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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.
8 The Independent JPEG Group's JPEG software
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.
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.
20 IJG is not affiliated with the ISO/IEC JTC1/SC29/WG1 standards committee
21 (also known as JPEG, together with ITU-T SG16).
24 DOCUMENTATION ROADMAP
27 This file contains the following sections:
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.
36 Other documentation files in the distribution are:
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.
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.
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.
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
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
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.
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.
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.
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.
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.
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.
112 LEGAL ISSUES
115 In plain English:
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.
124 In legalese:
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.
131 This software is copyright (C) 1991-2016, Thomas G. Lane, Guido Vollbeding.
132 All Rights Reserved except as specified below.
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
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.
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.
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
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.
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.
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."
178 We recommend reading one or more of these references before trying to
179 understand the innards of the JPEG software.
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.
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...
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).
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.
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
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.
238 ARCHIVE LOCATIONS
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".
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 email@example.com
251 with body
252 send usenet/news.answers/jpeg-faq/part1
253 send usenet/news.answers/jpeg-faq/part2
256 FILE FORMAT COMPATIBILITY
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.
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.
274 TO DO
277 Please send bug reports, offers of help, etc. to firstname.lastname@example.org.