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1 : chl 1.1 The Independent JPEG Group's JPEG software
2 :     ==========================================
3 :    
4 :     README for release 6b of 27-Mar-1998
5 :     ====================================
6 :    
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.
10 :    
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.
15 :    
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
19 :     Group.
20 :    
21 :     IJG is not affiliated with the official ISO JPEG standards committee.
22 :    
23 :    
24 :     DOCUMENTATION ROADMAP
25 :     =====================
26 :    
27 :     This file contains the following sections:
28 :    
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.
36 :    
37 :     Other documentation files in the distribution are:
38 :    
39 :     User 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.
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.
52 :    
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.
56 :    
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.
60 :    
61 :    
62 :     OVERVIEW
63 :     ========
64 :    
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.
76 :    
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.
83 :    
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.
88 :    
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.
98 :    
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.
105 :    
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.
109 :    
110 :    
111 :     LEGAL ISSUES
112 :     ============
113 :    
114 :     In plain English:
115 :    
116 :     1. We don't promise that this software works. (But if you find any bugs,
117 :     please let us know!)
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.
122 :    
123 :     In legalese:
124 :    
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.
129 :    
130 :     This software is copyright (C) 1991-1998, Thomas G. Lane.
131 :     All Rights Reserved except as specified below.
132 :    
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
135 :     conditions:
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.
146 :    
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
149 :     acknowledge us.
150 :    
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
154 :     software".
155 :    
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.
159 :    
160 :    
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.
169 :    
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.
175 :    
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
183 :     code.
184 :    
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
190 :     GIF decoders.
191 :    
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."
196 :    
197 :    
198 :     REFERENCES
199 :     ==========
200 :    
201 :     We highly recommend reading one or more of these references before trying to
202 :     understand the innards of the JPEG software.
203 :    
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.
215 :    
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...
224 :    
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.
231 :    
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.
247 :    
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.
251 :    
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.
257 :     1778 McCarthy Blvd.
258 :     Milpitas, CA 95035
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
263 :     the figures.
264 :    
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/.
276 :    
277 :    
278 :     ARCHIVE LOCATIONS
279 :     =================
280 :    
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.
287 :    
288 :     Numerous Internet sites maintain copies of the UUNET files. However, only
289 :     ftp.uu.net is guaranteed to have the latest official version.
290 :    
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
295 :     release.
296 :    
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
305 :     with body
306 :     send usenet/news.answers/jpeg-faq/part1
307 :     send usenet/news.answers/jpeg-faq/part2
308 :    
309 :    
310 :     RELATED SOFTWARE
311 :     ================
312 :    
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.
317 :    
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.
326 :    
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.)
333 :    
334 :    
335 :     FILE FORMAT WARS
336 :     ================
337 :    
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.)
344 :    
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.
352 :    
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.)
362 :    
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!
368 :    
369 :    
370 :     TO DO
371 :     =====
372 :    
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.
378 :    
379 :     In future versions, we are considering supporting some of the upcoming JPEG
380 :     Part 3 extensions --- principally, variable quantization and the SPIFF file
381 :     format.
382 :    
383 :     As always, speeding things up is of great interest.
384 :    
385 :     Please send bug reports, offers of help, etc. to jpeg-info@uunet.uu.net.

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