media/libjpeg/README@b8a032363ba2 (annotated)
media/libjpeg/README
Thu, 22 Jan 2015 13:21:57 +0100
- author
- Michael Schloh von Bennewitz <michael@schloh.com>
- date
- Thu, 22 Jan 2015 13:21:57 +0100
- branch
- TOR_BUG_9701
- changeset 15
- b8a032363ba2
- permissions
- -rw-r--r--
Incorporate requested changes from Mozilla in review:
https://bugzilla.mozilla.org/show_bug.cgi?id=1123480#c6
| michael@0 | 1 | libjpeg-turbo note: This file has been modified by The libjpeg-turbo Project |
| michael@0 | 2 | to include only information relevant to libjpeg-turbo, to wordsmith certain |
| michael@0 | 3 | sections, and to remove impolitic language that existed in the libjpeg v8 |
| michael@0 | 4 | README. It is included only for reference. Please see README-turbo.txt for |
| michael@0 | 5 | information specific to libjpeg-turbo. |
| michael@0 | 6 | |
| michael@0 | 7 | |
| michael@0 | 8 | The Independent JPEG Group's JPEG software |
| michael@0 | 9 | ========================================== |
| michael@0 | 10 | |
| michael@0 | 11 | This distribution contains a release of the Independent JPEG Group's free JPEG |
| michael@0 | 12 | software. You are welcome to redistribute this software and to use it for any |
| michael@0 | 13 | purpose, subject to the conditions under LEGAL ISSUES, below. |
| michael@0 | 14 | |
| michael@0 | 15 | This software is the work of Tom Lane, Guido Vollbeding, Philip Gladstone, |
| michael@0 | 16 | Bill Allombert, Jim Boucher, Lee Crocker, Bob Friesenhahn, Ben Jackson, |
| michael@0 | 17 | Julian Minguillon, Luis Ortiz, George Phillips, Davide Rossi, Ge' Weijers, |
| michael@0 | 18 | and other members of the Independent JPEG Group. |
| michael@0 | 19 | |
| michael@0 | 20 | IJG is not affiliated with the ISO/IEC JTC1/SC29/WG1 standards committee |
| michael@0 | 21 | (also known as JPEG, together with ITU-T SG16). |
| michael@0 | 22 | |
| michael@0 | 23 | |
| michael@0 | 24 | DOCUMENTATION ROADMAP |
| michael@0 | 25 | ===================== |
| michael@0 | 26 | |
| michael@0 | 27 | This file contains the following sections: |
| michael@0 | 28 | |
| michael@0 | 29 | OVERVIEW General description of JPEG and the IJG software. |
| michael@0 | 30 | LEGAL ISSUES Copyright, lack of warranty, terms of distribution. |
| michael@0 | 31 | REFERENCES Where to learn more about JPEG. |
| michael@0 | 32 | ARCHIVE LOCATIONS Where to find newer versions of this software. |
| michael@0 | 33 | FILE FORMAT WARS Software *not* to get. |
| michael@0 | 34 | TO DO Plans for future IJG releases. |
| michael@0 | 35 | |
| michael@0 | 36 | Other documentation files in the distribution are: |
| michael@0 | 37 | |
| michael@0 | 38 | User documentation: |
| michael@0 | 39 | install.txt How to configure and install the IJG software. |
| michael@0 | 40 | usage.txt Usage instructions for cjpeg, djpeg, jpegtran, |
| michael@0 | 41 | rdjpgcom, and wrjpgcom. |
| michael@0 | 42 | *.1 Unix-style man pages for programs (same info as usage.txt). |
| michael@0 | 43 | wizard.txt Advanced usage instructions for JPEG wizards only. |
| michael@0 | 44 | change.log Version-to-version change highlights. |
| michael@0 | 45 | Programmer and internal documentation: |
| michael@0 | 46 | libjpeg.txt How to use the JPEG library in your own programs. |
| michael@0 | 47 | example.c Sample code for calling the JPEG library. |
| michael@0 | 48 | structure.txt Overview of the JPEG library's internal structure. |
| michael@0 | 49 | coderules.txt Coding style rules --- please read if you contribute code. |
| michael@0 | 50 | |
| michael@0 | 51 | Please read at least the files install.txt and usage.txt. Some information |
| michael@0 | 52 | can also be found in the JPEG FAQ (Frequently Asked Questions) article. See |
| michael@0 | 53 | ARCHIVE LOCATIONS below to find out where to obtain the FAQ article. |
| michael@0 | 54 | |
| michael@0 | 55 | If you want to understand how the JPEG code works, we suggest reading one or |
| michael@0 | 56 | more of the REFERENCES, then looking at the documentation files (in roughly |
| michael@0 | 57 | the order listed) before diving into the code. |
| michael@0 | 58 | |
| michael@0 | 59 | |
| michael@0 | 60 | OVERVIEW |
| michael@0 | 61 | ======== |
| michael@0 | 62 | |
| michael@0 | 63 | This package contains C software to implement JPEG image encoding, decoding, |
| michael@0 | 64 | and transcoding. JPEG (pronounced "jay-peg") is a standardized compression |
| michael@0 | 65 | method for full-color and gray-scale images. JPEG's strong suit is compressing |
| michael@0 | 66 | photographic images or other types of images that have smooth color and |
| michael@0 | 67 | brightness transitions between neighboring pixels. Images with sharp lines or |
| michael@0 | 68 | other abrupt features may not compress well with JPEG, and a higher JPEG |
| michael@0 | 69 | quality may have to be used to avoid visible compression artifacts with such |
| michael@0 | 70 | images. |
| michael@0 | 71 | |
| michael@0 | 72 | JPEG is lossy, meaning that the output pixels are not necessarily identical to |
| michael@0 | 73 | the input pixels. However, on photographic content and other "smooth" images, |
| michael@0 | 74 | very good compression ratios can be obtained with no visible compression |
| michael@0 | 75 | artifacts, and extremely high compression ratios are possible if you are |
| michael@0 | 76 | willing to sacrifice image quality (by reducing the "quality" setting in the |
| michael@0 | 77 | compressor.) |
| michael@0 | 78 | |
| michael@0 | 79 | This software implements JPEG baseline, extended-sequential, and progressive |
| michael@0 | 80 | compression processes. Provision is made for supporting all variants of these |
| michael@0 | 81 | processes, although some uncommon parameter settings aren't implemented yet. |
| michael@0 | 82 | We have made no provision for supporting the hierarchical or lossless |
| michael@0 | 83 | processes defined in the standard. |
| michael@0 | 84 | |
| michael@0 | 85 | We provide a set of library routines for reading and writing JPEG image files, |
| michael@0 | 86 | plus two sample applications "cjpeg" and "djpeg", which use the library to |
| michael@0 | 87 | perform conversion between JPEG and some other popular image file formats. |
| michael@0 | 88 | The library is intended to be reused in other applications. |
| michael@0 | 89 | |
| michael@0 | 90 | In order to support file conversion and viewing software, we have included |
| michael@0 | 91 | considerable functionality beyond the bare JPEG coding/decoding capability; |
| michael@0 | 92 | for example, the color quantization modules are not strictly part of JPEG |
| michael@0 | 93 | decoding, but they are essential for output to colormapped file formats or |
| michael@0 | 94 | colormapped displays. These extra functions can be compiled out of the |
| michael@0 | 95 | library if not required for a particular application. |
| michael@0 | 96 | |
| michael@0 | 97 | We have also included "jpegtran", a utility for lossless transcoding between |
| michael@0 | 98 | different JPEG processes, and "rdjpgcom" and "wrjpgcom", two simple |
| michael@0 | 99 | applications for inserting and extracting textual comments in JFIF files. |
| michael@0 | 100 | |
| michael@0 | 101 | The emphasis in designing this software has been on achieving portability and |
| michael@0 | 102 | flexibility, while also making it fast enough to be useful. In particular, |
| michael@0 | 103 | the software is not intended to be read as a tutorial on JPEG. (See the |
| michael@0 | 104 | REFERENCES section for introductory material.) Rather, it is intended to |
| michael@0 | 105 | be reliable, portable, industrial-strength code. We do not claim to have |
| michael@0 | 106 | achieved that goal in every aspect of the software, but we strive for it. |
| michael@0 | 107 | |
| michael@0 | 108 | We welcome the use of this software as a component of commercial products. |
| michael@0 | 109 | No royalty is required, but we do ask for an acknowledgement in product |
| michael@0 | 110 | documentation, as described under LEGAL ISSUES. |
| michael@0 | 111 | |
| michael@0 | 112 | |
| michael@0 | 113 | LEGAL ISSUES |
| michael@0 | 114 | ============ |
| michael@0 | 115 | |
| michael@0 | 116 | In plain English: |
| michael@0 | 117 | |
| michael@0 | 118 | 1. We don't promise that this software works. (But if you find any bugs, |
| michael@0 | 119 | please let us know!) |
| michael@0 | 120 | 2. You can use this software for whatever you want. You don't have to pay us. |
| michael@0 | 121 | 3. You may not pretend that you wrote this software. If you use it in a |
| michael@0 | 122 | program, you must acknowledge somewhere in your documentation that |
| michael@0 | 123 | you've used the IJG code. |
| michael@0 | 124 | |
| michael@0 | 125 | In legalese: |
| michael@0 | 126 | |
| michael@0 | 127 | The authors make NO WARRANTY or representation, either express or implied, |
| michael@0 | 128 | with respect to this software, its quality, accuracy, merchantability, or |
| michael@0 | 129 | fitness for a particular purpose. This software is provided "AS IS", and you, |
| michael@0 | 130 | its user, assume the entire risk as to its quality and accuracy. |
| michael@0 | 131 | |
| michael@0 | 132 | This software is copyright (C) 1991-2012, Thomas G. Lane, Guido Vollbeding. |
| michael@0 | 133 | All Rights Reserved except as specified below. |
| michael@0 | 134 | |
| michael@0 | 135 | Permission is hereby granted to use, copy, modify, and distribute this |
| michael@0 | 136 | software (or portions thereof) for any purpose, without fee, subject to these |
| michael@0 | 137 | conditions: |
| michael@0 | 138 | (1) If any part of the source code for this software is distributed, then this |
| michael@0 | 139 | README file must be included, with this copyright and no-warranty notice |
| michael@0 | 140 | unaltered; and any additions, deletions, or changes to the original files |
| michael@0 | 141 | must be clearly indicated in accompanying documentation. |
| michael@0 | 142 | (2) If only executable code is distributed, then the accompanying |
| michael@0 | 143 | documentation must state that "this software is based in part on the work of |
| michael@0 | 144 | the Independent JPEG Group". |
| michael@0 | 145 | (3) Permission for use of this software is granted only if the user accepts |
| michael@0 | 146 | full responsibility for any undesirable consequences; the authors accept |
| michael@0 | 147 | NO LIABILITY for damages of any kind. |
| michael@0 | 148 | |
| michael@0 | 149 | These conditions apply to any software derived from or based on the IJG code, |
| michael@0 | 150 | not just to the unmodified library. If you use our work, you ought to |
| michael@0 | 151 | acknowledge us. |
| michael@0 | 152 | |
| michael@0 | 153 | Permission is NOT granted for the use of any IJG author's name or company name |
| michael@0 | 154 | in advertising or publicity relating to this software or products derived from |
| michael@0 | 155 | it. This software may be referred to only as "the Independent JPEG Group's |
| michael@0 | 156 | software". |
| michael@0 | 157 | |
| michael@0 | 158 | We specifically permit and encourage the use of this software as the basis of |
| michael@0 | 159 | commercial products, provided that all warranty or liability claims are |
| michael@0 | 160 | assumed by the product vendor. |
| michael@0 | 161 | |
| michael@0 | 162 | |
| michael@0 | 163 | The Unix configuration script "configure" was produced with GNU Autoconf. |
| michael@0 | 164 | It is copyright by the Free Software Foundation but is freely distributable. |
| michael@0 | 165 | The same holds for its supporting scripts (config.guess, config.sub, |
| michael@0 | 166 | ltmain.sh). Another support script, install-sh, is copyright by X Consortium |
| michael@0 | 167 | but is also freely distributable. |
| michael@0 | 168 | |
| michael@0 | 169 | The IJG distribution formerly included code to read and write GIF files. |
| michael@0 | 170 | To avoid entanglement with the Unisys LZW patent, GIF reading support has |
| michael@0 | 171 | been removed altogether, and the GIF writer has been simplified to produce |
| michael@0 | 172 | "uncompressed GIFs". This technique does not use the LZW algorithm; the |
| michael@0 | 173 | resulting GIF files are larger than usual, but are readable by all standard |
| michael@0 | 174 | GIF decoders. |
| michael@0 | 175 | |
| michael@0 | 176 | We are required to state that |
| michael@0 | 177 | "The Graphics Interchange Format(c) is the Copyright property of |
| michael@0 | 178 | CompuServe Incorporated. GIF(sm) is a Service Mark property of |
| michael@0 | 179 | CompuServe Incorporated." |
| michael@0 | 180 | |
| michael@0 | 181 | |
| michael@0 | 182 | REFERENCES |
| michael@0 | 183 | ========== |
| michael@0 | 184 | |
| michael@0 | 185 | We recommend reading one or more of these references before trying to |
| michael@0 | 186 | understand the innards of the JPEG software. |
| michael@0 | 187 | |
| michael@0 | 188 | The best short technical introduction to the JPEG compression algorithm is |
| michael@0 | 189 | Wallace, Gregory K. "The JPEG Still Picture Compression Standard", |
| michael@0 | 190 | Communications of the ACM, April 1991 (vol. 34 no. 4), pp. 30-44. |
| michael@0 | 191 | (Adjacent articles in that issue discuss MPEG motion picture compression, |
| michael@0 | 192 | applications of JPEG, and related topics.) If you don't have the CACM issue |
| michael@0 | 193 | handy, a PostScript file containing a revised version of Wallace's article is |
| michael@0 | 194 | available at http://www.ijg.org/files/wallace.ps.gz. The file (actually |
| michael@0 | 195 | a preprint for an article that appeared in IEEE Trans. Consumer Electronics) |
| michael@0 | 196 | omits the sample images that appeared in CACM, but it includes corrections |
| michael@0 | 197 | and some added material. Note: the Wallace article is copyright ACM and IEEE, |
| michael@0 | 198 | and it may not be used for commercial purposes. |
| michael@0 | 199 | |
| michael@0 | 200 | A somewhat less technical, more leisurely introduction to JPEG can be found in |
| michael@0 | 201 | "The Data Compression Book" by Mark Nelson and Jean-loup Gailly, published by |
| michael@0 | 202 | M&T Books (New York), 2nd ed. 1996, ISBN 1-55851-434-1. This book provides |
| michael@0 | 203 | good explanations and example C code for a multitude of compression methods |
| michael@0 | 204 | including JPEG. It is an excellent source if you are comfortable reading C |
| michael@0 | 205 | code but don't know much about data compression in general. The book's JPEG |
| michael@0 | 206 | sample code is far from industrial-strength, but when you are ready to look |
| michael@0 | 207 | at a full implementation, you've got one here... |
| michael@0 | 208 | |
| michael@0 | 209 | The best currently available description of JPEG is the textbook "JPEG Still |
| michael@0 | 210 | Image Data Compression Standard" by William B. Pennebaker and Joan L. |
| michael@0 | 211 | Mitchell, published by Van Nostrand Reinhold, 1993, ISBN 0-442-01272-1. |
| michael@0 | 212 | Price US$59.95, 638 pp. The book includes the complete text of the ISO JPEG |
| michael@0 | 213 | standards (DIS 10918-1 and draft DIS 10918-2). |
| michael@0 | 214 | |
| michael@0 | 215 | The original JPEG standard is divided into two parts, Part 1 being the actual |
| michael@0 | 216 | specification, while Part 2 covers compliance testing methods. Part 1 is |
| michael@0 | 217 | titled "Digital Compression and Coding of Continuous-tone Still Images, |
| michael@0 | 218 | Part 1: Requirements and guidelines" and has document numbers ISO/IEC IS |
| michael@0 | 219 | 10918-1, ITU-T T.81. Part 2 is titled "Digital Compression and Coding of |
| michael@0 | 220 | Continuous-tone Still Images, Part 2: Compliance testing" and has document |
| michael@0 | 221 | numbers ISO/IEC IS 10918-2, ITU-T T.83. |
| michael@0 | 222 | |
| michael@0 | 223 | The JPEG standard does not specify all details of an interchangeable file |
| michael@0 | 224 | format. For the omitted details we follow the "JFIF" conventions, revision |
| michael@0 | 225 | 1.02. JFIF 1.02 has been adopted as an Ecma International Technical Report |
| michael@0 | 226 | and thus received a formal publication status. It is available as a free |
| michael@0 | 227 | download in PDF format from |
| michael@0 | 228 | http://www.ecma-international.org/publications/techreports/E-TR-098.htm. |
| michael@0 | 229 | A PostScript version of the JFIF document is available at |
| michael@0 | 230 | http://www.ijg.org/files/jfif.ps.gz. There is also a plain text version at |
| michael@0 | 231 | http://www.ijg.org/files/jfif.txt.gz, but it is missing the figures. |
| michael@0 | 232 | |
| michael@0 | 233 | The TIFF 6.0 file format specification can be obtained by FTP from |
| michael@0 | 234 | ftp://ftp.sgi.com/graphics/tiff/TIFF6.ps.gz. The JPEG incorporation scheme |
| michael@0 | 235 | found in the TIFF 6.0 spec of 3-June-92 has a number of serious problems. |
| michael@0 | 236 | IJG does not recommend use of the TIFF 6.0 design (TIFF Compression tag 6). |
| michael@0 | 237 | Instead, we recommend the JPEG design proposed by TIFF Technical Note #2 |
| michael@0 | 238 | (Compression tag 7). Copies of this Note can be obtained from |
| michael@0 | 239 | http://www.ijg.org/files/. It is expected that the next revision |
| michael@0 | 240 | of the TIFF spec will replace the 6.0 JPEG design with the Note's design. |
| michael@0 | 241 | Although IJG's own code does not support TIFF/JPEG, the free libtiff library |
| michael@0 | 242 | uses our library to implement TIFF/JPEG per the Note. |
| michael@0 | 243 | |
| michael@0 | 244 | |
| michael@0 | 245 | ARCHIVE LOCATIONS |
| michael@0 | 246 | ================= |
| michael@0 | 247 | |
| michael@0 | 248 | The "official" archive site for this software is www.ijg.org. |
| michael@0 | 249 | The most recent released version can always be found there in |
| michael@0 | 250 | directory "files". This particular version will be archived as |
| michael@0 | 251 | http://www.ijg.org/files/jpegsrc.v8d.tar.gz, and in Windows-compatible |
| michael@0 | 252 | "zip" archive format as http://www.ijg.org/files/jpegsr8d.zip. |
| michael@0 | 253 | |
| michael@0 | 254 | The JPEG FAQ (Frequently Asked Questions) article is a source of some |
| michael@0 | 255 | general information about JPEG. |
| michael@0 | 256 | It is available on the World Wide Web at http://www.faqs.org/faqs/jpeg-faq/ |
| michael@0 | 257 | and other news.answers archive sites, including the official news.answers |
| michael@0 | 258 | archive at rtfm.mit.edu: ftp://rtfm.mit.edu/pub/usenet/news.answers/jpeg-faq/. |
| michael@0 | 259 | If you don't have Web or FTP access, send e-mail to mail-server@rtfm.mit.edu |
| michael@0 | 260 | with body |
| michael@0 | 261 | send usenet/news.answers/jpeg-faq/part1 |
| michael@0 | 262 | send usenet/news.answers/jpeg-faq/part2 |
| michael@0 | 263 | |
| michael@0 | 264 | |
| michael@0 | 265 | FILE FORMAT WARS |
| michael@0 | 266 | ================ |
| michael@0 | 267 | |
| michael@0 | 268 | The ISO/IEC JTC1/SC29/WG1 standards committee (also known as JPEG, together |
| michael@0 | 269 | with ITU-T SG16) currently promotes different formats containing the name |
| michael@0 | 270 | "JPEG" which are incompatible with original DCT-based JPEG. IJG therefore does |
| michael@0 | 271 | not support these formats (see REFERENCES). Indeed, one of the original |
| michael@0 | 272 | reasons for developing this free software was to help force convergence on |
| michael@0 | 273 | common, interoperable format standards for JPEG files. |
| michael@0 | 274 | Don't use an incompatible file format! |
| michael@0 | 275 | (In any case, our decoder will remain capable of reading existing JPEG |
| michael@0 | 276 | image files indefinitely.) |
| michael@0 | 277 | |
| michael@0 | 278 | |
| michael@0 | 279 | TO DO |
| michael@0 | 280 | ===== |
| michael@0 | 281 | |
| michael@0 | 282 | Please send bug reports, offers of help, etc. to jpeg-info@jpegclub.org. |
