1 libjpeg-turbo note: This file has been modified by The libjpeg-turbo Project 2 to include only information relevant to libjpeg-turbo, to wordsmith certain 3 sections, and to remove impolitic language that existed in the libjpeg v8 4 README. It is included only for reference. Please see README.md for 5 information specific to libjpeg-turbo. 6 7 8 The Independent JPEG Group's JPEG software 9 ========================================== 10 11 This distribution contains a release of the Independent JPEG Group's free JPEG 12 software. You are welcome to redistribute this software and to use it for any 13 purpose, subject to the conditions under LEGAL ISSUES, below. 14 15 This software is the work of Tom Lane, Guido Vollbeding, Philip Gladstone, 16 Bill Allombert, Jim Boucher, Lee Crocker, Bob Friesenhahn, Ben Jackson, 17 Julian Minguillon, Luis Ortiz, George Phillips, Davide Rossi, Ge' Weijers, 18 and other members of the Independent JPEG Group. 19 20 IJG is not affiliated with the ISO/IEC JTC1/SC29/WG1 standards committee 21 (also known as JPEG, together with ITU-T SG16). 22 23 24 DOCUMENTATION ROADMAP 25 ===================== 26 27 This file contains the following sections: 28 29 OVERVIEW General description of JPEG and the IJG software. 30 LEGAL ISSUES Copyright, lack of warranty, terms of distribution. 31 REFERENCES Where to learn more about JPEG. 32 ARCHIVE LOCATIONS Where to find newer versions of this software. 33 FILE FORMAT WARS Software *not* to get. 34 TO DO Plans for future IJG releases. 35 36 Other documentation files in the distribution are: 37 38 User 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. 44 Programmer and internal documentation: 45 libjpeg.txt How to use the JPEG library in your own programs. 46 example.c 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 50 Please read at least usage.txt. Some information can also be found in the JPEG 51 FAQ (Frequently Asked Questions) article. See ARCHIVE LOCATIONS below to find 52 out where to obtain the FAQ article. 53 54 If you want to understand how the JPEG code works, we suggest reading one or 55 more of the REFERENCES, then looking at the documentation files (in roughly 56 the order listed) before diving into the code. 57 58 59 OVERVIEW 60 ======== 61 62 This package contains C software to implement JPEG image encoding, decoding, 63 and transcoding. JPEG (pronounced "jay-peg") is a standardized compression 64 method for full-color and grayscale images. JPEG's strong suit is compressing 65 photographic images or other types of images that have smooth color and 66 brightness transitions between neighboring pixels. Images with sharp lines or 67 other abrupt features may not compress well with JPEG, and a higher JPEG 68 quality may have to be used to avoid visible compression artifacts with such 69 images. 70 71 JPEG is lossy, meaning that the output pixels are not necessarily identical to 72 the input pixels. However, on photographic content and other "smooth" images, 73 very good compression ratios can be obtained with no visible compression 74 artifacts, and extremely high compression ratios are possible if you are 75 willing to sacrifice image quality (by reducing the "quality" setting in the 76 compressor.) 77 78 This software implements JPEG baseline, extended-sequential, and progressive 79 compression processes. Provision is made for supporting all variants of these 80 processes, although some uncommon parameter settings aren't implemented yet. 81 We have made no provision for supporting the hierarchical or lossless 82 processes defined in the standard. 83 84 We provide a set of library routines for reading and writing JPEG image files, 85 plus two sample applications "cjpeg" and "djpeg", which use the library to 86 perform conversion between JPEG and some other popular image file formats. 87 The library is intended to be reused in other applications. 88 89 In order to support file conversion and viewing software, we have included 90 considerable functionality beyond the bare JPEG coding/decoding capability; 91 for example, the color quantization modules are not strictly part of JPEG 92 decoding, but they are essential for output to colormapped file formats or 93 colormapped displays. These extra functions can be compiled out of the 94 library if not required for a particular application. 95 96 We have also included "jpegtran", a utility for lossless transcoding between 97 different JPEG processes, and "rdjpgcom" and "wrjpgcom", two simple 98 applications for inserting and extracting textual comments in JFIF files. 99 100 The emphasis in designing this software has been on achieving portability and 101 flexibility, while also making it fast enough to be useful. In particular, 102 the software is not intended to be read as a tutorial on JPEG. (See the 103 REFERENCES section for introductory material.) Rather, it is intended to 104 be reliable, portable, industrial-strength code. We do not claim to have 105 achieved that goal in every aspect of the software, but we strive for it. 106 107 We welcome the use of this software as a component of commercial products. 108 No royalty is required, but we do ask for an acknowledgement in product 109 documentation, as described under LEGAL ISSUES. 110 111 112 LEGAL ISSUES 113 ============ 114 115 In plain English: 116 117 1. We don't promise that this software works. (But if you find any bugs, 118 please let us know!) 119 2. You can use this software for whatever you want. You don't have to pay us. 120 3. 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. 123 124 In legalese: 125 126 The authors make NO WARRANTY or representation, either express or implied, 127 with respect to this software, its quality, accuracy, merchantability, or 128 fitness for a particular purpose. This software is provided "AS IS", and you, 129 its user, assume the entire risk as to its quality and accuracy. 130 131 This software is copyright (C) 1991-2016, Thomas G. Lane, Guido Vollbeding. 132 All Rights Reserved except as specified below. 133 134 Permission is hereby granted to use, copy, modify, and distribute this 135 software (or portions thereof) for any purpose, without fee, subject to these 136 conditions: 137 (1) If any part of the source code for this software is distributed, then this 138 README file must be included, with this copyright and no-warranty notice 139 unaltered; and any additions, deletions, or changes to the original files 140 must be clearly indicated in accompanying documentation. 141 (2) If only executable code is distributed, then the accompanying 142 documentation must state that "this software is based in part on the work of 143 the Independent JPEG Group". 144 (3) Permission for use of this software is granted only if the user accepts 145 full responsibility for any undesirable consequences; the authors accept 146 NO LIABILITY for damages of any kind. 147 148 These conditions apply to any software derived from or based on the IJG code, 149 not just to the unmodified library. If you use our work, you ought to 150 acknowledge us. 151 152 Permission is NOT granted for the use of any IJG author's name or company name 153 in advertising or publicity relating to this software or products derived from 154 it. This software may be referred to only as "the Independent JPEG Group's 155 software". 156 157 We specifically permit and encourage the use of this software as the basis of 158 commercial products, provided that all warranty or liability claims are 159 assumed by the product vendor. 160 161 162 The Unix configuration script "configure" was produced with GNU Autoconf. 163 It is copyright by the Free Software Foundation but is freely distributable. 164 The same holds for its supporting scripts (config.guess, config.sub, 165 ltmain.sh). Another support script, install-sh, is copyright by X Consortium 166 but is also freely distributable. 167 168 The IJG distribution formerly included code to read and write GIF files. 169 To avoid entanglement with the Unisys LZW patent (now expired), GIF reading 170 support has been removed altogether, and the GIF writer has been simplified 171 to produce "uncompressed GIFs". This technique does not use the LZW 172 algorithm; the resulting GIF files are larger than usual, but are readable 173 by all standard GIF decoders. 174 175 We are required to state that 176 "The Graphics Interchange Format(c) is the Copyright property of 177 CompuServe Incorporated. GIF(sm) is a Service Mark property of 178 CompuServe Incorporated." 179 180 181 REFERENCES 182 ========== 183 184 We recommend reading one or more of these references before trying to 185 understand the innards of the JPEG software. 186 187 The best short technical introduction to the JPEG compression algorithm is 188 Wallace, Gregory K. "The JPEG Still Picture Compression Standard", 189 Communications of the ACM, April 1991 (vol. 34 no. 4), pp. 30-44. 190 (Adjacent articles in that issue discuss MPEG motion picture compression, 191 applications of JPEG, and related topics.) If you don't have the CACM issue 192 handy, a PDF file containing a revised version of Wallace's article is 193 available at http://www.ijg.org/files/Wallace.JPEG.pdf. The file (actually 194 a preprint for an article that appeared in IEEE Trans. Consumer Electronics) 195 omits the sample images that appeared in CACM, but it includes corrections 196 and some added material. Note: the Wallace article is copyright ACM and IEEE, 197 and it may not be used for commercial purposes. 198 199 A somewhat less technical, more leisurely introduction to JPEG can be found in 200 "The Data Compression Book" by Mark Nelson and Jean-loup Gailly, published by 201 M&T Books (New York), 2nd ed. 1996, ISBN 1-55851-434-1. This book provides 202 good explanations and example C code for a multitude of compression methods 203 including JPEG. It is an excellent source if you are comfortable reading C 204 code but don't know much about data compression in general. The book's JPEG 205 sample code is far from industrial-strength, but when you are ready to look 206 at a full implementation, you've got one here... 207 208 The best currently available description of JPEG is the textbook "JPEG Still 209 Image Data Compression Standard" by William B. Pennebaker and Joan L. 210 Mitchell, published by Van Nostrand Reinhold, 1993, ISBN 0-442-01272-1. 211 Price US$59.95, 638 pp. The book includes the complete text of the ISO JPEG 212 standards (DIS 10918-1 and draft DIS 10918-2). 213 214 The original JPEG standard is divided into two parts, Part 1 being the actual 215 specification, while Part 2 covers compliance testing methods. Part 1 is 216 titled "Digital Compression and Coding of Continuous-tone Still Images, 217 Part 1: Requirements and guidelines" and has document numbers ISO/IEC IS 218 10918-1, ITU-T T.81. Part 2 is titled "Digital Compression and Coding of 219 Continuous-tone Still Images, Part 2: Compliance testing" and has document 220 numbers ISO/IEC IS 10918-2, ITU-T T.83. 221 222 The JPEG standard does not specify all details of an interchangeable file 223 format. For the omitted details we follow the "JFIF" conventions, revision 224 1.02. JFIF 1.02 has been adopted as an Ecma International Technical Report 225 and thus received a formal publication status. It is available as a free 226 download in PDF format from 227 http://www.ecma-international.org/publications/techreports/E-TR-098.htm. 228 A PostScript version of the JFIF document is available at 229 http://www.ijg.org/files/jfif.ps.gz. There is also a plain text version at 230 http://www.ijg.org/files/jfif.txt.gz, but it is missing the figures. 231 232 The TIFF 6.0 file format specification can be obtained by FTP from 233 ftp://ftp.sgi.com/graphics/tiff/TIFF6.ps.gz. The JPEG incorporation scheme 234 found in the TIFF 6.0 spec of 3-June-92 has a number of serious problems. 235 IJG does not recommend use of the TIFF 6.0 design (TIFF Compression tag 6). 236 Instead, we recommend the JPEG design proposed by TIFF Technical Note #2 237 (Compression tag 7). Copies of this Note can be obtained from 238 http://www.ijg.org/files/. It is expected that the next revision 239 of the TIFF spec will replace the 6.0 JPEG design with the Note's design. 240 Although IJG's own code does not support TIFF/JPEG, the free libtiff library 241 uses our library to implement TIFF/JPEG per the Note. 242 243 244 ARCHIVE LOCATIONS 245 ================= 246 247 The "official" archive site for this software is www.ijg.org. 248 The most recent released version can always be found there in 249 directory "files". 250 251 The JPEG FAQ (Frequently Asked Questions) article is a source of some 252 general information about JPEG. 253 It is available on the World Wide Web at http://www.faqs.org/faqs/jpeg-faq/ 254 and other news.answers archive sites, including the official news.answers 255 archive at rtfm.mit.edu: ftp://rtfm.mit.edu/pub/usenet/news.answers/jpeg-faq/. 256 If you don't have Web or FTP access, send e-mail to mail-server (a] rtfm.mit.edu 257 with body 258 send usenet/news.answers/jpeg-faq/part1 259 send usenet/news.answers/jpeg-faq/part2 260 261 262 FILE FORMAT WARS 263 ================ 264 265 The ISO/IEC JTC1/SC29/WG1 standards committee (also known as JPEG, together 266 with ITU-T SG16) currently promotes different formats containing the name 267 "JPEG" which are incompatible with original DCT-based JPEG. IJG therefore does 268 not support these formats (see REFERENCES). Indeed, one of the original 269 reasons for developing this free software was to help force convergence on 270 common, interoperable format standards for JPEG files. 271 Don't use an incompatible file format! 272 (In any case, our decoder will remain capable of reading existing JPEG 273 image files indefinitely.) 274 275 276 TO DO 277 ===== 278 279 Please send bug reports, offers of help, etc. to jpeg-info (a] jpegclub.org. 280
