. 3953 3954Corresponding information about the pixels 3955 3956 PNG_IMAGE_PIXEL_CHANNELS(fmt) 3957 The number of separate channels (components) in a pixel; 1 for a 3958 color-mapped image. 3959 3960 PNG_IMAGE_PIXEL_COMPONENT_SIZE(fmt)\ 3961 The size, in bytes, of each component in a pixel; 1 for a color-mapped 3962 image. 3963 3964 PNG_IMAGE_PIXEL_SIZE(fmt) 3965 The size, in bytes, of a complete pixel; 1 for a color-mapped image. 3966 3967Information about the whole row, or whole image 3968 3969 PNG_IMAGE_ROW_STRIDE(image) 3970 Returns the total number of components in a single row of the image; this 3971 is the minimum 'row stride', the minimum count of components between each 3972 row. For a color-mapped image this is the minimum number of bytes in a 3973 row. 3974 3975 If you need the stride measured in bytes, row_stride_bytes is 3976 PNG_IMAGE_ROW_STRIDE(image) * PNG_IMAGE_PIXEL_COMPONENT_SIZE(fmt) 3977 plus any padding bytes that your application might need, for example 3978 to start the next row on a 4-byte boundary. 3979 3980 PNG_IMAGE_BUFFER_SIZE(image, row_stride) 3981 Return the size, in bytes, of an image buffer given a png_image and a row 3982 stride - the number of components to leave space for in each row. 3983 3984 PNG_IMAGE_SIZE(image) 3985 Return the size, in bytes, of the image in memory given just a png_image; 3986 the row stride is the minimum stride required for the image. 3987 3988 PNG_IMAGE_COLORMAP_SIZE(image) 3989 Return the size, in bytes, of the color-map of this image. If the image 3990 format is not a color-map format this will return a size sufficient for 3991 256 entries in the given format; check PNG_FORMAT_FLAG_COLORMAP if 3992 you don't want to allocate a color-map in this case. 3993 3994PNG_IMAGE_FLAG_* 3995 3996Flags containing additional information about the image are held in 3997the 'flags' field of png_image. 3998 3999 PNG_IMAGE_FLAG_COLORSPACE_NOT_sRGB == 0x01 4000 This indicates the the RGB values of the in-memory bitmap do not 4001 correspond to the red, green and blue end-points defined by sRGB. 4002 4003 PNG_IMAGE_FLAG_FAST == 0x02 4004 On write emphasise speed over compression; the resultant PNG file will be 4005 larger but will be produced significantly faster, particular for large 4006 images. Do not use this option for images which will be distributed, only 4007 used it when producing intermediate files that will be read back in 4008 repeatedly. For a typical 24-bit image the option will double the read 4009 speed at the cost of increasing the image size by 25%, however for many 4010 more compressible images the PNG file can be 10 times larger with only a 4011 slight speed gain. 4012 4013 PNG_IMAGE_FLAG_16BIT_sRGB == 0x04 4014 On read if the image is a 16-bit per component image and there is no gAMA 4015 or sRGB chunk assume that the components are sRGB encoded. Notice that 4016 images output by the simplified API always have gamma information; setting 4017 this flag only affects the interpretation of 16-bit images from an 4018 external source. It is recommended that the application expose this flag 4019 to the user; the user can normally easily recognize the difference between 4020 linear and sRGB encoding. This flag has no effect on write - the data 4021 passed to the write APIs must have the correct encoding (as defined 4022 above.) 4023 4024 If the flag is not set (the default) input 16-bit per component data is 4025 assumed to be linear. 4026 4027 NOTE: the flag can only be set after the png_image_begin_read_ call, 4028 because that call initializes the 'flags' field. 4029 4030READ APIs 4031 4032 The png_image passed to the read APIs must have been initialized by setting 4033 the png_controlp field 'opaque' to NULL (or, better, memset the whole thing.) 4034 4035 int png_image_begin_read_from_file( png_imagep image, 4036 const char *file_name) 4037 4038 The named file is opened for read and the image header 4039 is filled in from the PNG header in the file. 4040 4041 int png_image_begin_read_from_stdio (png_imagep image, 4042 FILE* file) 4043 4044 The PNG header is read from the stdio FILE object. 4045 4046 int png_image_begin_read_from_memory(png_imagep image, 4047 png_const_voidp memory, png_size_t size) 4048 4049 The PNG header is read from the given memory buffer. 4050 4051 int png_image_finish_read(png_imagep image, 4052 png_colorp background, void *buffer, 4053 png_int_32 row_stride, void *colormap)); 4054 4055 Finish reading the image into the supplied buffer and 4056 clean up the png_image structure. 4057 4058 row_stride is the step, in png_byte or png_uint_16 units 4059 as appropriate, between adjacent rows. A positive stride 4060 indicates that the top-most row is first in the buffer - 4061 the normal top-down arrangement. A negative stride 4062 indicates that the bottom-most row is first in the buffer. 4063 4064 background need only be supplied if an alpha channel must 4065 be removed from a png_byte format and the removal is to be 4066 done by compositing on a solid color; otherwise it may be 4067 NULL and any composition will be done directly onto the 4068 buffer. The value is an sRGB color to use for the 4069 background, for grayscale output the green channel is used. 4070 4071 For linear output removing the alpha channel is always done 4072 by compositing on black. 4073 4074 void png_image_free(png_imagep image) 4075 4076 Free any data allocated by libpng in image->opaque, 4077 setting the pointer to NULL. May be called at any time 4078 after the structure is initialized. 4079 4080When the simplified API needs to convert between sRGB and linear colorspaces, 4081the actual sRGB transfer curve defined in the sRGB specification (see the 4082article at http://en.wikipedia.org/wiki/SRGB) is used, not the gamma=1/2.2 4083approximation used elsewhere in libpng. 4084 4085WRITE APIS 4086 4087For write you must initialize a png_image structure to describe the image to 4088be written: 4089 4090 version: must be set to PNG_IMAGE_VERSION 4091 opaque: must be initialized to NULL 4092 width: image width in pixels 4093 height: image height in rows 4094 format: the format of the data you wish to write 4095 flags: set to 0 unless one of the defined flags applies; set 4096 PNG_IMAGE_FLAG_COLORSPACE_NOT_sRGB for color format images 4097 where the RGB values do not correspond to the colors in sRGB. 4098 colormap_entries: set to the number of entries in the color-map (0 to 256) 4099 4100 int png_image_write_to_file, (png_imagep image, 4101 const char *file, int convert_to_8bit, const void *buffer, 4102 png_int_32 row_stride, const void *colormap)); 4103 4104 Write the image to the named file. 4105 4106 int png_image_write_to_stdio(png_imagep image, FILE *file, 4107 int convert_to_8_bit, const void *buffer, 4108 png_int_32 row_stride, const void *colormap) 4109 4110 Write the image to the given (FILE*). 4111 4112With all write APIs if image is in one of the linear formats with 4113(png_uint_16) data then setting convert_to_8_bit will cause the output to be 4114a (png_byte) PNG gamma encoded according to the sRGB specification, otherwise 4115a 16-bit linear encoded PNG file is written. 4116 4117With all APIs row_stride is handled as in the read APIs - it is the spacing 4118from one row to the next in component sized units (float) and if negative 4119indicates a bottom-up row layout in the buffer. If you pass zero, libpng will 4120calculate the row_stride for you from the width and number of channels. 4121 4122Note that the write API does not support interlacing, sub-8-bit pixels, 4123indexed (paletted) images, or most ancillary chunks. 4124 4125VI. Modifying/Customizing libpng 4126 4127There are two issues here. The first is changing how libpng does 4128standard things like memory allocation, input/output, and error handling. 4129The second deals with more complicated things like adding new chunks, 4130adding new transformations, and generally changing how libpng works. 4131Both of those are compile-time issues; that is, they are generally 4132determined at the time the code is written, and there is rarely a need 4133to provide the user with a means of changing them. 4134 4135Memory allocation, input/output, and error handling 4136 4137All of the memory allocation, input/output, and error handling in libpng 4138goes through callbacks that are user-settable. The default routines are 4139in pngmem.c, pngrio.c, pngwio.c, and pngerror.c, respectively. To change 4140these functions, call the appropriate png_set_*_fn() function. 4141 4142Memory allocation is done through the functions png_malloc(), png_calloc(), 4143and png_free(). The png_malloc() and png_free() functions currently just 4144call the standard C functions and png_calloc() calls png_malloc() and then 4145clears the newly allocated memory to zero; note that png_calloc(png_ptr, size) 4146is not the same as the calloc(number, size) function provided by stdlib.h. 4147There is limited support for certain systems with segmented memory 4148architectures and the types of pointers declared by png.h match this; you 4149will have to use appropriate pointers in your application. If you prefer 4150to use a different method of allocating and freeing data, you can use 4151png_create_read_struct_2() or png_create_write_struct_2() to register your 4152own functions as described above. These functions also provide a void 4153pointer that can be retrieved via 4154 4155 mem_ptr=png_get_mem_ptr(png_ptr); 4156 4157Your replacement memory functions must have prototypes as follows: 4158 4159 png_voidp malloc_fn(png_structp png_ptr, 4160 png_alloc_size_t size); 4161 4162 void free_fn(png_structp png_ptr, png_voidp ptr); 4163 4164Your malloc_fn() must return NULL in case of failure. The png_malloc() 4165function will normally call png_error() if it receives a NULL from the 4166system memory allocator or from your replacement malloc_fn(). 4167 4168Your free_fn() will never be called with a NULL ptr, since libpng's 4169png_free() checks for NULL before calling free_fn(). 4170 4171Input/Output in libpng is done through png_read() and png_write(), 4172which currently just call fread() and fwrite(). The FILE * is stored in 4173png_struct and is initialized via png_init_io(). If you wish to change 4174the method of I/O, the library supplies callbacks that you can set 4175through the function png_set_read_fn() and png_set_write_fn() at run 4176time, instead of calling the png_init_io() function. These functions 4177also provide a void pointer that can be retrieved via the function 4178png_get_io_ptr(). For example: 4179 4180 png_set_read_fn(png_structp read_ptr, 4181 voidp read_io_ptr, png_rw_ptr read_data_fn) 4182 4183 png_set_write_fn(png_structp write_ptr, 4184 voidp write_io_ptr, png_rw_ptr write_data_fn, 4185 png_flush_ptr output_flush_fn); 4186 4187 voidp read_io_ptr = png_get_io_ptr(read_ptr); 4188 voidp write_io_ptr = png_get_io_ptr(write_ptr); 4189 4190The replacement I/O functions must have prototypes as follows: 4191 4192 void user_read_data(png_structp png_ptr, 4193 png_bytep data, png_size_t length); 4194 4195 void user_write_data(png_structp png_ptr, 4196 png_bytep data, png_size_t length); 4197 4198 void user_flush_data(png_structp png_ptr); 4199 4200The user_read_data() function is responsible for detecting and 4201handling end-of-data errors. 4202 4203Supplying NULL for the read, write, or flush functions sets them back 4204to using the default C stream functions, which expect the io_ptr to 4205point to a standard *FILE structure. It is probably a mistake 4206to use NULL for one of write_data_fn and output_flush_fn but not both 4207of them, unless you have built libpng with PNG_NO_WRITE_FLUSH defined. 4208It is an error to read from a write stream, and vice versa. 4209 4210Error handling in libpng is done through png_error() and png_warning(). 4211Errors handled through png_error() are fatal, meaning that png_error() 4212should never return to its caller. Currently, this is handled via 4213setjmp() and longjmp() (unless you have compiled libpng with 4214PNG_NO_SETJMP, in which case it is handled via PNG_ABORT()), 4215but you could change this to do things like exit() if you should wish, 4216as long as your function does not return. 4217 4218On non-fatal errors, png_warning() is called 4219to print a warning message, and then control returns to the calling code. 4220By default png_error() and png_warning() print a message on stderr via 4221fprintf() unless the library is compiled with PNG_NO_CONSOLE_IO defined 4222(because you don't want the messages) or PNG_NO_STDIO defined (because 4223fprintf() isn't available). If you wish to change the behavior of the error 4224functions, you will need to set up your own message callbacks. These 4225functions are normally supplied at the time that the png_struct is created. 4226It is also possible to redirect errors and warnings to your own replacement 4227functions after png_create_*_struct() has been called by calling: 4228 4229 png_set_error_fn(png_structp png_ptr, 4230 png_voidp error_ptr, png_error_ptr error_fn, 4231 png_error_ptr warning_fn); 4232 4233 png_voidp error_ptr = png_get_error_ptr(png_ptr); 4234 4235If NULL is supplied for either error_fn or warning_fn, then the libpng 4236default function will be used, calling fprintf() and/or longjmp() if a 4237problem is encountered. The replacement error functions should have 4238parameters as follows: 4239 4240 void user_error_fn(png_structp png_ptr, 4241 png_const_charp error_msg); 4242 4243 void user_warning_fn(png_structp png_ptr, 4244 png_const_charp warning_msg); 4245 4246The motivation behind using setjmp() and longjmp() is the C++ throw and 4247catch exception handling methods. This makes the code much easier to write, 4248as there is no need to check every return code of every function call. 4249However, there are some uncertainties about the status of local variables 4250after a longjmp, so the user may want to be careful about doing anything 4251after setjmp returns non-zero besides returning itself. Consult your 4252compiler documentation for more details. For an alternative approach, you 4253may wish to use the "cexcept" facility (see http://cexcept.sourceforge.net), 4254which is illustrated in pngvalid.c and in contrib/visupng. 4255 4256Beginning in libpng-1.4.0, the png_set_benign_errors() API became available. 4257You can use this to handle certain errors (normally handled as errors) 4258as warnings. 4259 4260 png_set_benign_errors (png_ptr, int allowed); 4261 4262 allowed: 0: treat png_benign_error() as an error. 4263 1: treat png_benign_error() as a warning. 4264 4265As of libpng-1.6.0, the default condition is to treat benign errors as 4266warnings while reading and as errors while writing. 4267 4268Custom chunks 4269 4270If you need to read or write custom chunks, you may need to get deeper 4271into the libpng code. The library now has mechanisms for storing 4272and writing chunks of unknown type; you can even declare callbacks 4273for custom chunks. However, this may not be good enough if the 4274library code itself needs to know about interactions between your 4275chunk and existing `intrinsic' chunks. 4276 4277If you need to write a new intrinsic chunk, first read the PNG 4278specification. Acquire a first level of understanding of how it works. 4279Pay particular attention to the sections that describe chunk names, 4280and look at how other chunks were designed, so you can do things 4281similarly. Second, check out the sections of libpng that read and 4282write chunks. Try to find a chunk that is similar to yours and use 4283it as a template. More details can be found in the comments inside 4284the code. It is best to handle private or unknown chunks in a generic method, 4285via callback functions, instead of by modifying libpng functions. This 4286is illustrated in pngtest.c, which uses a callback function to handle a 4287private "vpAg" chunk and the new "sTER" chunk, which are both unknown to 4288libpng. 4289 4290If you wish to write your own transformation for the data, look through 4291the part of the code that does the transformations, and check out some of 4292the simpler ones to get an idea of how they work. Try to find a similar 4293transformation to the one you want to add and copy off of it. More details 4294can be found in the comments inside the code itself. 4295 4296Configuring for gui/windowing platforms: 4297 4298You will need to write new error and warning functions that use the GUI 4299interface, as described previously, and set them to be the error and 4300warning functions at the time that png_create_*_struct() is called, 4301in order to have them available during the structure initialization. 4302They can be changed later via png_set_error_fn(). On some compilers, 4303you may also have to change the memory allocators (png_malloc, etc.). 4304 4305Configuring zlib: 4306 4307There are special functions to configure the compression. Perhaps the 4308most useful one changes the compression level, which currently uses 4309input compression values in the range 0 - 9. The library normally 4310uses the default compression level (Z_DEFAULT_COMPRESSION = 6). Tests 4311have shown that for a large majority of images, compression values in 4312the range 3-6 compress nearly as well as higher levels, and do so much 4313faster. For online applications it may be desirable to have maximum speed 4314(Z_BEST_SPEED = 1). With versions of zlib after v0.99, you can also 4315specify no compression (Z_NO_COMPRESSION = 0), but this would create 4316files larger than just storing the raw bitmap. You can specify the 4317compression level by calling: 4318 4319 #include zlib.h 4320 png_set_compression_level(png_ptr, level); 4321 4322Another useful one is to reduce the memory level used by the library. 4323The memory level defaults to 8, but it can be lowered if you are 4324short on memory (running DOS, for example, where you only have 640K). 4325Note that the memory level does have an effect on compression; among 4326other things, lower levels will result in sections of incompressible 4327data being emitted in smaller stored blocks, with a correspondingly 4328larger relative overhead of up to 15% in the worst case. 4329 4330 #include zlib.h 4331 png_set_compression_mem_level(png_ptr, level); 4332 4333The other functions are for configuring zlib. They are not recommended 4334for normal use and may result in writing an invalid PNG file. See 4335zlib.h for more information on what these mean. 4336 4337 #include zlib.h 4338 png_set_compression_strategy(png_ptr, 4339 strategy); 4340 4341 png_set_compression_window_bits(png_ptr, 4342 window_bits); 4343 4344 png_set_compression_method(png_ptr, method); 4345 4346This controls the size of the IDAT chunks (default 8192): 4347 4348 png_set_compression_buffer_size(png_ptr, size); 4349 4350As of libpng version 1.5.4, additional APIs became 4351available to set these separately for non-IDAT 4352compressed chunks such as zTXt, iTXt, and iCCP: 4353 4354 #include zlib.h 4355 #if PNG_LIBPNG_VER >= 10504 4356 png_set_text_compression_level(png_ptr, level); 4357 4358 png_set_text_compression_mem_level(png_ptr, level); 4359 4360 png_set_text_compression_strategy(png_ptr, 4361 strategy); 4362 4363 png_set_text_compression_window_bits(png_ptr, 4364 window_bits); 4365 4366 png_set_text_compression_method(png_ptr, method); 4367 #endif 4368 4369Controlling row filtering 4370 4371If you want to control whether libpng uses filtering or not, which 4372filters are used, and how it goes about picking row filters, you 4373can call one of these functions. The selection and configuration 4374of row filters can have a significant impact on the size and 4375encoding speed and a somewhat lesser impact on the decoding speed 4376of an image. Filtering is enabled by default for RGB and grayscale 4377images (with and without alpha), but not for paletted images nor 4378for any images with bit depths less than 8 bits/pixel. 4379 4380The 'method' parameter sets the main filtering method, which is 4381currently only '0' in the PNG 1.2 specification. The 'filters' 4382parameter sets which filter(s), if any, should be used for each 4383scanline. Possible values are PNG_ALL_FILTERS and PNG_NO_FILTERS 4384to turn filtering on and off, respectively. 4385 4386Individual filter types are PNG_FILTER_NONE, PNG_FILTER_SUB, 4387PNG_FILTER_UP, PNG_FILTER_AVG, PNG_FILTER_PAETH, which can be bitwise 4388ORed together with '|' to specify one or more filters to use. 4389These filters are described in more detail in the PNG specification. 4390If you intend to change the filter type during the course of writing 4391the image, you should start with flags set for all of the filters 4392you intend to use so that libpng can initialize its internal 4393structures appropriately for all of the filter types. (Note that this 4394means the first row must always be adaptively filtered, because libpng 4395currently does not allocate the filter buffers until png_write_row() 4396is called for the first time.) 4397 4398 filters = PNG_FILTER_NONE | PNG_FILTER_SUB 4399 PNG_FILTER_UP | PNG_FILTER_AVG | 4400 PNG_FILTER_PAETH | PNG_ALL_FILTERS; 4401 4402 png_set_filter(png_ptr, PNG_FILTER_TYPE_BASE, 4403 filters); 4404 The second parameter can also be 4405 PNG_INTRAPIXEL_DIFFERENCING if you are 4406 writing a PNG to be embedded in a MNG 4407 datastream. This parameter must be the 4408 same as the value of filter_method used 4409 in png_set_IHDR(). 4410 4411Requesting debug printout 4412 4413The macro definition PNG_DEBUG can be used to request debugging 4414printout. Set it to an integer value in the range 0 to 3. Higher 4415numbers result in increasing amounts of debugging information. The 4416information is printed to the "stderr" file, unless another file 4417name is specified in the PNG_DEBUG_FILE macro definition. 4418 4419When PNG_DEBUG > 0, the following functions (macros) become available: 4420 4421 png_debug(level, message) 4422 png_debug1(level, message, p1) 4423 png_debug2(level, message, p1, p2) 4424 4425in which "level" is compared to PNG_DEBUG to decide whether to print 4426the message, "message" is the formatted string to be printed, 4427and p1 and p2 are parameters that are to be embedded in the string 4428according to printf-style formatting directives. For example, 4429 4430 png_debug1(2, "foo=%d", foo); 4431 4432is expanded to 4433 4434 if (PNG_DEBUG > 2) 4435 fprintf(PNG_DEBUG_FILE, "foo=%d\n", foo); 4436 4437When PNG_DEBUG is defined but is zero, the macros aren't defined, but you 4438can still use PNG_DEBUG to control your own debugging: 4439 4440 #ifdef PNG_DEBUG 4441 fprintf(stderr, ... 4442 #endif 4443 4444When PNG_DEBUG = 1, the macros are defined, but only png_debug statements 4445having level = 0 will be printed. There aren't any such statements in 4446this version of libpng, but if you insert some they will be printed. 4447 4448VII. MNG support 4449 4450The MNG specification (available at http://www.libpng.org/pub/mng) allows 4451certain extensions to PNG for PNG images that are embedded in MNG datastreams. 4452Libpng can support some of these extensions. To enable them, use the 4453png_permit_mng_features() function: 4454 4455 feature_set = png_permit_mng_features(png_ptr, mask) 4456 4457 mask is a png_uint_32 containing the bitwise OR of the 4458 features you want to enable. These include 4459 PNG_FLAG_MNG_EMPTY_PLTE 4460 PNG_FLAG_MNG_FILTER_64 4461 PNG_ALL_MNG_FEATURES 4462 4463 feature_set is a png_uint_32 that is the bitwise AND of 4464 your mask with the set of MNG features that is 4465 supported by the version of libpng that you are using. 4466 4467It is an error to use this function when reading or writing a standalone 4468PNG file with the PNG 8-byte signature. The PNG datastream must be wrapped 4469in a MNG datastream. As a minimum, it must have the MNG 8-byte signature 4470and the MHDR and MEND chunks. Libpng does not provide support for these 4471or any other MNG chunks; your application must provide its own support for 4472them. You may wish to consider using libmng (available at 4473http://www.libmng.com) instead. 4474 4475VIII. Changes to Libpng from version 0.88 4476 4477It should be noted that versions of libpng later than 0.96 are not 4478distributed by the original libpng author, Guy Schalnat, nor by 4479Andreas Dilger, who had taken over from Guy during 1996 and 1997, and 4480distributed versions 0.89 through 0.96, but rather by another member 4481of the original PNG Group, Glenn Randers-Pehrson. Guy and Andreas are 4482still alive and well, but they have moved on to other things. 4483 4484The old libpng functions png_read_init(), png_write_init(), 4485png_info_init(), png_read_destroy(), and png_write_destroy() have been 4486moved to PNG_INTERNAL in version 0.95 to discourage their use. These 4487functions will be removed from libpng version 1.4.0. 4488 4489The preferred method of creating and initializing the libpng structures is 4490via the png_create_read_struct(), png_create_write_struct(), and 4491png_create_info_struct() because they isolate the size of the structures 4492from the application, allow version error checking, and also allow the 4493use of custom error handling routines during the initialization, which 4494the old functions do not. The functions png_read_destroy() and 4495png_write_destroy() do not actually free the memory that libpng 4496allocated for these structs, but just reset the data structures, so they 4497can be used instead of png_destroy_read_struct() and 4498png_destroy_write_struct() if you feel there is too much system overhead 4499allocating and freeing the png_struct for each image read. 4500 4501Setting the error callbacks via png_set_message_fn() before 4502png_read_init() as was suggested in libpng-0.88 is no longer supported 4503because this caused applications that do not use custom error functions 4504to fail if the png_ptr was not initialized to zero. It is still possible 4505to set the error callbacks AFTER png_read_init(), or to change them with 4506png_set_error_fn(), which is essentially the same function, but with a new 4507name to force compilation errors with applications that try to use the old 4508method. 4509 4510Support for the sCAL, iCCP, iTXt, and sPLT chunks was added at libpng-1.0.6; 4511however, iTXt support was not enabled by default. 4512 4513Starting with version 1.0.7, you can find out which version of the library 4514you are using at run-time: 4515 4516 png_uint_32 libpng_vn = png_access_version_number(); 4517 4518The number libpng_vn is constructed from the major version, minor 4519version with leading zero, and release number with leading zero, 4520(e.g., libpng_vn for version 1.0.7 is 10007). 4521 4522Note that this function does not take a png_ptr, so you can call it 4523before you've created one. 4524 4525You can also check which version of png.h you used when compiling your 4526application: 4527 4528 png_uint_32 application_vn = PNG_LIBPNG_VER; 4529 4530IX. Changes to Libpng from version 1.0.x to 1.2.x 4531 4532Support for user memory management was enabled by default. To 4533accomplish this, the functions png_create_read_struct_2(), 4534png_create_write_struct_2(), png_set_mem_fn(), png_get_mem_ptr(), 4535png_malloc_default(), and png_free_default() were added. 4536 4537Support for the iTXt chunk has been enabled by default as of 4538version 1.2.41. 4539 4540Support for certain MNG features was enabled. 4541 4542Support for numbered error messages was added. However, we never got 4543around to actually numbering the error messages. The function 4544png_set_strip_error_numbers() was added (Note: the prototype for this 4545function was inadvertently removed from png.h in PNG_NO_ASSEMBLER_CODE 4546builds of libpng-1.2.15. It was restored in libpng-1.2.36). 4547 4548The png_malloc_warn() function was added at libpng-1.2.3. This issues 4549a png_warning and returns NULL instead of aborting when it fails to 4550acquire the requested memory allocation. 4551 4552Support for setting user limits on image width and height was enabled 4553by default. The functions png_set_user_limits(), png_get_user_width_max(), 4554and png_get_user_height_max() were added at libpng-1.2.6. 4555 4556The png_set_add_alpha() function was added at libpng-1.2.7. 4557 4558The function png_set_expand_gray_1_2_4_to_8() was added at libpng-1.2.9. 4559Unlike png_set_gray_1_2_4_to_8(), the new function does not expand the 4560tRNS chunk to alpha. The png_set_gray_1_2_4_to_8() function is 4561deprecated. 4562 4563A number of macro definitions in support of runtime selection of 4564assembler code features (especially Intel MMX code support) were 4565added at libpng-1.2.0: 4566 4567 PNG_ASM_FLAG_MMX_SUPPORT_COMPILED 4568 PNG_ASM_FLAG_MMX_SUPPORT_IN_CPU 4569 PNG_ASM_FLAG_MMX_READ_COMBINE_ROW 4570 PNG_ASM_FLAG_MMX_READ_INTERLACE 4571 PNG_ASM_FLAG_MMX_READ_FILTER_SUB 4572 PNG_ASM_FLAG_MMX_READ_FILTER_UP 4573 PNG_ASM_FLAG_MMX_READ_FILTER_AVG 4574 PNG_ASM_FLAG_MMX_READ_FILTER_PAETH 4575 PNG_ASM_FLAGS_INITIALIZED 4576 PNG_MMX_READ_FLAGS 4577 PNG_MMX_FLAGS 4578 PNG_MMX_WRITE_FLAGS 4579 PNG_MMX_FLAGS 4580 4581We added the following functions in support of runtime 4582selection of assembler code features: 4583 4584 png_get_mmx_flagmask() 4585 png_set_mmx_thresholds() 4586 png_get_asm_flags() 4587 png_get_mmx_bitdepth_threshold() 4588 png_get_mmx_rowbytes_threshold() 4589 png_set_asm_flags() 4590 4591We replaced all of these functions with simple stubs in libpng-1.2.20, 4592when the Intel assembler code was removed due to a licensing issue. 4593 4594These macros are deprecated: 4595 4596 PNG_READ_TRANSFORMS_NOT_SUPPORTED 4597 PNG_PROGRESSIVE_READ_NOT_SUPPORTED 4598 PNG_NO_SEQUENTIAL_READ_SUPPORTED 4599 PNG_WRITE_TRANSFORMS_NOT_SUPPORTED 4600 PNG_READ_ANCILLARY_CHUNKS_NOT_SUPPORTED 4601 PNG_WRITE_ANCILLARY_CHUNKS_NOT_SUPPORTED 4602 4603They have been replaced, respectively, by: 4604 4605 PNG_NO_READ_TRANSFORMS 4606 PNG_NO_PROGRESSIVE_READ 4607 PNG_NO_SEQUENTIAL_READ 4608 PNG_NO_WRITE_TRANSFORMS 4609 PNG_NO_READ_ANCILLARY_CHUNKS 4610 PNG_NO_WRITE_ANCILLARY_CHUNKS 4611 4612PNG_MAX_UINT was replaced with PNG_UINT_31_MAX. It has been 4613deprecated since libpng-1.0.16 and libpng-1.2.6. 4614 4615The function 4616 png_check_sig(sig, num) 4617was replaced with 4618 !png_sig_cmp(sig, 0, num) 4619It has been deprecated since libpng-0.90. 4620 4621The function 4622 png_set_gray_1_2_4_to_8() 4623which also expands tRNS to alpha was replaced with 4624 png_set_expand_gray_1_2_4_to_8() 4625which does not. It has been deprecated since libpng-1.0.18 and 1.2.9. 4626 4627X. Changes to Libpng from version 1.0.x/1.2.x to 1.4.x 4628 4629Private libpng prototypes and macro definitions were moved from 4630png.h and pngconf.h into a new pngpriv.h header file. 4631 4632Functions png_set_benign_errors(), png_benign_error(), and 4633png_chunk_benign_error() were added. 4634 4635Support for setting the maximum amount of memory that the application 4636will allocate for reading chunks was added, as a security measure. 4637The functions png_set_chunk_cache_max() and png_get_chunk_cache_max() 4638were added to the library. 4639 4640We implemented support for I/O states by adding png_ptr member io_state 4641and functions png_get_io_chunk_name() and png_get_io_state() in pngget.c 4642 4643We added PNG_TRANSFORM_GRAY_TO_RGB to the available high-level 4644input transforms. 4645 4646Checking for and reporting of errors in the IHDR chunk is more thorough. 4647 4648Support for global arrays was removed, to improve thread safety. 4649 4650Some obsolete/deprecated macros and functions have been removed. 4651 4652Typecasted NULL definitions such as 4653 #define png_voidp_NULL (png_voidp)NULL 4654were eliminated. If you used these in your application, just use 4655NULL instead. 4656 4657The png_struct and info_struct members "trans" and "trans_values" were 4658changed to "trans_alpha" and "trans_color", respectively. 4659 4660The obsolete, unused pnggccrd.c and pngvcrd.c files and related makefiles 4661were removed. 4662 4663The PNG_1_0_X and PNG_1_2_X macros were eliminated. 4664 4665The PNG_LEGACY_SUPPORTED macro was eliminated. 4666 4667Many WIN32_WCE #ifdefs were removed. 4668 4669The functions png_read_init(info_ptr), png_write_init(info_ptr), 4670png_info_init(info_ptr), png_read_destroy(), and png_write_destroy() 4671have been removed. They have been deprecated since libpng-0.95. 4672 4673The png_permit_empty_plte() was removed. It has been deprecated 4674since libpng-1.0.9. Use png_permit_mng_features() instead. 4675 4676We removed the obsolete stub functions png_get_mmx_flagmask(), 4677png_set_mmx_thresholds(), png_get_asm_flags(), 4678png_get_mmx_bitdepth_threshold(), png_get_mmx_rowbytes_threshold(), 4679png_set_asm_flags(), and png_mmx_supported() 4680 4681We removed the obsolete png_check_sig(), png_memcpy_check(), and 4682png_memset_check() functions. Instead use !png_sig_cmp(), memcpy(), 4683and memset(), respectively. 4684 4685The function png_set_gray_1_2_4_to_8() was removed. It has been 4686deprecated since libpng-1.0.18 and 1.2.9, when it was replaced with 4687png_set_expand_gray_1_2_4_to_8() because the former function also 4688expanded any tRNS chunk to an alpha channel. 4689 4690Macros for png_get_uint_16, png_get_uint_32, and png_get_int_32 4691were added and are used by default instead of the corresponding 4692functions. Unfortunately, 4693from libpng-1.4.0 until 1.4.4, the png_get_uint_16 macro (but not the 4694function) incorrectly returned a value of type png_uint_32. 4695 4696We changed the prototype for png_malloc() from 4697 png_malloc(png_structp png_ptr, png_uint_32 size) 4698to 4699 png_malloc(png_structp png_ptr, png_alloc_size_t size) 4700 4701This also applies to the prototype for the user replacement malloc_fn(). 4702 4703The png_calloc() function was added and is used in place of 4704of "png_malloc(); memset();" except in the case in png_read_png() 4705where the array consists of pointers; in this case a "for" loop is used 4706after the png_malloc() to set the pointers to NULL, to give robust. 4707behavior in case the application runs out of memory part-way through 4708the process. 4709 4710We changed the prototypes of png_get_compression_buffer_size() and 4711png_set_compression_buffer_size() to work with png_size_t instead of 4712png_uint_32. 4713 4714Support for numbered error messages was removed by default, since we 4715never got around to actually numbering the error messages. The function 4716png_set_strip_error_numbers() was removed from the library by default. 4717 4718The png_zalloc() and png_zfree() functions are no longer exported. 4719The png_zalloc() function no longer zeroes out the memory that it 4720allocates. Applications that called png_zalloc(png_ptr, number, size) 4721can call png_calloc(png_ptr, number*size) instead, and can call 4722png_free() instead of png_zfree(). 4723 4724Support for dithering was disabled by default in libpng-1.4.0, because 4725it has not been well tested and doesn't actually "dither". 4726The code was not 4727removed, however, and could be enabled by building libpng with 4728PNG_READ_DITHER_SUPPORTED defined. In libpng-1.4.2, this support 4729was re-enabled, but the function was renamed png_set_quantize() to 4730reflect more accurately what it actually does. At the same time, 4731the PNG_DITHER_[RED,GREEN_BLUE]_BITS macros were also renamed to 4732PNG_QUANTIZE_[RED,GREEN,BLUE]_BITS, and PNG_READ_DITHER_SUPPORTED 4733was renamed to PNG_READ_QUANTIZE_SUPPORTED. 4734 4735We removed the trailing '.' from the warning and error messages. 4736 4737XI. Changes to Libpng from version 1.4.x to 1.5.x 4738 4739From libpng-1.4.0 until 1.4.4, the png_get_uint_16 macro (but not the 4740function) incorrectly returned a value of type png_uint_32. 4741The incorrect macro was removed from libpng-1.4.5. 4742 4743Checking for invalid palette index on write was added at libpng 47441.5.10. If a pixel contains an invalid (out-of-range) index libpng issues 4745a benign error. This is enabled by default because this condition is an 4746error according to the PNG specification, Clause 11.3.2, but the error can 4747be ignored in each png_ptr with 4748 4749 png_set_check_for_invalid_index(png_ptr, allowed); 4750 4751 allowed - one of 4752 0: disable benign error (accept the 4753 invalid data without warning). 4754 1: enable benign error (treat the 4755 invalid data as an error or a 4756 warning). 4757 4758If the error is ignored, or if png_benign_error() treats it as a warning, 4759any invalid pixels are decoded as opaque black by the decoder and written 4760as-is by the encoder. 4761 4762Retrieving the maximum palette index found was added at libpng-1.5.15. 4763This statement must appear after png_read_png() or png_read_image() while 4764reading, and after png_write_png() or png_write_image() while writing. 4765 4766 int max_palette = png_get_palette_max(png_ptr, info_ptr); 4767 4768This will return the maximum palette index found in the image, or "-1" if 4769the palette was not checked, or "0" if no palette was found. Note that this 4770does not account for any palette index used by ancillary chunks such as the 4771bKGD chunk; you must check those separately to determine the maximum 4772palette index actually used. 4773 4774There are no substantial API changes between the non-deprecated parts of 4775the 1.4.5 API and the 1.5.0 API; however, the ability to directly access 4776members of the main libpng control structures, png_struct and png_info, 4777deprecated in earlier versions of libpng, has been completely removed from 4778libpng 1.5. 4779 4780We no longer include zlib.h in png.h. The include statement has been moved 4781to pngstruct.h, where it is not accessible by applications. Applications that 4782need access to information in zlib.h will need to add the '#include "zlib.h"' 4783directive. It does not matter whether this is placed prior to or after 4784the '"#include png.h"' directive. 4785 4786The png_sprintf(), png_strcpy(), and png_strncpy() macros are no longer used 4787and were removed. 4788 4789We moved the png_strlen(), png_memcpy(), png_memset(), and png_memcmp() 4790macros into a private header file (pngpriv.h) that is not accessible to 4791applications. 4792 4793In png_get_iCCP, the type of "profile" was changed from png_charpp 4794to png_bytepp, and in png_set_iCCP, from png_charp to png_const_bytep. 4795 4796There are changes of form in png.h, including new and changed macros to 4797declare parts of the API. Some API functions with arguments that are 4798pointers to data not modified within the function have been corrected to 4799declare these arguments with PNG_CONST. 4800 4801Much of the internal use of C macros to control the library build has also 4802changed and some of this is visible in the exported header files, in 4803particular the use of macros to control data and API elements visible 4804during application compilation may require significant revision to 4805application code. (It is extremely rare for an application to do this.) 4806 4807Any program that compiled against libpng 1.4 and did not use deprecated 4808features or access internal library structures should compile and work 4809against libpng 1.5, except for the change in the prototype for 4810png_get_iCCP() and png_set_iCCP() API functions mentioned above. 4811 4812libpng 1.5.0 adds PNG_ PASS macros to help in the reading and writing of 4813interlaced images. The macros return the number of rows and columns in 4814each pass and information that can be used to de-interlace and (if 4815absolutely necessary) interlace an image. 4816 4817libpng 1.5.0 adds an API png_longjmp(png_ptr, value). This API calls 4818the application-provided png_longjmp_ptr on the internal, but application 4819initialized, longjmp buffer. It is provided as a convenience to avoid 4820the need to use the png_jmpbuf macro, which had the unnecessary side 4821effect of resetting the internal png_longjmp_ptr value. 4822 4823libpng 1.5.0 includes a complete fixed point API. By default this is 4824present along with the corresponding floating point API. In general the 4825fixed point API is faster and smaller than the floating point one because 4826the PNG file format used fixed point, not floating point. This applies 4827even if the library uses floating point in internal calculations. A new 4828macro, PNG_FLOATING_ARITHMETIC_SUPPORTED, reveals whether the library 4829uses floating point arithmetic (the default) or fixed point arithmetic 4830internally for performance critical calculations such as gamma correction. 4831In some cases, the gamma calculations may produce slightly different 4832results. This has changed the results in png_rgb_to_gray and in alpha 4833composition (png_set_background for example). This applies even if the 4834original image was already linear (gamma == 1.0) and, therefore, it is 4835not necessary to linearize the image. This is because libpng has *not* 4836been changed to optimize that case correctly, yet. 4837 4838Fixed point support for the sCAL chunk comes with an important caveat; 4839the sCAL specification uses a decimal encoding of floating point values 4840and the accuracy of PNG fixed point values is insufficient for 4841representation of these values. Consequently a "string" API 4842(png_get_sCAL_s and png_set_sCAL_s) is the only reliable way of reading 4843arbitrary sCAL chunks in the absence of either the floating point API or 4844internal floating point calculations. Starting with libpng-1.5.0, both 4845of these functions are present when PNG_sCAL_SUPPORTED is defined. Prior 4846to libpng-1.5.0, their presence also depended upon PNG_FIXED_POINT_SUPPORTED 4847being defined and PNG_FLOATING_POINT_SUPPORTED not being defined. 4848 4849Applications no longer need to include the optional distribution header 4850file pngusr.h or define the corresponding macros during application 4851build in order to see the correct variant of the libpng API. From 1.5.0 4852application code can check for the corresponding _SUPPORTED macro: 4853 4854#ifdef PNG_INCH_CONVERSIONS_SUPPORTED 4855 /* code that uses the inch conversion APIs. */ 4856#endif 4857 4858This macro will only be defined if the inch conversion functions have been 4859compiled into libpng. The full set of macros, and whether or not support 4860has been compiled in, are available in the header file pnglibconf.h. 4861This header file is specific to the libpng build. Notice that prior to 48621.5.0 the _SUPPORTED macros would always have the default definition unless 4863reset by pngusr.h or by explicit settings on the compiler command line. 4864These settings may produce compiler warnings or errors in 1.5.0 because 4865of macro redefinition. 4866 4867Applications can now choose whether to use these macros or to call the 4868corresponding function by defining PNG_USE_READ_MACROS or 4869PNG_NO_USE_READ_MACROS before including png.h. Notice that this is 4870only supported from 1.5.0; defining PNG_NO_USE_READ_MACROS prior to 1.5.0 4871will lead to a link failure. 4872 4873Prior to libpng-1.5.4, the zlib compressor used the same set of parameters 4874when compressing the IDAT data and textual data such as zTXt and iCCP. 4875In libpng-1.5.4 we reinitialized the zlib stream for each type of data. 4876We added five png_set_text_*() functions for setting the parameters to 4877use with textual data. 4878 4879Prior to libpng-1.5.4, the PNG_READ_16_TO_8_ACCURATE_SCALE_SUPPORTED 4880option was off by default, and slightly inaccurate scaling occurred. 4881This option can no longer be turned off, and the choice of accurate 4882or inaccurate 16-to-8 scaling is by using the new png_set_scale_16_to_8() 4883API for accurate scaling or the old png_set_strip_16_to_8() API for simple 4884chopping. In libpng-1.5.4, the PNG_READ_16_TO_8_ACCURATE_SCALE_SUPPORTED 4885macro became PNG_READ_SCALE_16_TO_8_SUPPORTED, and the PNG_READ_16_TO_8 4886macro became PNG_READ_STRIP_16_TO_8_SUPPORTED, to enable the two 4887png_set_*_16_to_8() functions separately. 4888 4889Prior to libpng-1.5.4, the png_set_user_limits() function could only be 4890used to reduce the width and height limits from the value of 4891PNG_USER_WIDTH_MAX and PNG_USER_HEIGHT_MAX, although this document said 4892that it could be used to override them. Now this function will reduce or 4893increase the limits. 4894 4895Starting in libpng-1.5.10, the user limits can be set en masse with the 4896configuration option PNG_SAFE_LIMITS_SUPPORTED. If this option is enabled, 4897a set of "safe" limits is applied in pngpriv.h. These can be overridden by 4898application calls to png_set_user_limits(), png_set_user_chunk_cache_max(), 4899and/or png_set_user_malloc_max() that increase or decrease the limits. Also, 4900in libpng-1.5.10 the default width and height limits were increased 4901from 1,000,000 to 0x7fffffff (i.e., made unlimited). Therefore, the 4902limits are now 4903 default safe 4904 png_user_width_max 0x7fffffff 1,000,000 4905 png_user_height_max 0x7fffffff 1,000,000 4906 png_user_chunk_cache_max 0 (unlimited) 128 4907 png_user_chunk_malloc_max 0 (unlimited) 8,000,000 4908 4909The png_set_option() function (and the "options" member of the png struct) was 4910added to libpng-1.5.15, with option PNG_ARM_NEON. 4911 4912The library now supports a complete fixed point implementation and can 4913thus be used on systems that have no floating point support or very 4914limited or slow support. Previously gamma correction, an essential part 4915of complete PNG support, required reasonably fast floating point. 4916 4917As part of this the choice of internal implementation has been made 4918independent of the choice of fixed versus floating point APIs and all the 4919missing fixed point APIs have been implemented. 4920 4921The exact mechanism used to control attributes of API functions has 4922changed, as described in the INSTALL file. 4923 4924A new test program, pngvalid, is provided in addition to pngtest. 4925pngvalid validates the arithmetic accuracy of the gamma correction 4926calculations and includes a number of validations of the file format. 4927A subset of the full range of tests is run when "make check" is done 4928(in the 'configure' build.) pngvalid also allows total allocated memory 4929usage to be evaluated and performs additional memory overwrite validation. 4930 4931Many changes to individual feature macros have been made. The following 4932are the changes most likely to be noticed by library builders who 4933configure libpng: 4934 49351) All feature macros now have consistent naming: 4936 4937#define PNG_NO_feature turns the feature off 4938#define PNG_feature_SUPPORTED turns the feature on 4939 4940pnglibconf.h contains one line for each feature macro which is either: 4941 4942#define PNG_feature_SUPPORTED 4943 4944if the feature is supported or: 4945 4946/*#undef PNG_feature_SUPPORTED*/ 4947 4948if it is not. Library code consistently checks for the 'SUPPORTED' macro. 4949It does not, and libpng applications should not, check for the 'NO' macro 4950which will not normally be defined even if the feature is not supported. 4951The 'NO' macros are only used internally for setting or not setting the 4952corresponding 'SUPPORTED' macros. 4953 4954Compatibility with the old names is provided as follows: 4955 4956PNG_INCH_CONVERSIONS turns on PNG_INCH_CONVERSIONS_SUPPORTED 4957 4958And the following definitions disable the corresponding feature: 4959 4960PNG_SETJMP_NOT_SUPPORTED disables SETJMP 4961PNG_READ_TRANSFORMS_NOT_SUPPORTED disables READ_TRANSFORMS 4962PNG_NO_READ_COMPOSITED_NODIV disables READ_COMPOSITE_NODIV 4963PNG_WRITE_TRANSFORMS_NOT_SUPPORTED disables WRITE_TRANSFORMS 4964PNG_READ_ANCILLARY_CHUNKS_NOT_SUPPORTED disables READ_ANCILLARY_CHUNKS 4965PNG_WRITE_ANCILLARY_CHUNKS_NOT_SUPPORTED disables WRITE_ANCILLARY_CHUNKS 4966 4967Library builders should remove use of the above, inconsistent, names. 4968 49692) Warning and error message formatting was previously conditional on 4970the STDIO feature. The library has been changed to use the 4971CONSOLE_IO feature instead. This means that if CONSOLE_IO is disabled 4972the library no longer uses the printf(3) functions, even though the 4973default read/write implementations use (FILE) style stdio.h functions. 4974 49753) Three feature macros now control the fixed/floating point decisions: 4976 4977PNG_FLOATING_POINT_SUPPORTED enables the floating point APIs 4978 4979PNG_FIXED_POINT_SUPPORTED enables the fixed point APIs; however, in 4980practice these are normally required internally anyway (because the PNG 4981file format is fixed point), therefore in most cases PNG_NO_FIXED_POINT 4982merely stops the function from being exported. 4983 4984PNG_FLOATING_ARITHMETIC_SUPPORTED chooses between the internal floating 4985point implementation or the fixed point one. Typically the fixed point 4986implementation is larger and slower than the floating point implementation 4987on a system that supports floating point; however, it may be faster on a 4988system which lacks floating point hardware and therefore uses a software 4989emulation. 4990 49914) Added PNG_{READ,WRITE}_INT_FUNCTIONS_SUPPORTED. This allows the 4992functions to read and write ints to be disabled independently of 4993PNG_USE_READ_MACROS, which allows libpng to be built with the functions 4994even though the default is to use the macros - this allows applications 4995to choose at app buildtime whether or not to use macros (previously 4996impossible because the functions weren't in the default build.) 4997 4998XII. Changes to Libpng from version 1.5.x to 1.6.x 4999 5000A "simplified API" has been added (see documentation in png.h and a simple 5001example in contrib/examples/pngtopng.c). The new publicly visible API 5002includes the following: 5003 5004 macros: 5005 PNG_FORMAT_* 5006 PNG_IMAGE_* 5007 structures: 5008 png_control 5009 png_image 5010 read functions 5011 png_image_begin_read_from_file() 5012 png_image_begin_read_from_stdio() 5013 png_image_begin_read_from_memory() 5014 png_image_finish_read() 5015 png_image_free() 5016 write functions 5017 png_image_write_to_file() 5018 png_image_write_to_stdio() 5019 5020Starting with libpng-1.6.0, you can configure libpng to prefix all exported 5021symbols, using the PNG_PREFIX macro. 5022 5023We no longer include string.h in png.h. The include statement has been moved 5024to pngpriv.h, where it is not accessible by applications. Applications that 5025need access to information in string.h must add an '#include <string.h>' 5026directive. It does not matter whether this is placed prior to or after 5027the '#include "png.h"' directive. 5028 5029The following API are now DEPRECATED: 5030 png_info_init_3() 5031 png_convert_to_rfc1123() which has been replaced 5032 with png_convert_to_rfc1123_buffer() 5033 png_malloc_default() 5034 png_free_default() 5035 png_reset_zstream() 5036 5037The following have been removed: 5038 png_get_io_chunk_name(), which has been replaced 5039 with png_get_io_chunk_type(). The new 5040 function returns a 32-bit integer instead of 5041 a string. 5042 The png_sizeof(), png_strlen(), png_memcpy(), png_memcmp(), and 5043 png_memset() macros are no longer used in the libpng sources and 5044 have been removed. These had already been made invisible to applications 5045 (i.e., defined in the private pngpriv.h header file) since libpng-1.5.0. 5046 5047The signatures of many exported functions were changed, such that 5048 png_structp became png_structrp or png_const_structrp 5049 png_infop became png_inforp or png_const_inforp 5050where "rp" indicates a "restricted pointer". 5051 5052Dropped support for 16-bit platforms. The support for FAR/far types has 5053been eliminated and the definition of png_alloc_size_t is now controlled 5054by a flag so that 'small size_t' systems can select it if necessary. 5055 5056Error detection in some chunks has improved; in particular the iCCP chunk 5057reader now does pretty complete validation of the basic format. Some bad 5058profiles that were previously accepted are now accepted with a warning or 5059rejected, depending upon the png_set_benign_errors() setting, in particular 5060the very old broken Microsoft/HP 3144-byte sRGB profile. Starting with 5061libpng-1.6.11, recognizing and checking sRGB profiles can be avoided by 5062means of 5063 5064 #if defined(PNG_SKIP_sRGB_CHECK_PROFILE) && \ 5065 defined(PNG_SET_OPTION_SUPPORTED) 5066 png_set_option(png_ptr, PNG_SKIP_sRGB_CHECK_PROFILE, 5067 PNG_OPTION_ON); 5068 #endif 5069 5070It's not a good idea to do this if you are using the "simplified API", 5071which needs to be able to recognize sRGB profiles conveyed via the iCCP 5072chunk. 5073 5074The PNG spec requirement that only grayscale profiles may appear in images 5075with color type 0 or 4 and that even if the image only contains gray pixels, 5076only RGB profiles may appear in images with color type 2, 3, or 6, is now 5077enforced. The sRGB chunk is allowed to appear in images with any color type 5078and is interpreted by libpng to convey a one-tracer-curve gray profile or a 5079three-tracer-curve RGB profile as appropriate. 5080 5081Prior to libpng-1.6.0 a warning would be issued if the iTXt chunk contained 5082an empty language field or an empty translated keyword. Both of these 5083are allowed by the PNG specification, so these warnings are no longer issued. 5084 5085The library now issues an error if the application attempts to set a 5086transform after it calls png_read_update_info() or if it attempts to call 5087both png_read_update_info() and png_start_read_image() or to call either 5088of them more than once. 5089 5090The default condition for benign_errors is now to treat benign errors as 5091warnings while reading and as errors while writing. 5092 5093The library now issues a warning if both background processing and RGB to 5094gray are used when gamma correction happens. As with previous versions of 5095the library the results are numerically very incorrect in this case. 5096 5097There are some minor arithmetic changes in some transforms such as 5098png_set_background(), that might be detected by certain regression tests. 5099 5100Unknown chunk handling has been improved internally, without any API change. 5101This adds more correct option control of the unknown handling, corrects 5102a pre-existing bug where the per-chunk 'keep' setting is ignored, and makes 5103it possible to skip IDAT chunks in the sequential reader. 5104 5105The machine-generated configure files are no longer included in branches 5106libpng16 and later of the GIT repository. They continue to be included 5107in the tarball releases, however. 5108 5109Libpng-1.6.0 through 1.6.2 used the CMF bytes at the beginning of the IDAT 5110stream to set the size of the sliding window for reading instead of using the 5111default 32-kbyte sliding window size. It was discovered that there are 5112hundreds of PNG files in the wild that have incorrect CMF bytes that caused 5113zlib to issue the "invalid distance too far back" error and reject the file. 5114Libpng-1.6.3 and later calculate their own safe CMF from the image dimensions, 5115provide a way to revert to the libpng-1.5.x behavior (ignoring the CMF bytes 5116and using a 32-kbyte sliding window), by using 5117 5118 png_set_option(png_ptr, PNG_MAXIMUM_INFLATE_WINDOW, 5119 PNG_OPTION_ON); 5120 5121and provide a tool (contrib/tools/pngfix) for rewriting a PNG file while 5122optimizing the CMF bytes in its IDAT chunk correctly. 5123 5124Libpng-1.6.0 and libpng-1.6.1 wrote uncompressed iTXt chunks with the wrong 5125length, which resulted in PNG files that cannot be read beyond the bad iTXt 5126chunk. This error was fixed in libpng-1.6.3, and a tool (called 5127contrib/tools/png-fix-itxt) has been added to the libpng distribution. 5128 5129Starting with libpng-1.6.17, the PNG_SAFE_LIMITS macro was eliminated 5130and safe limits are used by default (users who need larger limits 5131can still override them at compile time or run time, as described above). 5132 5133The new limits are 5134 default spec limit 5135 png_user_width_max 1,000,000 2,147,483,647 5136 png_user_height_max 1,000,000 2,147,483,647 5137 png_user_chunk_cache_max 128 unlimited 5138 png_user_chunk_malloc_max 8,000,000 unlimited 5139 5140Starting with libpng-1.6.18, a PNG_RELEASE_BUILD macro was added, which allows 5141library builders to control compilation for an installed system (a release build). 5142It can be set for testing debug or beta builds to ensure that they will compile 5143when the build type is switched to RC or STABLE. In essence this overrides the 5144PNG_LIBPNG_BUILD_BASE_TYPE definition which is not directly user controllable. 5145 5146Starting with libpng-1.6.19, attempting to set an over-length PLTE chunk 5147is an error. Previously this requirement of the PNG specification was not 5148enforced, and the palette was always limited to 256 entries. An over-length 5149PLTE chunk found in an input PNG is silently truncated. 5150 5151XIII. Detecting libpng 5152 5153The png_get_io_ptr() function has been present since libpng-0.88, has never 5154changed, and is unaffected by conditional compilation macros. It is the 5155best choice for use in configure scripts for detecting the presence of any 5156libpng version since 0.88. In an autoconf "configure.in" you could use 5157 5158 AC_CHECK_LIB(png, png_get_io_ptr, ... 5159 5160XV. Source code repository 5161 5162Since about February 2009, version 1.2.34, libpng has been under "git" source 5163control. The git repository was built from old libpng-x.y.z.tar.gz files 5164going back to version 0.70. You can access the git repository (read only) 5165at 5166 5167 git://git.code.sf.net/p/libpng/code 5168 5169or you can browse it with a web browser by selecting the "code" button at 5170 5171 https://sourceforge.net/projects/libpng 5172 5173Patches can be sent to glennrp at users.sourceforge.net or to 5174png-mng-implement at lists.sourceforge.net or you can upload them to 5175the libpng bug tracker at 5176 5177 http://libpng.sourceforge.net 5178 5179We also accept patches built from the tar or zip distributions, and 5180simple verbal discriptions of bug fixes, reported either to the 5181SourceForge bug tracker, to the png-mng-implement at lists.sf.net 5182mailing list, or directly to glennrp. 5183 5184XV. Coding style 5185 5186Our coding style is similar to the "Allman" style 5187(See http://en.wikipedia.org/wiki/Indent_style#Allman_style), with curly 5188braces on separate lines: 5189 5190 if (condition) 5191 { 5192 action; 5193 } 5194 5195 else if (another condition) 5196 { 5197 another action; 5198 } 5199 5200The braces can be omitted from simple one-line actions: 5201 5202 if (condition) 5203 return (0); 5204 5205We use 3-space indentation, except for continued statements which 5206are usually indented the same as the first line of the statement 5207plus four more spaces. 5208 5209For macro definitions we use 2-space indentation, always leaving the "#" 5210in the first column. 5211 5212 #ifndef PNG_NO_FEATURE 5213 # ifndef PNG_FEATURE_SUPPORTED 5214 # define PNG_FEATURE_SUPPORTED 5215 # endif 5216 #endif 5217 5218Comments appear with the leading "/*" at the same indentation as 5219the statement that follows the comment: 5220 5221 /* Single-line comment */ 5222 statement; 5223 5224 /* This is a multiple-line 5225 * comment. 5226 */ 5227 statement; 5228 5229Very short comments can be placed after the end of the statement 5230to which they pertain: 5231 5232 statement; /* comment */ 5233 5234We don't use C++ style ("//") comments. We have, however, 5235used them in the past in some now-abandoned MMX assembler 5236code. 5237 5238Functions and their curly braces are not indented, and 5239exported functions are marked with PNGAPI: 5240 5241 /* This is a public function that is visible to 5242 * application programmers. It does thus-and-so. 5243 */ 5244 void PNGAPI 5245 png_exported_function(png_ptr, png_info, foo) 5246 { 5247 body; 5248 } 5249 5250The return type and decorations are placed on a separate line 5251ahead of the function name, as illustrated above. 5252 5253The prototypes for all exported functions appear in png.h, 5254above the comment that says 5255 5256 /* Maintainer: Put new public prototypes here ... */ 5257 5258We mark all non-exported functions with "/* PRIVATE */"": 5259 5260 void /* PRIVATE */ 5261 png_non_exported_function(png_ptr, png_info, foo) 5262 { 5263 body; 5264 } 5265 5266The prototypes for non-exported functions (except for those in 5267pngtest) appear in pngpriv.h above the comment that says 5268 5269 /* Maintainer: Put new private prototypes here ^ */ 5270 5271To avoid polluting the global namespace, the names of all exported 5272functions and variables begin with "png_", and all publicly visible C 5273preprocessor macros begin with "PNG". We request that applications that 5274use libpng *not* begin any of their own symbols with either of these strings. 5275 5276We put a space after the "sizeof" operator and we omit the 5277optional parentheses around its argument when the argument 5278is an expression, not a type name, and we always enclose the 5279sizeof operator, with its argument, in parentheses: 5280 5281 (sizeof (png_uint_32)) 5282 (sizeof array) 5283 5284Prior to libpng-1.6.0 we used a "png_sizeof()" macro, formatted as 5285though it were a function. 5286 5287Control keywords if, for, while, and switch are always followed by a space 5288to distinguish them from function calls, which have no trailing space. 5289 5290We put a space after each comma and after each semicolon 5291in "for" statements, and we put spaces before and after each 5292C binary operator and after "for" or "while", and before 5293"?". We don't put a space between a typecast and the expression 5294being cast, nor do we put one between a function name and the 5295left parenthesis that follows it: 5296 5297 for (i = 2; i > 0; --i) 5298 y[i] = a(x) + (int)b; 5299 5300We prefer #ifdef and #ifndef to #if defined() and #if !defined() 5301when there is only one macro being tested. We always use parentheses 5302with "defined". 5303 5304We express integer constants that are used as bit masks in hex format, 5305with an even number of lower-case hex digits, and to make them unsigned 5306(e.g., 0x00U, 0xffU, 0x0100U) and long if they are greater than 0x7fff 5307(e.g., 0xffffUL). 5308 5309We prefer to use underscores rather than camelCase in names, except 5310for a few type names that we inherit from zlib.h. 5311 5312We prefer "if (something != 0)" and "if (something == 0)" 5313over "if (something)" and if "(!something)", respectively. 5314 5315We do not use the TAB character for indentation in the C sources. 5316 5317Lines do not exceed 80 characters. 5318 5319Other rules can be inferred by inspecting the libpng source. 5320 5321XVI. Y2K Compliance in libpng 5322 5323Since the PNG Development group is an ad-hoc body, we can't make 5324an official declaration. 5325 5326This is your unofficial assurance that libpng from version 0.71 and 5327upward through 1.6.20 are Y2K compliant. It is my belief that earlier 5328versions were also Y2K compliant. 5329 5330Libpng only has two year fields. One is a 2-byte unsigned integer 5331that will hold years up to 65535. The other, which is deprecated, 5332holds the date in text format, and will hold years up to 9999. 5333 5334The integer is 5335 "png_uint_16 year" in png_time_struct. 5336 5337The string is 5338 "char time_buffer[29]" in png_struct. This is no longer used 5339in libpng-1.6.x and will be removed from libpng-1.7.0. 5340 5341There are seven time-related functions: 5342 5343 png_convert_to_rfc_1123_buffer() in png.c 5344 (formerly png_convert_to_rfc_1152() in error, and 5345 also formerly png_convert_to_rfc_1123()) 5346 png_convert_from_struct_tm() in pngwrite.c, called 5347 in pngwrite.c 5348 png_convert_from_time_t() in pngwrite.c 5349 png_get_tIME() in pngget.c 5350 png_handle_tIME() in pngrutil.c, called in pngread.c 5351 png_set_tIME() in pngset.c 5352 png_write_tIME() in pngwutil.c, called in pngwrite.c 5353 5354All appear to handle dates properly in a Y2K environment. The 5355png_convert_from_time_t() function calls gmtime() to convert from system 5356clock time, which returns (year - 1900), which we properly convert to 5357the full 4-digit year. There is a possibility that applications using 5358libpng are not passing 4-digit years into the png_convert_to_rfc_1123() 5359function, or that they are incorrectly passing only a 2-digit year 5360instead of "year - 1900" into the png_convert_from_struct_tm() function, 5361but this is not under our control. The libpng documentation has always 5362stated that it works with 4-digit years, and the APIs have been 5363documented as such. 5364 5365The tIME chunk itself is also Y2K compliant. It uses a 2-byte unsigned 5366integer to hold the year, and can hold years as large as 65535. 5367 5368zlib, upon which libpng depends, is also Y2K compliant. It contains 5369no date-related code. 5370 5371 5372 Glenn Randers-Pehrson 5373 libpng maintainer 5374 PNG Development Group 5375