source: trunk/libjpeg/README @ 15

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1The Independent JPEG Group's JPEG software
2==========================================
3
4README for release 6b of 27-Mar-1998
5====================================
6
7This distribution contains the sixth public release of the Independent JPEG
8Group's free JPEG software.  You are welcome to redistribute this software and
9to use it for any purpose, subject to the conditions under LEGAL ISSUES, below.
10
11Serious users of this software (particularly those incorporating it into
12larger programs) should contact IJG at jpeg-info@uunet.uu.net to be added to
13our electronic mailing list.  Mailing list members are notified of updates
14and have a chance to participate in technical discussions, etc.
15
16This software is the work of Tom Lane, Philip Gladstone, Jim Boucher,
17Lee Crocker, Julian Minguillon, Luis Ortiz, George Phillips, Davide Rossi,
18Guido Vollbeding, Ge' Weijers, and other members of the Independent JPEG
19Group.
20
21IJG is not affiliated with the official ISO JPEG standards committee.
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.
33RELATED SOFTWARE    Other stuff you should get.
34FILE FORMAT WARS    Software *not* to get.
35TO DO               Plans for future IJG releases.
36
37Other documentation files in the distribution are:
38
39User documentation:
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.
46Programmer 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.
52
53Please read at least the files install.doc and usage.doc.  Useful information
54can also be found in the JPEG FAQ (Frequently Asked Questions) article.  See
55ARCHIVE LOCATIONS below to find out where to obtain the FAQ article.
56
57If you want to understand how the JPEG code works, we suggest reading one or
58more of the REFERENCES, then looking at the documentation files (in roughly
59the order listed) before diving into the code.
60
61
62OVERVIEW
63========
64
65This package contains C software to implement JPEG image compression and
66decompression.  JPEG (pronounced "jay-peg") is a standardized compression
67method for full-color and gray-scale images.  JPEG is intended for compressing
68"real-world" scenes; line drawings, cartoons and other non-realistic images
69are not its strong suit.  JPEG is lossy, meaning that the output image is not
70exactly identical to the input image.  Hence you must not use JPEG if you
71have to have identical output bits.  However, on typical photographic images,
72very good compression levels can be obtained with no visible change, and
73remarkably high compression levels are possible if you can tolerate a
74low-quality image.  For more details, see the references, or just experiment
75with various compression settings.
76
77This software implements JPEG baseline, extended-sequential, and progressive
78compression processes.  Provision is made for supporting all variants of these
79processes, although some uncommon parameter settings aren't implemented yet.
80For legal reasons, we are not distributing code for the arithmetic-coding
81variants of JPEG; see LEGAL ISSUES.  We have made no provision for supporting
82the hierarchical or lossless processes 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.  We have also included
95"jpegtran", a utility for lossless transcoding between different JPEG
96processes, and "rdjpgcom" and "wrjpgcom", two simple applications for
97inserting and extracting textual comments in JFIF files.
98
99The emphasis in designing this software has been on achieving portability and
100flexibility, while also making it fast enough to be useful.  In particular,
101the software is not intended to be read as a tutorial on JPEG.  (See the
102REFERENCES section for introductory material.)  Rather, it is intended to
103be reliable, portable, industrial-strength code.  We do not claim to have
104achieved that goal in every aspect of the software, but we strive for it.
105
106We welcome the use of this software as a component of commercial products.
107No royalty is required, but we do ask for an acknowledgement in product
108documentation, as described under LEGAL ISSUES.
109
110
111LEGAL ISSUES
112============
113
114In plain English:
115
1161. We don't promise that this software works.  (But if you find any bugs,
117   please let us know!)
1182. You can use this software for whatever you want.  You don't have to pay us.
1193. 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.
122
123In legalese:
124
125The authors make NO WARRANTY or representation, either express or implied,
126with respect to this software, its quality, accuracy, merchantability, or
127fitness for a particular purpose.  This software is provided "AS IS", and you,
128its user, assume the entire risk as to its quality and accuracy.
129
130This software is copyright (C) 1991-1998, Thomas G. Lane.
131All Rights Reserved except as specified below.
132
133Permission is hereby granted to use, copy, modify, and distribute this
134software (or portions thereof) for any purpose, without fee, subject to these
135conditions:
136(1) If any part of the source code for this software is distributed, then this
137README file must be included, with this copyright and no-warranty notice
138unaltered; and any additions, deletions, or changes to the original files
139must be clearly indicated in accompanying documentation.
140(2) If only executable code is distributed, then the accompanying
141documentation must state that "this software is based in part on the work of
142the Independent JPEG Group".
143(3) Permission for use of this software is granted only if the user accepts
144full responsibility for any undesirable consequences; the authors accept
145NO LIABILITY for damages of any kind.
146
147These conditions apply to any software derived from or based on the IJG code,
148not just to the unmodified library.  If you use our work, you ought to
149acknowledge us.
150
151Permission is NOT granted for the use of any IJG author's name or company name
152in advertising or publicity relating to this software or products derived from
153it.  This software may be referred to only as "the Independent JPEG Group's
154software".
155
156We specifically permit and encourage the use of this software as the basis of
157commercial products, provided that all warranty or liability claims are
158assumed by the product vendor.
159
160
161ansi2knr.c is included in this distribution by permission of L. Peter Deutsch,
162sole proprietor of its copyright holder, Aladdin Enterprises of Menlo Park, CA.
163ansi2knr.c is NOT covered by the above copyright and conditions, but instead
164by the usual distribution terms of the Free Software Foundation; principally,
165that you must include source code if you redistribute it.  (See the file
166ansi2knr.c for full details.)  However, since ansi2knr.c is not needed as part
167of any program generated from the IJG code, this does not limit you more than
168the foregoing paragraphs do.
169
170The Unix configuration script "configure" was produced with GNU Autoconf.
171It is copyright by the Free Software Foundation but is freely distributable.
172The same holds for its supporting scripts (config.guess, config.sub,
173ltconfig, ltmain.sh).  Another support script, install-sh, is copyright
174by M.I.T. but is also freely distributable.
175
176It appears that the arithmetic coding option of the JPEG spec is covered by
177patents owned by IBM, AT&T, and Mitsubishi.  Hence arithmetic coding cannot
178legally be used without obtaining one or more licenses.  For this reason,
179support for arithmetic coding has been removed from the free JPEG software.
180(Since arithmetic coding provides only a marginal gain over the unpatented
181Huffman mode, it is unlikely that very many implementations will support it.)
182So far as we are aware, there are no patent restrictions on the remaining
183code.
184
185The IJG distribution formerly included code to read and write GIF files.
186To avoid entanglement with the Unisys LZW patent, GIF reading support has
187been removed altogether, and the GIF writer has been simplified to produce
188"uncompressed GIFs".  This technique does not use the LZW algorithm; the
189resulting GIF files are larger than usual, but are readable by all standard
190GIF decoders.
191
192We 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."
196
197
198REFERENCES
199==========
200
201We highly recommend reading one or more of these references before trying to
202understand the innards of the JPEG software.
203
204The 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,
208applications of JPEG, and related topics.)  If you don't have the CACM issue
209handy, a PostScript file containing a revised version of Wallace's article is
210available at ftp://ftp.uu.net/graphics/jpeg/wallace.ps.gz.  The file (actually
211a preprint for an article that appeared in IEEE Trans. Consumer Electronics)
212omits the sample images that appeared in CACM, but it includes corrections
213and some added material.  Note: the Wallace article is copyright ACM and IEEE,
214and it may not be used for commercial purposes.
215
216A 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
218M&T Books (New York), 2nd ed. 1996, ISBN 1-55851-434-1.  This book provides
219good explanations and example C code for a multitude of compression methods
220including JPEG.  It is an excellent source if you are comfortable reading C
221code but don't know much about data compression in general.  The book's JPEG
222sample code is far from industrial-strength, but when you are ready to look
223at a full implementation, you've got one here...
224
225The best full description of JPEG is the textbook "JPEG Still Image Data
226Compression Standard" by William B. Pennebaker and Joan L. Mitchell, published
227by Van Nostrand Reinhold, 1993, ISBN 0-442-01272-1.  Price US$59.95, 638 pp.
228The book includes the complete text of the ISO JPEG standards (DIS 10918-1
229and draft DIS 10918-2).  This is by far the most complete exposition of JPEG
230in existence, and we highly recommend it.
231
232The JPEG standard itself is not available electronically; you must order a
233paper copy through ISO or ITU.  (Unless you feel a need to own a certified
234official copy, we recommend buying the Pennebaker and Mitchell book instead;
235it's much cheaper and includes a great deal of useful explanatory material.)
236In the USA, copies of the standard may be ordered from ANSI Sales at (212)
237642-4900, or from Global Engineering Documents at (800) 854-7179.  (ANSI
238doesn't take credit card orders, but Global does.)  It's not cheap: as of
2391992, ANSI was charging $95 for Part 1 and $47 for Part 2, plus 7%
240shipping/handling.  The standard is divided into two parts, Part 1 being the
241actual specification, while Part 2 covers compliance testing methods.  Part 1
242is titled "Digital Compression and Coding of Continuous-tone Still Images,
243Part 1: Requirements and guidelines" and has document numbers ISO/IEC IS
24410918-1, ITU-T T.81.  Part 2 is titled "Digital Compression and Coding of
245Continuous-tone Still Images, Part 2: Compliance testing" and has document
246numbers ISO/IEC IS 10918-2, ITU-T T.83.
247
248Some extensions to the original JPEG standard are defined in JPEG Part 3,
249a newer ISO standard numbered ISO/IEC IS 10918-3 and ITU-T T.84.  IJG
250currently does not support any Part 3 extensions.
251
252The JPEG standard does not specify all details of an interchangeable file
253format.  For the omitted details we follow the "JFIF" conventions, revision
2541.02.  A copy of the JFIF spec is available from:
255        Literature Department
256        C-Cube Microsystems, Inc.
257        1778 McCarthy Blvd.
258        Milpitas, CA 95035
259        phone (408) 944-6300,  fax (408) 944-6314
260A PostScript version of this document is available by FTP at
261ftp://ftp.uu.net/graphics/jpeg/jfif.ps.gz.  There is also a plain text
262version at ftp://ftp.uu.net/graphics/jpeg/jfif.txt.gz, but it is missing
263the figures.
264
265The TIFF 6.0 file format specification can be obtained by FTP from
266ftp://ftp.sgi.com/graphics/tiff/TIFF6.ps.gz.  The JPEG incorporation scheme
267found in the TIFF 6.0 spec of 3-June-92 has a number of serious problems.
268IJG does not recommend use of the TIFF 6.0 design (TIFF Compression tag 6).
269Instead, 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
271from ftp://ftp.uu.net/graphics/jpeg/.  It is expected that the next revision
272of the TIFF spec will replace the 6.0 JPEG design with the Note's design.
273Although IJG's own code does not support TIFF/JPEG, the free libtiff library
274uses our library to implement TIFF/JPEG per the Note.  libtiff is available
275from ftp://ftp.sgi.com/graphics/tiff/.
276
277
278ARCHIVE LOCATIONS
279=================
280
281The "official" archive site for this software is ftp.uu.net (Internet
282address 192.48.96.9).  The most recent released version can always be found
283there in directory graphics/jpeg.  This particular version will be archived
284as ftp://ftp.uu.net/graphics/jpeg/jpegsrc.v6b.tar.gz.  If you don't have
285direct Internet access, UUNET's archives are also available via UUCP; contact
286help@uunet.uu.net for information on retrieving files that way.
287
288Numerous Internet sites maintain copies of the UUNET files.  However, only
289ftp.uu.net is guaranteed to have the latest official version.
290
291You can also obtain this software in DOS-compatible "zip" archive format from
292the SimTel archives (ftp://ftp.simtel.net/pub/simtelnet/msdos/graphics/), or
293on 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
295release.
296
297The JPEG FAQ (Frequently Asked Questions) article is a useful source of
298general information about JPEG.  It is updated constantly and therefore is
299not included in this distribution.  The FAQ is posted every two weeks to
300Usenet newsgroups comp.graphics.misc, news.answers, and other groups.
301It is available on the World Wide Web at http://www.faqs.org/faqs/jpeg-faq/
302and other news.answers archive sites, including the official news.answers
303archive at rtfm.mit.edu: ftp://rtfm.mit.edu/pub/usenet/news.answers/jpeg-faq/.
304If you don't have Web or FTP access, send e-mail to mail-server@rtfm.mit.edu
305with body
306        send usenet/news.answers/jpeg-faq/part1
307        send usenet/news.answers/jpeg-faq/part2
308
309
310RELATED SOFTWARE
311================
312
313Numerous viewing and image manipulation programs now support JPEG.  (Quite a
314few of them use this library to do so.)  The JPEG FAQ described above lists
315some of the more popular free and shareware viewers, and tells where to
316obtain them on Internet.
317
318If you are on a Unix machine, we highly recommend Jef Poskanzer's free
319PBMPLUS software, which provides many useful operations on PPM-format image
320files.  In particular, it can convert PPM images to and from a wide range of
321other formats, thus making cjpeg/djpeg considerably more useful.  The latest
322version is distributed by the NetPBM group, and is available from numerous
323sites, notably ftp://wuarchive.wustl.edu/graphics/graphics/packages/NetPBM/.
324Unfortunately PBMPLUS/NETPBM is not nearly as portable as the IJG software is;
325you are likely to have difficulty making it work on any non-Unix machine.
326
327A different free JPEG implementation, written by the PVRG group at Stanford,
328is available from ftp://havefun.stanford.edu/pub/jpeg/.  This program
329is designed for research and experimentation rather than production use;
330it is slower, harder to use, and less portable than the IJG code, but it
331is easier to read and modify.  Also, the PVRG code supports lossless JPEG,
332which we do not.  (On the other hand, it doesn't do progressive JPEG.)
333
334
335FILE FORMAT WARS
336================
337
338Some JPEG programs produce files that are not compatible with our library.
339The root of the problem is that the ISO JPEG committee failed to specify a
340concrete file format.  Some vendors "filled in the blanks" on their own,
341creating proprietary formats that no one else could read.  (For example, none
342of the early commercial JPEG implementations for the Macintosh were able to
343exchange compressed files.)
344
345The file format we have adopted is called JFIF (see REFERENCES).  This format
346has been agreed to by a number of major commercial JPEG vendors, and it has
347become the de facto standard.  JFIF is a minimal or "low end" representation.
348We recommend the use of TIFF/JPEG (TIFF revision 6.0 as modified by TIFF
349Technical Note #2) for "high end" applications that need to record a lot of
350additional data about an image.  TIFF/JPEG is fairly new and not yet widely
351supported, unfortunately.
352
353The upcoming JPEG Part 3 standard defines a file format called SPIFF.
354SPIFF is interoperable with JFIF, in the sense that most JFIF decoders should
355be able to read the most common variant of SPIFF.  SPIFF has some technical
356advantages over JFIF, but its major claim to fame is simply that it is an
357official standard rather than an informal one.  At this point it is unclear
358whether SPIFF will supersede JFIF or whether JFIF will remain the de-facto
359standard.  IJG intends to support SPIFF once the standard is frozen, but we
360have 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.)
362
363Various proprietary file formats incorporating JPEG compression also exist.
364We have little or no sympathy for the existence of these formats.  Indeed,
365one of the original reasons for developing this free software was to help
366force convergence on common, open format standards for JPEG files.  Don't
367use a proprietary file format!
368
369
370TO DO
371=====
372
373The major thrust for v7 will probably be improvement of visual quality.
374The current method for scaling the quantization tables is known not to be
375very good at low Q values.  We also intend to investigate block boundary
376smoothing, "poor man's variable quantization", and other means of improving
377quality-vs-file-size performance without sacrificing compatibility.
378
379In future versions, we are considering supporting some of the upcoming JPEG
380Part 3 extensions --- principally, variable quantization and the SPIFF file
381format.
382
383As always, speeding things up is of great interest.
384
385Please send bug reports, offers of help, etc. to jpeg-info@uunet.uu.net.
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