1libjpeg-turbo note: This file has been modified by The libjpeg-turbo Project 2to include only information relevant to libjpeg-turbo, to wordsmith certain 3sections, and to remove impolitic language that existed in the libjpeg v8 4README. It is included only for reference. Please see README.md for 5information specific to libjpeg-turbo. 6 7 8The Independent JPEG Group's JPEG software 9========================================== 10 11This distribution contains a release of the Independent JPEG Group's free JPEG 12software. You are welcome to redistribute this software and to use it for any 13purpose, subject to the conditions under LEGAL ISSUES, below. 14 15This software is the work of Tom Lane, Guido Vollbeding, Philip Gladstone, 16Bill Allombert, Jim Boucher, Lee Crocker, Bob Friesenhahn, Ben Jackson, 17Julian Minguillon, Luis Ortiz, George Phillips, Davide Rossi, Ge' Weijers, 18and other members of the Independent JPEG Group. 19 20IJG is not affiliated with the ISO/IEC JTC1/SC29/WG1 standards committee 21(also known as JPEG, together with ITU-T SG16). 22 23 24DOCUMENTATION ROADMAP 25===================== 26 27This file contains the following sections: 28 29OVERVIEW General description of JPEG and the IJG software. 30LEGAL ISSUES Copyright, lack of warranty, terms of distribution. 31REFERENCES Where to learn more about JPEG. 32ARCHIVE LOCATIONS Where to find newer versions of this software. 33FILE FORMAT WARS Software *not* to get. 34TO DO Plans for future IJG releases. 35 36Other documentation files in the distribution are: 37 38User documentation: 39 usage.txt Usage instructions for cjpeg, djpeg, jpegtran, 40 rdjpgcom, and wrjpgcom. 41 *.1 Unix-style man pages for programs (same info as usage.txt). 42 wizard.txt Advanced usage instructions for JPEG wizards only. 43 change.log Version-to-version change highlights. 44Programmer and internal documentation: 45 libjpeg.txt How to use the JPEG library in your own programs. 46 example.txt Sample code for calling the JPEG library. 47 structure.txt Overview of the JPEG library's internal structure. 48 coderules.txt Coding style rules --- please read if you contribute code. 49 50Please read at least usage.txt. Some information can also be found in the JPEG 51FAQ (Frequently Asked Questions) article. See ARCHIVE LOCATIONS below to find 52out where to obtain the FAQ article. 53 54If you want to understand how the JPEG code works, we suggest reading one or 55more of the REFERENCES, then looking at the documentation files (in roughly 56the order listed) before diving into the code. 57 58 59OVERVIEW 60======== 61 62This package contains C software to implement JPEG image encoding, decoding, 63and transcoding. JPEG (pronounced "jay-peg") is a standardized compression 64method for full-color and grayscale images. JPEG's strong suit is compressing 65photographic images or other types of images that have smooth color and 66brightness transitions between neighboring pixels. Images with sharp lines or 67other abrupt features may not compress well with JPEG, and a higher JPEG 68quality may have to be used to avoid visible compression artifacts with such 69images. 70 71JPEG is lossy, meaning that the output pixels are not necessarily identical to 72the input pixels. However, on photographic content and other "smooth" images, 73very good compression ratios can be obtained with no visible compression 74artifacts, and extremely high compression ratios are possible if you are 75willing to sacrifice image quality (by reducing the "quality" setting in the 76compressor.) 77 78This software implements JPEG baseline, extended-sequential, and progressive 79compression processes. Provision is made for supporting all variants of these 80processes, although some uncommon parameter settings aren't implemented yet. 81We have made no provision for supporting the hierarchical or lossless 82processes defined in the standard. 83 84We provide a set of library routines for reading and writing JPEG image files, 85plus two sample applications "cjpeg" and "djpeg", which use the library to 86perform conversion between JPEG and some other popular image file formats. 87The library is intended to be reused in other applications. 88 89In order to support file conversion and viewing software, we have included 90considerable functionality beyond the bare JPEG coding/decoding capability; 91for example, the color quantization modules are not strictly part of JPEG 92decoding, but they are essential for output to colormapped file formats or 93colormapped displays. These extra functions can be compiled out of the 94library if not required for a particular application. 95 96We have also included "jpegtran", a utility for lossless transcoding between 97different JPEG processes, and "rdjpgcom" and "wrjpgcom", two simple 98applications for inserting and extracting textual comments in JFIF files. 99 100The emphasis in designing this software has been on achieving portability and 101flexibility, while also making it fast enough to be useful. In particular, 102the software is not intended to be read as a tutorial on JPEG. (See the 103REFERENCES section for introductory material.) Rather, it is intended to 104be reliable, portable, industrial-strength code. We do not claim to have 105achieved that goal in every aspect of the software, but we strive for it. 106 107We welcome the use of this software as a component of commercial products. 108No royalty is required, but we do ask for an acknowledgement in product 109documentation, as described under LEGAL ISSUES. 110 111 112LEGAL ISSUES 113============ 114 115In plain English: 116 1171. We don't promise that this software works. (But if you find any bugs, 118 please let us know!) 1192. You can use this software for whatever you want. You don't have to pay us. 1203. You may not pretend that you wrote this software. If you use it in a 121 program, you must acknowledge somewhere in your documentation that 122 you've used the IJG code. 123 124In legalese: 125 126The authors make NO WARRANTY or representation, either express or implied, 127with respect to this software, its quality, accuracy, merchantability, or 128fitness for a particular purpose. This software is provided "AS IS", and you, 129its user, assume the entire risk as to its quality and accuracy. 130 131This software is copyright (C) 1991-2020, Thomas G. Lane, Guido Vollbeding. 132All Rights Reserved except as specified below. 133 134Permission is hereby granted to use, copy, modify, and distribute this 135software (or portions thereof) for any purpose, without fee, subject to these 136conditions: 137(1) If any part of the source code for this software is distributed, then this 138README file must be included, with this copyright and no-warranty notice 139unaltered; and any additions, deletions, or changes to the original files 140must be clearly indicated in accompanying documentation. 141(2) If only executable code is distributed, then the accompanying 142documentation must state that "this software is based in part on the work of 143the Independent JPEG Group". 144(3) Permission for use of this software is granted only if the user accepts 145full responsibility for any undesirable consequences; the authors accept 146NO LIABILITY for damages of any kind. 147 148These conditions apply to any software derived from or based on the IJG code, 149not just to the unmodified library. If you use our work, you ought to 150acknowledge us. 151 152Permission is NOT granted for the use of any IJG author's name or company name 153in advertising or publicity relating to this software or products derived from 154it. This software may be referred to only as "the Independent JPEG Group's 155software". 156 157We specifically permit and encourage the use of this software as the basis of 158commercial products, provided that all warranty or liability claims are 159assumed by the product vendor. 160 161 162REFERENCES 163========== 164 165We recommend reading one or more of these references before trying to 166understand the innards of the JPEG software. 167 168The best short technical introduction to the JPEG compression algorithm is 169 Wallace, Gregory K. "The JPEG Still Picture Compression Standard", 170 Communications of the ACM, April 1991 (vol. 34 no. 4), pp. 30-44. 171(Adjacent articles in that issue discuss MPEG motion picture compression, 172applications of JPEG, and related topics.) If you don't have the CACM issue 173handy, a PDF file containing a revised version of Wallace's article is 174available at http://www.ijg.org/files/Wallace.JPEG.pdf. The file (actually 175a preprint for an article that appeared in IEEE Trans. Consumer Electronics) 176omits the sample images that appeared in CACM, but it includes corrections 177and some added material. Note: the Wallace article is copyright ACM and IEEE, 178and it may not be used for commercial purposes. 179 180A somewhat less technical, more leisurely introduction to JPEG can be found in 181"The Data Compression Book" by Mark Nelson and Jean-loup Gailly, published by 182M&T Books (New York), 2nd ed. 1996, ISBN 1-55851-434-1. This book provides 183good explanations and example C code for a multitude of compression methods 184including JPEG. It is an excellent source if you are comfortable reading C 185code but don't know much about data compression in general. The book's JPEG 186sample code is far from industrial-strength, but when you are ready to look 187at a full implementation, you've got one here... 188 189The best currently available description of JPEG is the textbook "JPEG Still 190Image Data Compression Standard" by William B. Pennebaker and Joan L. 191Mitchell, published by Van Nostrand Reinhold, 1993, ISBN 0-442-01272-1. 192Price US$59.95, 638 pp. The book includes the complete text of the ISO JPEG 193standards (DIS 10918-1 and draft DIS 10918-2). 194 195The original JPEG standard is divided into two parts, Part 1 being the actual 196specification, while Part 2 covers compliance testing methods. Part 1 is 197titled "Digital Compression and Coding of Continuous-tone Still Images, 198Part 1: Requirements and guidelines" and has document numbers ISO/IEC IS 19910918-1, ITU-T T.81. Part 2 is titled "Digital Compression and Coding of 200Continuous-tone Still Images, Part 2: Compliance testing" and has document 201numbers ISO/IEC IS 10918-2, ITU-T T.83. 202 203The JPEG standard does not specify all details of an interchangeable file 204format. For the omitted details, we follow the "JFIF" conventions, revision 2051.02. JFIF version 1 has been adopted as ISO/IEC 10918-5 (05/2013) and 206Recommendation ITU-T T.871 (05/2011): Information technology - Digital 207compression and coding of continuous-tone still images: JPEG File Interchange 208Format (JFIF). It is available as a free download in PDF file format from 209https://www.iso.org/standard/54989.html and http://www.itu.int/rec/T-REC-T.871. 210A PDF file of the older JFIF 1.02 specification is available at 211http://www.w3.org/Graphics/JPEG/jfif3.pdf. 212 213The TIFF 6.0 file format specification can be obtained from 214http://mirrors.ctan.org/graphics/tiff/TIFF6.ps.gz. The JPEG incorporation 215scheme found in the TIFF 6.0 spec of 3-June-92 has a number of serious 216problems. IJG does not recommend use of the TIFF 6.0 design (TIFF Compression 217tag 6). Instead, we recommend the JPEG design proposed by TIFF Technical Note 218#2 (Compression tag 7). Copies of this Note can be obtained from 219http://www.ijg.org/files/. It is expected that the next revision 220of the TIFF spec will replace the 6.0 JPEG design with the Note's design. 221Although IJG's own code does not support TIFF/JPEG, the free libtiff library 222uses our library to implement TIFF/JPEG per the Note. 223 224 225ARCHIVE LOCATIONS 226================= 227 228The "official" archive site for this software is www.ijg.org. 229The most recent released version can always be found there in 230directory "files". 231 232The JPEG FAQ (Frequently Asked Questions) article is a source of some 233general information about JPEG. It is available at 234http://www.faqs.org/faqs/jpeg-faq. 235 236 237FILE FORMAT COMPATIBILITY 238========================= 239 240This software implements ITU T.81 | ISO/IEC 10918 with some extensions from 241ITU T.871 | ISO/IEC 10918-5 (JPEG File Interchange Format-- see REFERENCES). 242Informally, the term "JPEG image" or "JPEG file" most often refers to JFIF or 243a subset thereof, but there are other formats containing the name "JPEG" that 244are incompatible with the DCT-based JPEG standard or with JFIF (for instance, 245JPEG 2000 and JPEG XR). This software therefore does not support these 246formats. Indeed, one of the original reasons for developing this free software 247was to help force convergence on a common, interoperable format standard for 248JPEG files. 249 250JFIF is a minimal or "low end" representation. TIFF/JPEG (TIFF revision 6.0 as 251modified by TIFF Technical Note #2) can be used for "high end" applications 252that need to record a lot of additional data about an image. 253 254 255TO DO 256===== 257 258Please send bug reports, offers of help, etc. to jpeg-info@jpegclub.org. 259
