1 The Independent JPEG Group's JPEG software
2 ==========================================
4 README for release 6b of 27-Mar-1998
5 ====================================
7 This distribution contains the sixth public release of the Independent JPEG
8 Group's free JPEG software. You are welcome to redistribute this software and
9 to use it for any purpose, subject to the conditions under LEGAL ISSUES, below.
11 Serious users of this software (particularly those incorporating it into
12 larger programs) should contact IJG at jpeg-info@uunet.uu.net to be added to
13 our electronic mailing list. Mailing list members are notified of updates
14 and have a chance to participate in technical discussions, etc.
16 This software is the work of Tom Lane, Philip Gladstone, Jim Boucher,
17 Lee Crocker, Julian Minguillon, Luis Ortiz, George Phillips, Davide Rossi,
18 Guido Vollbeding, Ge' Weijers, and other members of the Independent JPEG
21 IJG is not affiliated with the official ISO JPEG standards committee.
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 RELATED SOFTWARE Other stuff you should get.
34 FILE FORMAT WARS Software *not* to get.
35 TO DO Plans for future IJG releases.
37 Other documentation files in the distribution are:
40 install.doc How to configure and install the IJG software.
41 usage.doc Usage instructions for cjpeg, djpeg, jpegtran,
42 rdjpgcom, and wrjpgcom.
43 *.1 Unix-style man pages for programs (same info as usage.doc).
44 wizard.doc Advanced usage instructions for JPEG wizards only.
45 change.log Version-to-version change highlights.
46 Programmer and internal documentation:
47 libjpeg.doc How to use the JPEG library in your own programs.
48 example.c Sample code for calling the JPEG library.
49 structure.doc Overview of the JPEG library's internal structure.
50 filelist.doc Road map of IJG files.
51 coderules.doc Coding style rules --- please read if you contribute code.
53 Please read at least the files install.doc and usage.doc. Useful information
54 can also be found in the JPEG FAQ (Frequently Asked Questions) article. See
55 ARCHIVE LOCATIONS below to find out where to obtain the FAQ article.
57 If you want to understand how the JPEG code works, we suggest reading one or
58 more of the REFERENCES, then looking at the documentation files (in roughly
59 the order listed) before diving into the code.
65 This package contains C software to implement JPEG image compression and
66 decompression. JPEG (pronounced "jay-peg") is a standardized compression
67 method for full-color and gray-scale images. JPEG is intended for compressing
68 "real-world" scenes; line drawings, cartoons and other non-realistic images
69 are not its strong suit. JPEG is lossy, meaning that the output image is not
70 exactly identical to the input image. Hence you must not use JPEG if you
71 have to have identical output bits. However, on typical photographic images,
72 very good compression levels can be obtained with no visible change, and
73 remarkably high compression levels are possible if you can tolerate a
74 low-quality image. For more details, see the references, or just experiment
75 with various compression settings.
77 This software implements JPEG baseline, extended-sequential, and progressive
78 compression processes. Provision is made for supporting all variants of these
79 processes, although some uncommon parameter settings aren't implemented yet.
80 For legal reasons, we are not distributing code for the arithmetic-coding
81 variants of JPEG; see LEGAL ISSUES. We have made no provision for supporting
82 the hierarchical or lossless 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. We have also included
95 "jpegtran", a utility for lossless transcoding between different JPEG
96 processes, and "rdjpgcom" and "wrjpgcom", two simple applications for
97 inserting and extracting textual comments in JFIF files.
99 The emphasis in designing this software has been on achieving portability and
100 flexibility, while also making it fast enough to be useful. In particular,
101 the software is not intended to be read as a tutorial on JPEG. (See the
102 REFERENCES section for introductory material.) Rather, it is intended to
103 be reliable, portable, industrial-strength code. We do not claim to have
104 achieved that goal in every aspect of the software, but we strive for it.
106 We welcome the use of this software as a component of commercial products.
107 No royalty is required, but we do ask for an acknowledgement in product
108 documentation, as described under LEGAL ISSUES.
116 1. We don't promise that this software works. (But if you find any bugs,
118 2. You can use this software for whatever you want. You don't have to pay us.
119 3. You may not pretend that you wrote this software. If you use it in a
120 program, you must acknowledge somewhere in your documentation that
121 you've used the IJG code.
125 The authors make NO WARRANTY or representation, either express or implied,
126 with respect to this software, its quality, accuracy, merchantability, or
127 fitness for a particular purpose. This software is provided "AS IS", and you,
128 its user, assume the entire risk as to its quality and accuracy.
130 This software is copyright (C) 1991-1998, Thomas G. Lane.
131 All Rights Reserved except as specified below.
133 Permission is hereby granted to use, copy, modify, and distribute this
134 software (or portions thereof) for any purpose, without fee, subject to these
136 (1) If any part of the source code for this software is distributed, then this
137 README file must be included, with this copyright and no-warranty notice
138 unaltered; and any additions, deletions, or changes to the original files
139 must be clearly indicated in accompanying documentation.
140 (2) If only executable code is distributed, then the accompanying
141 documentation must state that "this software is based in part on the work of
142 the Independent JPEG Group".
143 (3) Permission for use of this software is granted only if the user accepts
144 full responsibility for any undesirable consequences; the authors accept
145 NO LIABILITY for damages of any kind.
147 These conditions apply to any software derived from or based on the IJG code,
148 not just to the unmodified library. If you use our work, you ought to
151 Permission is NOT granted for the use of any IJG author's name or company name
152 in advertising or publicity relating to this software or products derived from
153 it. This software may be referred to only as "the Independent JPEG Group's
156 We specifically permit and encourage the use of this software as the basis of
157 commercial products, provided that all warranty or liability claims are
158 assumed by the product vendor.
161 ansi2knr.c is included in this distribution by permission of L. Peter Deutsch,
162 sole proprietor of its copyright holder, Aladdin Enterprises of Menlo Park, CA.
163 ansi2knr.c is NOT covered by the above copyright and conditions, but instead
164 by the usual distribution terms of the Free Software Foundation; principally,
165 that you must include source code if you redistribute it. (See the file
166 ansi2knr.c for full details.) However, since ansi2knr.c is not needed as part
167 of any program generated from the IJG code, this does not limit you more than
168 the foregoing paragraphs do.
170 The Unix configuration script "configure" was produced with GNU Autoconf.
171 It is copyright by the Free Software Foundation but is freely distributable.
172 The same holds for its supporting scripts (config.guess, config.sub,
173 ltconfig, ltmain.sh). Another support script, install-sh, is copyright
174 by M.I.T. but is also freely distributable.
176 It appears that the arithmetic coding option of the JPEG spec is covered by
177 patents owned by IBM, AT&T, and Mitsubishi. Hence arithmetic coding cannot
178 legally be used without obtaining one or more licenses. For this reason,
179 support for arithmetic coding has been removed from the free JPEG software.
180 (Since arithmetic coding provides only a marginal gain over the unpatented
181 Huffman mode, it is unlikely that very many implementations will support it.)
182 So far as we are aware, there are no patent restrictions on the remaining
185 The IJG distribution formerly included code to read and write GIF files.
186 To avoid entanglement with the Unisys LZW patent, GIF reading support has
187 been removed altogether, and the GIF writer has been simplified to produce
188 "uncompressed GIFs". This technique does not use the LZW algorithm; the
189 resulting GIF files are larger than usual, but are readable by all standard
192 We are required to state that
193 "The Graphics Interchange Format(c) is the Copyright property of
194 CompuServe Incorporated. GIF(sm) is a Service Mark property of
195 CompuServe Incorporated."
201 We highly recommend reading one or more of these references before trying to
202 understand the innards of the JPEG software.
204 The best short technical introduction to the JPEG compression algorithm is
205 Wallace, Gregory K. "The JPEG Still Picture Compression Standard",
206 Communications of the ACM, April 1991 (vol. 34 no. 4), pp. 30-44.
207 (Adjacent articles in that issue discuss MPEG motion picture compression,
208 applications of JPEG, and related topics.) If you don't have the CACM issue
209 handy, a PostScript file containing a revised version of Wallace's article is
210 available at ftp://ftp.uu.net/graphics/jpeg/wallace.ps.gz. The file (actually
211 a preprint for an article that appeared in IEEE Trans. Consumer Electronics)
212 omits the sample images that appeared in CACM, but it includes corrections
213 and some added material. Note: the Wallace article is copyright ACM and IEEE,
214 and it may not be used for commercial purposes.
216 A somewhat less technical, more leisurely introduction to JPEG can be found in
217 "The Data Compression Book" by Mark Nelson and Jean-loup Gailly, published by
218 M&T Books (New York), 2nd ed. 1996, ISBN 1-55851-434-1. This book provides
219 good explanations and example C code for a multitude of compression methods
220 including JPEG. It is an excellent source if you are comfortable reading C
221 code but don't know much about data compression in general. The book's JPEG
222 sample code is far from industrial-strength, but when you are ready to look
223 at a full implementation, you've got one here...
225 The best full description of JPEG is the textbook "JPEG Still Image Data
226 Compression Standard" by William B. Pennebaker and Joan L. Mitchell, published
227 by Van Nostrand Reinhold, 1993, ISBN 0-442-01272-1. Price US$59.95, 638 pp.
228 The book includes the complete text of the ISO JPEG standards (DIS 10918-1
229 and draft DIS 10918-2). This is by far the most complete exposition of JPEG
230 in existence, and we highly recommend it.
232 The JPEG standard itself is not available electronically; you must order a
233 paper copy through ISO or ITU. (Unless you feel a need to own a certified
234 official copy, we recommend buying the Pennebaker and Mitchell book instead;
235 it's much cheaper and includes a great deal of useful explanatory material.)
236 In the USA, copies of the standard may be ordered from ANSI Sales at (212)
237 642-4900, or from Global Engineering Documents at (800) 854-7179. (ANSI
238 doesn't take credit card orders, but Global does.) It's not cheap: as of
239 1992, ANSI was charging $95 for Part 1 and $47 for Part 2, plus 7%
240 shipping/handling. The standard is divided into two parts, Part 1 being the
241 actual specification, while Part 2 covers compliance testing methods. Part 1
242 is titled "Digital Compression and Coding of Continuous-tone Still Images,
243 Part 1: Requirements and guidelines" and has document numbers ISO/IEC IS
244 10918-1, ITU-T T.81. Part 2 is titled "Digital Compression and Coding of
245 Continuous-tone Still Images, Part 2: Compliance testing" and has document
246 numbers ISO/IEC IS 10918-2, ITU-T T.83.
248 Some extensions to the original JPEG standard are defined in JPEG Part 3,
249 a newer ISO standard numbered ISO/IEC IS 10918-3 and ITU-T T.84. IJG
250 currently does not support any Part 3 extensions.
252 The JPEG standard does not specify all details of an interchangeable file
253 format. For the omitted details we follow the "JFIF" conventions, revision
254 1.02. A copy of the JFIF spec is available from:
255 Literature Department
256 C-Cube Microsystems, Inc.
259 phone (408) 944-6300, fax (408) 944-6314
260 A PostScript version of this document is available by FTP at
261 ftp://ftp.uu.net/graphics/jpeg/jfif.ps.gz. There is also a plain text
262 version at ftp://ftp.uu.net/graphics/jpeg/jfif.txt.gz, but it is missing
265 The TIFF 6.0 file format specification can be obtained by FTP from
266 ftp://ftp.sgi.com/graphics/tiff/TIFF6.ps.gz. The JPEG incorporation scheme
267 found in the TIFF 6.0 spec of 3-June-92 has a number of serious problems.
268 IJG does not recommend use of the TIFF 6.0 design (TIFF Compression tag 6).
269 Instead, we recommend the JPEG design proposed by TIFF Technical Note #2
270 (Compression tag 7). Copies of this Note can be obtained from ftp.sgi.com or
271 from ftp://ftp.uu.net/graphics/jpeg/. It is expected that the next revision
272 of the TIFF spec will replace the 6.0 JPEG design with the Note's design.
273 Although IJG's own code does not support TIFF/JPEG, the free libtiff library
274 uses our library to implement TIFF/JPEG per the Note. libtiff is available
275 from ftp://ftp.sgi.com/graphics/tiff/.
281 The "official" archive site for this software is ftp.uu.net (Internet
282 address 192.48.96.9). The most recent released version can always be found
283 there in directory graphics/jpeg. This particular version will be archived
284 as ftp://ftp.uu.net/graphics/jpeg/jpegsrc.v6b.tar.gz. If you don't have
285 direct Internet access, UUNET's archives are also available via UUCP; contact
286 help@uunet.uu.net for information on retrieving files that way.
288 Numerous Internet sites maintain copies of the UUNET files. However, only
289 ftp.uu.net is guaranteed to have the latest official version.
291 You can also obtain this software in DOS-compatible "zip" archive format from
292 the SimTel archives (ftp://ftp.simtel.net/pub/simtelnet/msdos/graphics/), or
293 on CompuServe in the Graphics Support forum (GO CIS:GRAPHSUP), library 12
294 "JPEG Tools". Again, these versions may sometimes lag behind the ftp.uu.net
297 The JPEG FAQ (Frequently Asked Questions) article is a useful source of
298 general information about JPEG. It is updated constantly and therefore is
299 not included in this distribution. The FAQ is posted every two weeks to
300 Usenet newsgroups comp.graphics.misc, news.answers, and other groups.
301 It is available on the World Wide Web at http://www.faqs.org/faqs/jpeg-faq/
302 and other news.answers archive sites, including the official news.answers
303 archive at rtfm.mit.edu: ftp://rtfm.mit.edu/pub/usenet/news.answers/jpeg-faq/.
304 If you don't have Web or FTP access, send e-mail to mail-server@rtfm.mit.edu
306 send usenet/news.answers/jpeg-faq/part1
307 send usenet/news.answers/jpeg-faq/part2
313 Numerous viewing and image manipulation programs now support JPEG. (Quite a
314 few of them use this library to do so.) The JPEG FAQ described above lists
315 some of the more popular free and shareware viewers, and tells where to
316 obtain them on Internet.
318 If you are on a Unix machine, we highly recommend Jef Poskanzer's free
319 PBMPLUS software, which provides many useful operations on PPM-format image
320 files. In particular, it can convert PPM images to and from a wide range of
321 other formats, thus making cjpeg/djpeg considerably more useful. The latest
322 version is distributed by the NetPBM group, and is available from numerous
323 sites, notably ftp://wuarchive.wustl.edu/graphics/graphics/packages/NetPBM/.
324 Unfortunately PBMPLUS/NETPBM is not nearly as portable as the IJG software is;
325 you are likely to have difficulty making it work on any non-Unix machine.
327 A different free JPEG implementation, written by the PVRG group at Stanford,
328 is available from ftp://havefun.stanford.edu/pub/jpeg/. This program
329 is designed for research and experimentation rather than production use;
330 it is slower, harder to use, and less portable than the IJG code, but it
331 is easier to read and modify. Also, the PVRG code supports lossless JPEG,
332 which we do not. (On the other hand, it doesn't do progressive JPEG.)
338 Some JPEG programs produce files that are not compatible with our library.
339 The root of the problem is that the ISO JPEG committee failed to specify a
340 concrete file format. Some vendors "filled in the blanks" on their own,
341 creating proprietary formats that no one else could read. (For example, none
342 of the early commercial JPEG implementations for the Macintosh were able to
343 exchange compressed files.)
345 The file format we have adopted is called JFIF (see REFERENCES). This format
346 has been agreed to by a number of major commercial JPEG vendors, and it has
347 become the de facto standard. JFIF is a minimal or "low end" representation.
348 We recommend the use of TIFF/JPEG (TIFF revision 6.0 as modified by TIFF
349 Technical Note #2) for "high end" applications that need to record a lot of
350 additional data about an image. TIFF/JPEG is fairly new and not yet widely
351 supported, unfortunately.
353 The upcoming JPEG Part 3 standard defines a file format called SPIFF.
354 SPIFF is interoperable with JFIF, in the sense that most JFIF decoders should
355 be able to read the most common variant of SPIFF. SPIFF has some technical
356 advantages over JFIF, but its major claim to fame is simply that it is an
357 official standard rather than an informal one. At this point it is unclear
358 whether SPIFF will supersede JFIF or whether JFIF will remain the de-facto
359 standard. IJG intends to support SPIFF once the standard is frozen, but we
360 have not decided whether it should become our default output format or not.
361 (In any case, our decoder will remain capable of reading JFIF indefinitely.)
363 Various proprietary file formats incorporating JPEG compression also exist.
364 We have little or no sympathy for the existence of these formats. Indeed,
365 one of the original reasons for developing this free software was to help
366 force convergence on common, open format standards for JPEG files. Don't
367 use a proprietary file format!
373 The major thrust for v7 will probably be improvement of visual quality.
374 The current method for scaling the quantization tables is known not to be
375 very good at low Q values. We also intend to investigate block boundary
376 smoothing, "poor man's variable quantization", and other means of improving
377 quality-vs-file-size performance without sacrificing compatibility.
379 In future versions, we are considering supporting some of the upcoming JPEG
380 Part 3 extensions --- principally, variable quantization and the SPIFF file
383 As always, speeding things up is of great interest.
385 Please send bug reports, offers of help, etc. to jpeg-info@uunet.uu.net.
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