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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) 1991-2016, 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
162The IJG distribution formerly included code to read and write GIF files.
163To avoid entanglement with the Unisys LZW patent (now expired), GIF reading
164support has been removed altogether, and the GIF writer has been simplified
165to produce "uncompressed GIFs".  This technique does not use the LZW
166algorithm; the resulting GIF files are larger than usual, but are readable
167by all standard GIF decoders.
168
169We 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
175REFERENCES
176==========
177
178We recommend reading one or more of these references before trying to
179understand the innards of the JPEG software.
180
181The 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,
185applications of JPEG, and related topics.)  If you don't have the CACM issue
186handy, a PDF file containing a revised version of Wallace's article is
187available at http://www.ijg.org/files/Wallace.JPEG.pdf.  The file (actually
188a preprint for an article that appeared in IEEE Trans. Consumer Electronics)
189omits the sample images that appeared in CACM, but it includes corrections
190and some added material.  Note: the Wallace article is copyright ACM and IEEE,
191and it may not be used for commercial purposes.
192
193A 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
195M&T Books (New York), 2nd ed. 1996, ISBN 1-55851-434-1.  This book provides
196good explanations and example C code for a multitude of compression methods
197including JPEG.  It is an excellent source if you are comfortable reading C
198code but don't know much about data compression in general.  The book's JPEG
199sample code is far from industrial-strength, but when you are ready to look
200at a full implementation, you've got one here...
201
202The best currently available description of JPEG is the textbook "JPEG Still
203Image Data Compression Standard" by William B. Pennebaker and Joan L.
204Mitchell, published by Van Nostrand Reinhold, 1993, ISBN 0-442-01272-1.
205Price US$59.95, 638 pp.  The book includes the complete text of the ISO JPEG
206standards (DIS 10918-1 and draft DIS 10918-2).
207
208The original JPEG standard is divided into two parts, Part 1 being the actual
209specification, while Part 2 covers compliance testing methods.  Part 1 is
210titled "Digital Compression and Coding of Continuous-tone Still Images,
211Part 1: Requirements and guidelines" and has document numbers ISO/IEC IS
21210918-1, ITU-T T.81.  Part 2 is titled "Digital Compression and Coding of
213Continuous-tone Still Images, Part 2: Compliance testing" and has document
214numbers ISO/IEC IS 10918-2, ITU-T T.83.
215
216The JPEG standard does not specify all details of an interchangeable file
217format.  For the omitted details, we follow the "JFIF" conventions, revision
2181.02.  JFIF version 1 has been adopted as ISO/IEC 10918-5 (05/2013) and
219Recommendation ITU-T T.871 (05/2011): Information technology - Digital
220compression and coding of continuous-tone still images: JPEG File Interchange
221Format (JFIF).  It is available as a free download in PDF file format from
222https://www.iso.org/standard/54989.html and http://www.itu.int/rec/T-REC-T.871.
223A PDF file of the older JFIF 1.02 specification is available at
224http://www.w3.org/Graphics/JPEG/jfif3.pdf.
225
226The TIFF 6.0 file format specification can be obtained by FTP from
227ftp://ftp.sgi.com/graphics/tiff/TIFF6.ps.gz.  The JPEG incorporation scheme
228found in the TIFF 6.0 spec of 3-June-92 has a number of serious problems.
229IJG does not recommend use of the TIFF 6.0 design (TIFF Compression tag 6).
230Instead, we recommend the JPEG design proposed by TIFF Technical Note #2
231(Compression tag 7).  Copies of this Note can be obtained from
232http://www.ijg.org/files/.  It is expected that the next revision
233of the TIFF spec will replace the 6.0 JPEG design with the Note's design.
234Although IJG's own code does not support TIFF/JPEG, the free libtiff library
235uses our library to implement TIFF/JPEG per the Note.
236
237
238ARCHIVE LOCATIONS
239=================
240
241The "official" archive site for this software is www.ijg.org.
242The most recent released version can always be found there in
243directory "files".
244
245The JPEG FAQ (Frequently Asked Questions) article is a source of some
246general information about JPEG.
247It is available on the World Wide Web at http://www.faqs.org/faqs/jpeg-faq/
248and other news.answers archive sites, including the official news.answers
249archive at rtfm.mit.edu: ftp://rtfm.mit.edu/pub/usenet/news.answers/jpeg-faq/.
250If you don't have Web or FTP access, send e-mail to mail-server@rtfm.mit.edu
251with body
252        send usenet/news.answers/jpeg-faq/part1
253        send usenet/news.answers/jpeg-faq/part2
254
255
256FILE FORMAT COMPATIBILITY
257=========================
258
259This software implements ITU T.81 | ISO/IEC 10918 with some extensions from
260ITU T.871 | ISO/IEC 10918-5 (JPEG File Interchange Format-- see REFERENCES).
261Informally, the term "JPEG image" or "JPEG file" most often refers to JFIF or
262a subset thereof, but there are other formats containing the name "JPEG" that
263are incompatible with the DCT-based JPEG standard or with JFIF (for instance,
264JPEG 2000 and JPEG XR).  This software therefore does not support these
265formats.  Indeed, one of the original reasons for developing this free software
266was to help force convergence on a common, interoperable format standard for
267JPEG files.
268
269JFIF is a minimal or "low end" representation.  TIFF/JPEG (TIFF revision 6.0 as
270modified by TIFF Technical Note #2) can be used for "high end" applications
271that need to record a lot of additional data about an image.
272
273
274TO DO
275=====
276
277Please send bug reports, offers of help, etc. to jpeg-info@jpegclub.org.
278