1NOTE: This file was modified by The libjpeg-turbo Project to include only 2information relevant to libjpeg-turbo and to wordsmith certain sections. 3 4USAGE instructions for the Independent JPEG Group's JPEG software 5================================================================= 6 7This file describes usage of the JPEG conversion programs cjpeg and djpeg, 8as well as the utility programs jpegtran, rdjpgcom and wrjpgcom. (See 9the other documentation files if you wish to use the JPEG library within 10your own programs.) 11 12If you are on a Unix machine you may prefer to read the Unix-style manual 13pages in files cjpeg.1, djpeg.1, jpegtran.1, rdjpgcom.1, wrjpgcom.1. 14 15 16INTRODUCTION 17 18These programs implement JPEG image encoding, decoding, and transcoding. 19JPEG (pronounced "jay-peg") is a standardized compression method for 20full-color and grayscale images. 21 22 23GENERAL USAGE 24 25We provide two programs, cjpeg to compress an image file into JPEG format, 26and djpeg to decompress a JPEG file back into a conventional image format. 27 28On Unix-like systems, you say: 29 cjpeg [switches] [imagefile] >jpegfile 30or 31 djpeg [switches] [jpegfile] >imagefile 32The programs read the specified input file, or standard input if none is 33named. They always write to standard output (with trace/error messages to 34standard error). These conventions are handy for piping images between 35programs. 36 37On most non-Unix systems, you say: 38 cjpeg [switches] imagefile jpegfile 39or 40 djpeg [switches] jpegfile imagefile 41i.e., both the input and output files are named on the command line. This 42style is a little more foolproof, and it loses no functionality if you don't 43have pipes. (You can get this style on Unix too, if you prefer, by defining 44TWO_FILE_COMMANDLINE when you compile the programs; see install.txt.) 45 46You can also say: 47 cjpeg [switches] -outfile jpegfile imagefile 48or 49 djpeg [switches] -outfile imagefile jpegfile 50This syntax works on all systems, so it is useful for scripts. 51 52The currently supported image file formats are: PPM (PBMPLUS color format), 53PGM (PBMPLUS grayscale format), BMP, Targa, and RLE (Utah Raster Toolkit 54format). (RLE is supported only if the URT library is available, which it 55isn't on most non-Unix systems.) cjpeg recognizes the input image format 56automatically, with the exception of some Targa files. You have to tell djpeg 57which format to generate. 58 59JPEG files are in the defacto standard JFIF file format. There are other, 60less widely used JPEG-based file formats, but we don't support them. 61 62All switch names may be abbreviated; for example, -grayscale may be written 63-gray or -gr. Most of the "basic" switches can be abbreviated to as little as 64one letter. Upper and lower case are equivalent (-BMP is the same as -bmp). 65British spellings are also accepted (e.g., -greyscale), though for brevity 66these are not mentioned below. 67 68 69CJPEG DETAILS 70 71The basic command line switches for cjpeg are: 72 73 -quality N[,...] Scale quantization tables to adjust image quality. 74 Quality is 0 (worst) to 100 (best); default is 75. 75 (See below for more info.) 76 77 -grayscale Create monochrome JPEG file from color input. 78 Be sure to use this switch when compressing a grayscale 79 BMP file, because cjpeg isn't bright enough to notice 80 whether a BMP file uses only shades of gray. By 81 saying -grayscale, you'll get a smaller JPEG file that 82 takes less time to process. 83 84 -rgb Create RGB JPEG file. 85 Using this switch suppresses the conversion from RGB 86 colorspace input to the default YCbCr JPEG colorspace. 87 88 -optimize Perform optimization of entropy encoding parameters. 89 Without this, default encoding parameters are used. 90 -optimize usually makes the JPEG file a little smaller, 91 but cjpeg runs somewhat slower and needs much more 92 memory. Image quality and speed of decompression are 93 unaffected by -optimize. 94 95 -progressive Create progressive JPEG file (see below). 96 97 -targa Input file is Targa format. Targa files that contain 98 an "identification" field will not be automatically 99 recognized by cjpeg; for such files you must specify 100 -targa to make cjpeg treat the input as Targa format. 101 For most Targa files, you won't need this switch. 102 103The -quality switch lets you trade off compressed file size against quality of 104the reconstructed image: the higher the quality setting, the larger the JPEG 105file, and the closer the output image will be to the original input. Normally 106you want to use the lowest quality setting (smallest file) that decompresses 107into something visually indistinguishable from the original image. For this 108purpose the quality setting should generally be between 50 and 95 (the default 109is 75) for photographic images. If you see defects at -quality 75, then go up 1105 or 10 counts at a time until you are happy with the output image. (The 111optimal setting will vary from one image to another.) 112 113-quality 100 will generate a quantization table of all 1's, minimizing loss 114in the quantization step (but there is still information loss in subsampling, 115as well as roundoff error.) For most images, specifying a quality value above 116about 95 will increase the size of the compressed file dramatically, and while 117the quality gain from these higher quality values is measurable (using metrics 118such as PSNR or SSIM), it is rarely perceivable by human vision. 119 120In the other direction, quality values below 50 will produce very small files 121of low image quality. Settings around 5 to 10 might be useful in preparing an 122index of a large image library, for example. Try -quality 2 (or so) for some 123amusing Cubist effects. (Note: quality values below about 25 generate 2-byte 124quantization tables, which are considered optional in the JPEG standard. 125cjpeg emits a warning message when you give such a quality value, because some 126other JPEG programs may be unable to decode the resulting file. Use -baseline 127if you need to ensure compatibility at low quality values.) 128 129The -quality option has been extended in this version of cjpeg to support 130separate quality settings for luminance and chrominance (or, in general, 131separate settings for every quantization table slot.) The principle is the 132same as chrominance subsampling: since the human eye is more sensitive to 133spatial changes in brightness than spatial changes in color, the chrominance 134components can be quantized more than the luminance components without 135incurring any visible image quality loss. However, unlike subsampling, this 136feature reduces data in the frequency domain instead of the spatial domain, 137which allows for more fine-grained control. This option is useful in 138quality-sensitive applications, for which the artifacts generated by 139subsampling may be unacceptable. 140 141The -quality option accepts a comma-separated list of parameters, which 142respectively refer to the quality levels that should be assigned to the 143quantization table slots. If there are more q-table slots than parameters, 144then the last parameter is replicated. Thus, if only one quality parameter is 145given, this is used for both luminance and chrominance (slots 0 and 1, 146respectively), preserving the legacy behavior of cjpeg v6b and prior. More (or 147customized) quantization tables can be set with the -qtables option and 148assigned to components with the -qslots option (see the "wizard" switches 149below.) 150 151JPEG files generated with separate luminance and chrominance quality are 152fully compliant with standard JPEG decoders. 153 154CAUTION: For this setting to be useful, be sure to pass an argument of 155-sample 1x1 to cjpeg to disable chrominance subsampling. Otherwise, the 156default subsampling level (2x2, AKA "4:2:0") will be used. 157 158The -progressive switch creates a "progressive JPEG" file. In this type of 159JPEG file, the data is stored in multiple scans of increasing quality. If the 160file is being transmitted over a slow communications link, the decoder can use 161the first scan to display a low-quality image very quickly, and can then 162improve the display with each subsequent scan. The final image is exactly 163equivalent to a standard JPEG file of the same quality setting, and the total 164file size is about the same --- often a little smaller. 165 166Switches for advanced users: 167 168 -arithmetic Use arithmetic coding. CAUTION: arithmetic coded JPEG 169 is not yet widely implemented, so many decoders will 170 be unable to view an arithmetic coded JPEG file at 171 all. 172 173 -dct int Use integer DCT method (default). 174 -dct fast Use fast integer DCT (less accurate). 175 In libjpeg-turbo, the fast method is generally about 176 5-15% faster than the int method when using the 177 x86/x86-64 SIMD extensions (results may vary with other 178 SIMD implementations, or when using libjpeg-turbo 179 without SIMD extensions.) For quality levels of 90 and 180 below, there should be little or no perceptible 181 difference between the two algorithms. For quality 182 levels above 90, however, the difference between 183 the fast and the int methods becomes more pronounced. 184 With quality=97, for instance, the fast method incurs 185 generally about a 1-3 dB loss (in PSNR) relative to 186 the int method, but this can be larger for some images. 187 Do not use the fast method with quality levels above 188 97. The algorithm often degenerates at quality=98 and 189 above and can actually produce a more lossy image than 190 if lower quality levels had been used. Also, in 191 libjpeg-turbo, the fast method is not fully accerated 192 for quality levels above 97, so it will be slower than 193 the int method. 194 -dct float Use floating-point DCT method. 195 The float method is mainly a legacy feature. It does 196 not produce significantly more accurate results than 197 the int method, and it is much slower. The float 198 method may also give different results on different 199 machines due to varying roundoff behavior, whereas the 200 integer methods should give the same results on all 201 machines. 202 203 -restart N Emit a JPEG restart marker every N MCU rows, or every 204 N MCU blocks if "B" is attached to the number. 205 -restart 0 (the default) means no restart markers. 206 207 -smooth N Smooth the input image to eliminate dithering noise. 208 N, ranging from 1 to 100, indicates the strength of 209 smoothing. 0 (the default) means no smoothing. 210 211 -maxmemory N Set limit for amount of memory to use in processing 212 large images. Value is in thousands of bytes, or 213 millions of bytes if "M" is attached to the number. 214 For example, -max 4m selects 4000000 bytes. If more 215 space is needed, an error will occur. 216 217 -verbose Enable debug printout. More -v's give more printout. 218 or -debug Also, version information is printed at startup. 219 220The -restart option inserts extra markers that allow a JPEG decoder to 221resynchronize after a transmission error. Without restart markers, any damage 222to a compressed file will usually ruin the image from the point of the error 223to the end of the image; with restart markers, the damage is usually confined 224to the portion of the image up to the next restart marker. Of course, the 225restart markers occupy extra space. We recommend -restart 1 for images that 226will be transmitted across unreliable networks such as Usenet. 227 228The -smooth option filters the input to eliminate fine-scale noise. This is 229often useful when converting dithered images to JPEG: a moderate smoothing 230factor of 10 to 50 gets rid of dithering patterns in the input file, resulting 231in a smaller JPEG file and a better-looking image. Too large a smoothing 232factor will visibly blur the image, however. 233 234Switches for wizards: 235 236 -baseline Force baseline-compatible quantization tables to be 237 generated. This clamps quantization values to 8 bits 238 even at low quality settings. (This switch is poorly 239 named, since it does not ensure that the output is 240 actually baseline JPEG. For example, you can use 241 -baseline and -progressive together.) 242 243 -qtables file Use the quantization tables given in the specified 244 text file. 245 246 -qslots N[,...] Select which quantization table to use for each color 247 component. 248 249 -sample HxV[,...] Set JPEG sampling factors for each color component. 250 251 -scans file Use the scan script given in the specified text file. 252 253The "wizard" switches are intended for experimentation with JPEG. If you 254don't know what you are doing, DON'T USE THEM. These switches are documented 255further in the file wizard.txt. 256 257 258DJPEG DETAILS 259 260The basic command line switches for djpeg are: 261 262 -colors N Reduce image to at most N colors. This reduces the 263 or -quantize N number of colors used in the output image, so that it 264 can be displayed on a colormapped display or stored in 265 a colormapped file format. For example, if you have 266 an 8-bit display, you'd need to reduce to 256 or fewer 267 colors. (-colors is the recommended name, -quantize 268 is provided only for backwards compatibility.) 269 270 -fast Select recommended processing options for fast, low 271 quality output. (The default options are chosen for 272 highest quality output.) Currently, this is equivalent 273 to "-dct fast -nosmooth -onepass -dither ordered". 274 275 -grayscale Force grayscale output even if JPEG file is color. 276 Useful for viewing on monochrome displays; also, 277 djpeg runs noticeably faster in this mode. 278 279 -rgb Force RGB output even if JPEG file is grayscale. 280 281 -scale M/N Scale the output image by a factor M/N. Currently 282 the scale factor must be M/8, where M is an integer 283 between 1 and 16 inclusive, or any reduced fraction 284 thereof (such as 1/2, 3/4, etc. Scaling is handy if 285 the image is larger than your screen; also, djpeg runs 286 much faster when scaling down the output. 287 288 -bmp Select BMP output format (Windows flavor). 8-bit 289 colormapped format is emitted if -colors or -grayscale 290 is specified, or if the JPEG file is grayscale; 291 otherwise, 24-bit full-color format is emitted. 292 293 -gif Select GIF output format. Since GIF does not support 294 more than 256 colors, -colors 256 is assumed (unless 295 you specify a smaller number of colors). If you 296 specify -fast, the default number of colors is 216. 297 298 -os2 Select BMP output format (OS/2 1.x flavor). 8-bit 299 colormapped format is emitted if -colors or -grayscale 300 is specified, or if the JPEG file is grayscale; 301 otherwise, 24-bit full-color format is emitted. 302 303 -pnm Select PBMPLUS (PPM/PGM) output format (this is the 304 default format). PGM is emitted if the JPEG file is 305 grayscale or if -grayscale is specified; otherwise 306 PPM is emitted. 307 308 -rle Select RLE output format. (Requires URT library.) 309 310 -targa Select Targa output format. Grayscale format is 311 emitted if the JPEG file is grayscale or if 312 -grayscale is specified; otherwise, colormapped format 313 is emitted if -colors is specified; otherwise, 24-bit 314 full-color format is emitted. 315 316Switches for advanced users: 317 318 -dct int Use integer DCT method (default). 319 -dct fast Use fast integer DCT (less accurate). 320 In libjpeg-turbo, the fast method is generally about 321 5-15% faster than the int method when using the 322 x86/x86-64 SIMD extensions (results may vary with other 323 SIMD implementations, or when using libjpeg-turbo 324 without SIMD extensions.) If the JPEG image was 325 compressed using a quality level of 85 or below, then 326 there should be little or no perceptible difference 327 between the two algorithms. When decompressing images 328 that were compressed using quality levels above 85, 329 however, the difference between the fast and int 330 methods becomes more pronounced. With images 331 compressed using quality=97, for instance, the fast 332 method incurs generally about a 4-6 dB loss (in PSNR) 333 relative to the int method, but this can be larger for 334 some images. If you can avoid it, do not use the fast 335 method when decompressing images that were compressed 336 using quality levels above 97. The algorithm often 337 degenerates for such images and can actually produce 338 a more lossy output image than if the JPEG image had 339 been compressed using lower quality levels. 340 -dct float Use floating-point DCT method. 341 The float method is mainly a legacy feature. It does 342 not produce significantly more accurate results than 343 the int method, and it is much slower. The float 344 method may also give different results on different 345 machines due to varying roundoff behavior, whereas the 346 integer methods should give the same results on all 347 machines. 348 349 -dither fs Use Floyd-Steinberg dithering in color quantization. 350 -dither ordered Use ordered dithering in color quantization. 351 -dither none Do not use dithering in color quantization. 352 By default, Floyd-Steinberg dithering is applied when 353 quantizing colors; this is slow but usually produces 354 the best results. Ordered dither is a compromise 355 between speed and quality; no dithering is fast but 356 usually looks awful. Note that these switches have 357 no effect unless color quantization is being done. 358 Ordered dither is only available in -onepass mode. 359 360 -map FILE Quantize to the colors used in the specified image 361 file. This is useful for producing multiple files 362 with identical color maps, or for forcing a predefined 363 set of colors to be used. The FILE must be a GIF 364 or PPM file. This option overrides -colors and 365 -onepass. 366 367 -nosmooth Use a faster, lower-quality upsampling routine. 368 369 -onepass Use one-pass instead of two-pass color quantization. 370 The one-pass method is faster and needs less memory, 371 but it produces a lower-quality image. -onepass is 372 ignored unless you also say -colors N. Also, 373 the one-pass method is always used for grayscale 374 output (the two-pass method is no improvement then). 375 376 -maxmemory N Set limit for amount of memory to use in processing 377 large images. Value is in thousands of bytes, or 378 millions of bytes if "M" is attached to the number. 379 For example, -max 4m selects 4000000 bytes. If more 380 space is needed, an error will occur. 381 382 -verbose Enable debug printout. More -v's give more printout. 383 or -debug Also, version information is printed at startup. 384 385 386HINTS FOR CJPEG 387 388Color GIF files are not the ideal input for JPEG; JPEG is really intended for 389compressing full-color (24-bit) images. In particular, don't try to convert 390cartoons, line drawings, and other images that have only a few distinct 391colors. GIF works great on these, JPEG does not. If you want to convert a 392GIF to JPEG, you should experiment with cjpeg's -quality and -smooth options 393to get a satisfactory conversion. -smooth 10 or so is often helpful. 394 395Avoid running an image through a series of JPEG compression/decompression 396cycles. Image quality loss will accumulate; after ten or so cycles the image 397may be noticeably worse than it was after one cycle. It's best to use a 398lossless format while manipulating an image, then convert to JPEG format when 399you are ready to file the image away. 400 401The -optimize option to cjpeg is worth using when you are making a "final" 402version for posting or archiving. It's also a win when you are using low 403quality settings to make very small JPEG files; the percentage improvement 404is often a lot more than it is on larger files. (At present, -optimize 405mode is always selected when generating progressive JPEG files.) 406 407Support for GIF input files was removed in cjpeg v6b due to concerns over 408the Unisys LZW patent. Although this patent expired in 2006, cjpeg still 409lacks GIF support, for these historical reasons. (Conversion of GIF files to 410JPEG is usually a bad idea anyway.) 411 412 413HINTS FOR DJPEG 414 415To get a quick preview of an image, use the -grayscale and/or -scale switches. 416"-grayscale -scale 1/8" is the fastest case. 417 418Several options are available that trade off image quality to gain speed. 419"-fast" turns on the recommended settings. 420 421"-dct fast" and/or "-nosmooth" gain speed at a small sacrifice in quality. 422When producing a color-quantized image, "-onepass -dither ordered" is fast but 423much lower quality than the default behavior. "-dither none" may give 424acceptable results in two-pass mode, but is seldom tolerable in one-pass mode. 425 426To avoid the Unisys LZW patent (now expired), djpeg produces uncompressed GIF 427files. These are larger than they should be, but are readable by standard GIF 428decoders. 429 430 431HINTS FOR BOTH PROGRAMS 432 433If the memory needed by cjpeg or djpeg exceeds the limit specified by 434-maxmemory, an error will occur. You can leave out -progressive and -optimize 435(for cjpeg) or specify -onepass (for djpeg) to reduce memory usage. 436 437On machines that have "environment" variables, you can define the environment 438variable JPEGMEM to set the default memory limit. The value is specified as 439described for the -maxmemory switch. JPEGMEM overrides the default value 440specified when the program was compiled, and itself is overridden by an 441explicit -maxmemory switch. 442 443 444JPEGTRAN 445 446jpegtran performs various useful transformations of JPEG files. 447It can translate the coded representation from one variant of JPEG to another, 448for example from baseline JPEG to progressive JPEG or vice versa. It can also 449perform some rearrangements of the image data, for example turning an image 450from landscape to portrait format by rotation. For EXIF files and JPEG files 451containing Exif data, you may prefer to use exiftran instead. 452 453jpegtran works by rearranging the compressed data (DCT coefficients), without 454ever fully decoding the image. Therefore, its transformations are lossless: 455there is no image degradation at all, which would not be true if you used 456djpeg followed by cjpeg to accomplish the same conversion. But by the same 457token, jpegtran cannot perform lossy operations such as changing the image 458quality. However, while the image data is losslessly transformed, metadata 459can be removed. See the -copy option for specifics. 460 461jpegtran uses a command line syntax similar to cjpeg or djpeg. 462On Unix-like systems, you say: 463 jpegtran [switches] [inputfile] >outputfile 464On most non-Unix systems, you say: 465 jpegtran [switches] inputfile outputfile 466where both the input and output files are JPEG files. 467 468To specify the coded JPEG representation used in the output file, 469jpegtran accepts a subset of the switches recognized by cjpeg: 470 -optimize Perform optimization of entropy encoding parameters. 471 -progressive Create progressive JPEG file. 472 -arithmetic Use arithmetic coding. 473 -restart N Emit a JPEG restart marker every N MCU rows, or every 474 N MCU blocks if "B" is attached to the number. 475 -scans file Use the scan script given in the specified text file. 476See the previous discussion of cjpeg for more details about these switches. 477If you specify none of these switches, you get a plain baseline-JPEG output 478file. The quality setting and so forth are determined by the input file. 479 480The image can be losslessly transformed by giving one of these switches: 481 -flip horizontal Mirror image horizontally (left-right). 482 -flip vertical Mirror image vertically (top-bottom). 483 -rotate 90 Rotate image 90 degrees clockwise. 484 -rotate 180 Rotate image 180 degrees. 485 -rotate 270 Rotate image 270 degrees clockwise (or 90 ccw). 486 -transpose Transpose image (across UL-to-LR axis). 487 -transverse Transverse transpose (across UR-to-LL axis). 488 489The transpose transformation has no restrictions regarding image dimensions. 490The other transformations operate rather oddly if the image dimensions are not 491a multiple of the iMCU size (usually 8 or 16 pixels), because they can only 492transform complete blocks of DCT coefficient data in the desired way. 493 494jpegtran's default behavior when transforming an odd-size image is designed 495to preserve exact reversibility and mathematical consistency of the 496transformation set. As stated, transpose is able to flip the entire image 497area. Horizontal mirroring leaves any partial iMCU column at the right edge 498untouched, but is able to flip all rows of the image. Similarly, vertical 499mirroring leaves any partial iMCU row at the bottom edge untouched, but is 500able to flip all columns. The other transforms can be built up as sequences 501of transpose and flip operations; for consistency, their actions on edge 502pixels are defined to be the same as the end result of the corresponding 503transpose-and-flip sequence. 504 505For practical use, you may prefer to discard any untransformable edge pixels 506rather than having a strange-looking strip along the right and/or bottom edges 507of a transformed image. To do this, add the -trim switch: 508 -trim Drop non-transformable edge blocks. 509Obviously, a transformation with -trim is not reversible, so strictly speaking 510jpegtran with this switch is not lossless. Also, the expected mathematical 511equivalences between the transformations no longer hold. For example, 512"-rot 270 -trim" trims only the bottom edge, but "-rot 90 -trim" followed by 513"-rot 180 -trim" trims both edges. 514 515If you are only interested in perfect transformations, add the -perfect switch: 516 -perfect Fail with an error if the transformation is not 517 perfect. 518For example, you may want to do 519 jpegtran -rot 90 -perfect foo.jpg || djpeg foo.jpg | pnmflip -r90 | cjpeg 520to do a perfect rotation, if available, or an approximated one if not. 521 522This version of jpegtran also offers a lossless crop option, which discards 523data outside of a given image region but losslessly preserves what is inside. 524Like the rotate and flip transforms, lossless crop is restricted by the current 525JPEG format; the upper left corner of the selected region must fall on an iMCU 526boundary. If it doesn't, then it is silently moved up and/or left to the 527nearest iMCU boundary (the lower right corner is unchanged.) Thus, the output 528image covers at least the requested region, but it may cover more. The 529adjustment of the region dimensions may be optionally disabled by attaching an 530'f' character ("force") to the width or height number. 531 532The image can be losslessly cropped by giving the switch: 533 -crop WxH+X+Y Crop to a rectangular region of width W and height H, 534 starting at point X,Y. 535 536Other not-strictly-lossless transformation switches are: 537 538 -grayscale Force grayscale output. 539This option discards the chrominance channels if the input image is YCbCr 540(ie, a standard color JPEG), resulting in a grayscale JPEG file. The 541luminance channel is preserved exactly, so this is a better method of reducing 542to grayscale than decompression, conversion, and recompression. This switch 543is particularly handy for fixing a monochrome picture that was mistakenly 544encoded as a color JPEG. (In such a case, the space savings from getting rid 545of the near-empty chroma channels won't be large; but the decoding time for 546a grayscale JPEG is substantially less than that for a color JPEG.) 547 548jpegtran also recognizes these switches that control what to do with "extra" 549markers, such as comment blocks: 550 -copy none Copy no extra markers from source file. This setting 551 suppresses all comments and other metadata in the 552 source file. 553 -copy comments Copy only comment markers. This setting copies 554 comments from the source file but discards any other 555 metadata. 556 -copy all Copy all extra markers. This setting preserves 557 miscellaneous markers found in the source file, such 558 as JFIF thumbnails, Exif data, and Photoshop settings. 559 In some files, these extra markers can be sizable. 560 Note that this option will copy thumbnails as-is; 561 they will not be transformed. 562The default behavior is -copy comments. (Note: in IJG releases v6 and v6a, 563jpegtran always did the equivalent of -copy none.) 564 565Additional switches recognized by jpegtran are: 566 -outfile filename 567 -maxmemory N 568 -verbose 569 -debug 570These work the same as in cjpeg or djpeg. 571 572 573THE COMMENT UTILITIES 574 575The JPEG standard allows "comment" (COM) blocks to occur within a JPEG file. 576Although the standard doesn't actually define what COM blocks are for, they 577are widely used to hold user-supplied text strings. This lets you add 578annotations, titles, index terms, etc to your JPEG files, and later retrieve 579them as text. COM blocks do not interfere with the image stored in the JPEG 580file. The maximum size of a COM block is 64K, but you can have as many of 581them as you like in one JPEG file. 582 583We provide two utility programs to display COM block contents and add COM 584blocks to a JPEG file. 585 586rdjpgcom searches a JPEG file and prints the contents of any COM blocks on 587standard output. The command line syntax is 588 rdjpgcom [-raw] [-verbose] [inputfilename] 589The switch "-raw" (or just "-r") causes rdjpgcom to output non-printable 590characters in JPEG comments. These characters are normally escaped for 591security reasons. 592The switch "-verbose" (or just "-v") causes rdjpgcom to also display the JPEG 593image dimensions. If you omit the input file name from the command line, 594the JPEG file is read from standard input. (This may not work on some 595operating systems, if binary data can't be read from stdin.) 596 597wrjpgcom adds a COM block, containing text you provide, to a JPEG file. 598Ordinarily, the COM block is added after any existing COM blocks, but you 599can delete the old COM blocks if you wish. wrjpgcom produces a new JPEG 600file; it does not modify the input file. DO NOT try to overwrite the input 601file by directing wrjpgcom's output back into it; on most systems this will 602just destroy your file. 603 604The command line syntax for wrjpgcom is similar to cjpeg's. On Unix-like 605systems, it is 606 wrjpgcom [switches] [inputfilename] 607The output file is written to standard output. The input file comes from 608the named file, or from standard input if no input file is named. 609 610On most non-Unix systems, the syntax is 611 wrjpgcom [switches] inputfilename outputfilename 612where both input and output file names must be given explicitly. 613 614wrjpgcom understands three switches: 615 -replace Delete any existing COM blocks from the file. 616 -comment "Comment text" Supply new COM text on command line. 617 -cfile name Read text for new COM block from named file. 618(Switch names can be abbreviated.) If you have only one line of comment text 619to add, you can provide it on the command line with -comment. The comment 620text must be surrounded with quotes so that it is treated as a single 621argument. Longer comments can be read from a text file. 622 623If you give neither -comment nor -cfile, then wrjpgcom will read the comment 624text from standard input. (In this case an input image file name MUST be 625supplied, so that the source JPEG file comes from somewhere else.) You can 626enter multiple lines, up to 64KB worth. Type an end-of-file indicator 627(usually control-D or control-Z) to terminate the comment text entry. 628 629wrjpgcom will not add a COM block if the provided comment string is empty. 630Therefore -replace -comment "" can be used to delete all COM blocks from a 631file. 632 633These utility programs do not depend on the IJG JPEG library. In 634particular, the source code for rdjpgcom is intended as an illustration of 635the minimum amount of code required to parse a JPEG file header correctly. 636
