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.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 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 IJG distribution formerly included code to read and write GIF files. 163 To avoid entanglement with the Unisys LZW patent (now expired), GIF reading 164 support has been removed altogether, and the GIF writer has been simplified 165 to produce "uncompressed GIFs". This technique does not use the LZW 166 algorithm; the resulting GIF files are larger than usual, but are readable 167 by all standard GIF decoders. 168 169 We are required to state that 170 "The Graphics Interchange Format(c) is the Copyright property of 171 CompuServe Incorporated. GIF(sm) is a Service Mark property of 172 CompuServe Incorporated." 173 174 175 REFERENCES 176 ========== 177 178 We recommend reading one or more of these references before trying to 179 understand the innards of the JPEG software. 180 181 The best short technical introduction to the JPEG compression algorithm is 182 Wallace, Gregory K. "The JPEG Still Picture Compression Standard", 183 Communications of the ACM, April 1991 (vol. 34 no. 4), pp. 30-44. 184 (Adjacent articles in that issue discuss MPEG motion picture compression, 185 applications of JPEG, and related topics.) If you don't have the CACM issue 186 handy, a PDF file containing a revised version of Wallace's article is 187 available at http://www.ijg.org/files/Wallace.JPEG.pdf. The file (actually 188 a preprint for an article that appeared in IEEE Trans. Consumer Electronics) 189 omits the sample images that appeared in CACM, but it includes corrections 190 and some added material. Note: the Wallace article is copyright ACM and IEEE, 191 and it may not be used for commercial purposes. 192 193 A somewhat less technical, more leisurely introduction to JPEG can be found in 194 "The Data Compression Book" by Mark Nelson and Jean-loup Gailly, published by 195 M&T Books (New York), 2nd ed. 1996, ISBN 1-55851-434-1. This book provides 196 good explanations and example C code for a multitude of compression methods 197 including JPEG. It is an excellent source if you are comfortable reading C 198 code but don't know much about data compression in general. The book's JPEG 199 sample code is far from industrial-strength, but when you are ready to look 200 at a full implementation, you've got one here... 201 202 The best currently available description of JPEG is the textbook "JPEG Still 203 Image Data Compression Standard" by William B. Pennebaker and Joan L. 204 Mitchell, published by Van Nostrand Reinhold, 1993, ISBN 0-442-01272-1. 205 Price US$59.95, 638 pp. The book includes the complete text of the ISO JPEG 206 standards (DIS 10918-1 and draft DIS 10918-2). 207 208 The original JPEG standard is divided into two parts, Part 1 being the actual 209 specification, while Part 2 covers compliance testing methods. Part 1 is 210 titled "Digital Compression and Coding of Continuous-tone Still Images, 211 Part 1: Requirements and guidelines" and has document numbers ISO/IEC IS 212 10918-1, ITU-T T.81. Part 2 is titled "Digital Compression and Coding of 213 Continuous-tone Still Images, Part 2: Compliance testing" and has document 214 numbers ISO/IEC IS 10918-2, ITU-T T.83. 215 216 The JPEG standard does not specify all details of an interchangeable file 217 format. For the omitted details, we follow the "JFIF" conventions, revision 218 1.02. JFIF version 1 has been adopted as ISO/IEC 10918-5 (05/2013) and 219 Recommendation ITU-T T.871 (05/2011): Information technology - Digital 220 compression and coding of continuous-tone still images: JPEG File Interchange 221 Format (JFIF). It is available as a free download in PDF file format from 222 https://www.iso.org/standard/54989.html and http://www.itu.int/rec/T-REC-T.871. 223 A PDF file of the older JFIF 1.02 specification is available at 224 http://www.w3.org/Graphics/JPEG/jfif3.pdf. 225 226 The TIFF 6.0 file format specification can be obtained by FTP from 227 ftp://ftp.sgi.com/graphics/tiff/TIFF6.ps.gz. The JPEG incorporation scheme 228 found in the TIFF 6.0 spec of 3-June-92 has a number of serious problems. 229 IJG does not recommend use of the TIFF 6.0 design (TIFF Compression tag 6). 230 Instead, we recommend the JPEG design proposed by TIFF Technical Note #2 231 (Compression tag 7). Copies of this Note can be obtained from 232 http://www.ijg.org/files/. It is expected that the next revision 233 of the TIFF spec will replace the 6.0 JPEG design with the Note's design. 234 Although IJG's own code does not support TIFF/JPEG, the free libtiff library 235 uses our library to implement TIFF/JPEG per the Note. 236 237 238 ARCHIVE LOCATIONS 239 ================= 240 241 The "official" archive site for this software is www.ijg.org. 242 The most recent released version can always be found there in 243 directory "files". 244 245 The JPEG FAQ (Frequently Asked Questions) article is a source of some 246 general information about JPEG. 247 It is available on the World Wide Web at http://www.faqs.org/faqs/jpeg-faq/ 248 and other news.answers archive sites, including the official news.answers 249 archive at rtfm.mit.edu: ftp://rtfm.mit.edu/pub/usenet/news.answers/jpeg-faq/. 250 If you don't have Web or FTP access, send e-mail to mail-server (a] rtfm.mit.edu 251 with body 252 send usenet/news.answers/jpeg-faq/part1 253 send usenet/news.answers/jpeg-faq/part2 254 255 256 FILE FORMAT COMPATIBILITY 257 ========================= 258 259 This software implements ITU T.81 | ISO/IEC 10918 with some extensions from 260 ITU T.871 | ISO/IEC 10918-5 (JPEG File Interchange Format-- see REFERENCES). 261 Informally, the term "JPEG image" or "JPEG file" most often refers to JFIF or 262 a subset thereof, but there are other formats containing the name "JPEG" that 263 are incompatible with the DCT-based JPEG standard or with JFIF (for instance, 264 JPEG 2000 and JPEG XR). This software therefore does not support these 265 formats. Indeed, one of the original reasons for developing this free software 266 was to help force convergence on a common, interoperable format standard for 267 JPEG files. 268 269 JFIF is a minimal or "low end" representation. TIFF/JPEG (TIFF revision 6.0 as 270 modified by TIFF Technical Note #2) can be used for "high end" applications 271 that need to record a lot of additional data about an image. 272 273 274 TO DO 275 ===== 276 277 Please send bug reports, offers of help, etc. to jpeg-info (a] jpegclub.org. 278