1 The Independent JPEG Group's JPEG software
2 ==========================================
3
4 README for release 4 of 10-Dec-92
5 =================================
6
7 This distribution contains the fourth 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 For installation instructions, see file SETUP.
12
13 For usage instructions, see file USAGE (or the cjpeg.1 and djpeg.1 manual
14 pages; but USAGE contains a "hints" section not found in the manual pages).
15 Useful information can also be found in the JPEG FAQ (Frequently Asked
16 Questions) article; see ARCHIVE LOCATIONS below to obtain the FAQ article.
17
18 This software is still undergoing revision. Updated versions may be obtained
19 by FTP or UUCP to UUNET and other archive sites; see ARCHIVE LOCATIONS below
20 for details.
21
22 Serious users of this software (particularly those incorporating it into
23 larger programs) should contact jpeg-info@uunet.uu.net to be added to our
24 electronic mailing list. Mailing list members are notified of updates and
25 have a chance to participate in technical discussions, etc.
26
27 This software is the work of Tom Lane, Philip Gladstone, Luis Ortiz,
28 Lee Crocker, Ge' Weijers, and other members of the Independent JPEG Group.
29
30
31 DISCLAIMER
32 ==========
33
34 THIS SOFTWARE IS NOT COMPLETE NOR FULLY DEBUGGED. It is not guaranteed to be
35 useful for anything, nor to be compatible with subsequent releases, nor to be
36 an accurate implementation of the JPEG standard. (See LEGAL ISSUES for even
37 more disclaimers.)
38
39 Despite that, we believe that this software is pretty good, and if you find
40 any problems with it, we'd like to know about them. Please report problems
41 by e-mail to jpeg-info@uunet.uu.net.
42
43
44 WHAT'S HERE
45 ===========
46
47 This distribution contains C software to implement JPEG image compression and
48 decompression. JPEG (pronounced "jay-peg") is a standardized compression
49 method for full-color and gray-scale images. JPEG is intended for compressing
50 "real-world" scenes; cartoons and other non-realistic images are not its
51 strong suit. JPEG is lossy, meaning that the output image is not necessarily
52 identical to the input image. Hence you must not use JPEG if you have to have
53 identical output bits. However, on typical images of real-world scenes, very
54 good compression levels can be obtained with no visible change, and amazingly
55 high compression levels are possible if you can tolerate a low-quality image.
56 For more details, see the references, or just experiment with various
57 compression settings.
58
59 The software implements JPEG baseline and extended-sequential compression
60 processes. Provision is made for supporting all variants of these processes,
61 although some uncommon parameter settings aren't implemented yet. For legal
62 reasons, we are not distributing code for the arithmetic-coding process; see
63 LEGAL ISSUES. At present we have made no provision for supporting the
64 progressive, hierarchical, or lossless processes defined in the standard.
65
66 In order to support file conversion and viewing software, we have included
67 considerable functionality beyond the bare JPEG coding/decoding capability;
68 for example, the color quantization modules are not strictly part of JPEG
69 decoding, but they are essential for output to colormapped file formats or
70 colormapped displays. These extra functions can be compiled out if not
71 required for a particular application.
72
73 The emphasis in designing this software has been on achieving portability and
74 flexibility, while also making it fast enough to be useful. In particular,
75 the software is not intended to be read as a tutorial on JPEG. (See the
76 REFERENCES section for introductory material.) While we hope that the entire
77 package will someday be industrial-strength code, much remains to be done in
78 performance tuning and in improving the capabilities of individual modules.
79
80
81 This software can be used on several levels:
82
83 * As canned software for JPEG compression and decompression. Just edit the
84 Makefile and configuration files as needed (see file SETUP), compile and go.
85 Members of the Independent JPEG Group will improve the out-of-the-box
86 functionality and speed as time goes on.
87
88 * As the basis for other JPEG programs. For example, you could incorporate
89 the decompressor into a general image viewing package by replacing the
90 output module with write-to-screen functions. For an implementation on
91 specific hardware, you might want to replace some of the inner loops with
92 assembly code. For a non-command-line-driven system, you might want a
93 different user interface. (Members of the group will be producing Macintosh
94 and Amiga versions with more appropriate user interfaces, for example.)
95
96 * As a toolkit for experimentation with JPEG and JPEG-like algorithms. Most
97 of the individual decisions you might want to mess with are packaged up into
98 separate modules. For example, the details of color-space conversion and
99 subsampling techniques are each localized in one compressor and one
100 decompressor module. You'd probably also want to extend the user interface
101 to give you more detailed control over the JPEG compression parameters.
102
103 In particular, we welcome the use of this software as a component of commercial
104 products; no royalty is required.
105
106
107 ARCHIVE LOCATIONS
108 =================
109
110 The "official" archive site for this software is ftp.uu.net (Internet
111 address 137.39.1.9 or 192.48.96.9). The most recent released version can
112 always be found there in directory graphics/jpeg. This particular version
113 will be archived as jpegsrc.v4.tar.Z. If you are on the Internet, you can
114 retrieve files from UUNET by anonymous FTP. If you don't have FTP access,
115 UUNET's archives are also available via UUCP; contact postmaster@uunet.uu.net
116 for information on retrieving files that way.
117
118 Numerous Internet sites maintain copies of the UUNET files; in particular,
119 you can probably find a copy at any site that archives comp.sources.misc
120 submissions. However, only ftp.uu.net is guaranteed to have the latest
121 official version.
122
123 You can also obtain this software from CompuServe, in the GRAPHSUPPORT forum
124 (GO PICS), library 15; this version will be file jpsrc4.zip. Again,
125 CompuServe is not guaranteed to have the very latest version.
126
127 The JPEG FAQ (Frequently Asked Questions) article is a useful source of
128 general information about JPEG. It is updated constantly and therefore
129 is not included in this distribution. The FAQ is posted every two weeks
130 to Usenet newsgroups comp.graphics, news.answers, and other groups. You
131 can always obtain the latest version from the news.answers archive at
132 rtfm.mit.edu (18.172.1.27). By FTP, fetch /pub/usenet/news.answers/jpeg-faq.
133 If you don't have FTP, send e-mail to mail-server@rtfm.mit.edu with body
134 "send usenet/news.answers/jpeg-faq".
135
136
137 SUPPORTING SOFTWARE
138 ===================
139
140 You will probably want Jef Poskanzer's PBMPLUS image software, which provides
141 many useful operations on PPM-format image files. In particular, it can
142 convert PPM images to and from a wide range of other formats. You can FTP
143 this free software from export.lcs.mit.edu (contrib/pbmplus*.tar.Z) or
144 ftp.ee.lbl.gov (pbmplus*.tar.Z). Unfortunately PBMPLUS is not nearly as
145 portable as the JPEG software is; you are likely to have difficulty making it
146 work on any non-Unix machine.
147
148 If you are using X Windows you might want to use the xv or xloadimage viewers
149 to save yourself the trouble of converting PPM to some other format. Both of
150 these can be found in the contrib directory at export.lcs.mit.edu. Actually,
151 xv version 2.00 and up incorporates our software and thus can read and write
152 JPEG files directly. (NOTE: since xv internally reduces all images to 8
153 bits/pixel, a JPEG file written by xv will not be very high quality; and xv
154 cannot fully exploit a 24-bit display. These problems are expected to go away
155 in the next xv release, planned for early 1993. In the meantime, use
156 xloadimage for 24-bit displays.)
157
158 For DOS machines, Lee Crocker's free Piclab program is a useful companion to
159 the JPEG software. The latest version, currently 1.91, is available by FTP
160 from SIMTEL20 and its various mirror sites, file <msdos.graphics>piclb191.zip.
161 CompuServe also has it, in the same library as the JPEG software.
162
163
164 SOFTWARE THAT'S NO HELP AT ALL
165 ==============================
166
167 Handmade Software's shareware PC program GIF2JPG produces files that are
168 totally incompatible with our programs. They use a proprietary format that is
169 an amalgam of GIF and JPEG representations. However, you can force GIF2JPG
170 to produce compatible files with its -j switch, and their decompression
171 program JPG2GIF can read our files (at least ones produced with our default
172 option settings).
173
174 Some commercial JPEG implementations are also incompatible as of this writing,
175 especially programs released before summer 1991. The root of the problem is
176 that the ISO JPEG committee failed to specify a concrete file format. Some
177 vendors "filled in the blanks" on their own, creating proprietary formats that
178 no one else could read. (For example, none of the early commercial JPEG
179 implementations for the Macintosh were able to exchange compressed files.)
180
181 The file format we have adopted is called JFIF (see REFERENCES). This format
182 has been agreed to by a number of major commercial JPEG vendors, and we expect
183 that it will become the de facto standard. JFIF is a minimal representation;
184 work is also going forward to incorporate JPEG compression into the TIFF 6.0
185 standard, for use in "high end" applications that need to record a lot of
186 additional data about an image. We intend to support TIFF 6.0 in the future.
187 We hope that these two formats will be sufficient and that other, incompatible
188 JPEG file formats will not proliferate.
189
190 Indeed, part of the reason for developing and releasing this free software is
191 to help force rapid convergence to de facto standards for JPEG file formats.
192 SUPPORT STANDARD, NON-PROPRIETARY FORMATS: demand JFIF or TIFF 6.0!
193
194
195 USING JPEG AS A SUBROUTINE IN A LARGER PROGRAM
196 ==============================================
197
198 You can readily incorporate the JPEG compression and decompression routines in
199 a larger program. The file example.c provides a skeleton of the interface
200 routines you'll need for this purpose. Essentially, you replace jcmain.c (for
201 compression) and/or jdmain.c (for decompression) with your own code. Note
202 that the fewer JPEG options you allow the user to twiddle, the less code you
203 need; all the default options are set up automatically. (Alternately, if you
204 know a lot about JPEG or have a special application, you may want to twiddle
205 the default options even more extensively than jcmain/jdmain do.)
206
207 Most likely, you will want the uncompressed image to come from memory (for
208 compression) or go to memory or the screen (for decompression). For this
209 purpose you must provide image reading or writing routines that match the
210 interface used by the image file I/O modules (jrdXXX or jwrXXX); again,
211 example.c shows a skeleton of what is required. In this situation, you
212 won't need any of the non-JPEG image file I/O modules used by cjpeg and djpeg.
213
214 By default, any error detected inside the JPEG routines will cause a message
215 to be printed on stderr, followed by exit(). You can override this behavior
216 by supplying your own message-printing and/or error-exit routines; again,
217 example.c shows how.
218
219 We recommend you create libjpeg.a as shown in the Makefile, then link that
220 with your surrounding program. (If your linker is at all reasonable, only the
221 code you actually need will get loaded.) Include the files jconfig.h and
222 jpegdata.h in C files that need to call the JPEG routines.
223
224 CAUTION: some people have tried to compile JPEG and their surrounding code
225 with different compilers, e.g., cc for JPEG and c++ or gcc for the rest. This
226 is a Real Bad Move and you will deserve what happens to you if you try it.
227 (Hint: the parameter structures can get laid out differently with no warning.)
228
229 Read our "architecture" file for more info. If it seems to you that the
230 software structure doesn't accommodate what you want to do, please contact
231 the authors.
232
233 Beginning with version 3, we will endeavor to hold the interface described by
234 example.c constant, so that you can plug in updated versions of the JPEG code
235 just by recompiling. However, we can't guarantee this, especially if you
236 choose to twiddle any JPEG options not listed in example.c. Check the
237 CHANGELOG when installing any new version, and compare example.c against the
238 prior version. Recompile your calling software (don't just relink), as we may
239 add or subtract fields in the parameter structures.
240
241
242 REFERENCES
243 ==========
244
245 We highly recommend reading one or more of these references before trying to
246 understand the innards of any JPEG software.
247
248 The best short technical introduction to the JPEG compression algorithm is
249 Wallace, Gregory K. "The JPEG Still Picture Compression Standard",
250 Communications of the ACM, April 1991 (vol. 34 no. 4), pp. 30-44.
251 (Adjacent articles in that issue discuss MPEG motion picture compression,
252 applications of JPEG, and related topics.) If you don't have the CACM issue
253 handy, a PostScript file containing a revised version of the article is
254 available at ftp.uu.net, graphics/jpeg/wallace.ps.Z. The file (actually a
255 preprint for an article to appear in IEEE Trans. Consumer Electronics) omits
256 the sample images that appeared in CACM, but it includes corrections and some
257 added material. Note: the Wallace article is copyright ACM and IEEE, and it
258 may not be used for commercial purposes.
259
260 A somewhat less technical, more leisurely introduction to JPEG can be found in
261 "The Data Compression Book" by Mark Nelson, published by M&T Books (Redwood
262 City, CA), 1991, ISBN 1-55851-216-0. This book provides good explanations and
263 example C code for a multitude of compression methods including JPEG. It is
264 an excellent source if you are comfortable reading C code but don't know much
265 about data compression in general. The book's JPEG sample code is far from
266 industrial-strength, but when you are ready to look at a full implementation,
267 you've got one here...
268
269 A new textbook about JPEG is "JPEG Still Image Data Compression Standard" by
270 William B. Pennebaker and Joan L. Mitchell, published by Van Nostrand
271 Reinhold, 1993, ISBN 0-442-01272-1. Price US$59.95. This book includes the
272 complete text of the ISO JPEG standards (DIS 10918-1 and draft DIS 10918-2).
273 This is by far the most complete exposition of JPEG in existence, and I highly
274 recommend it. If you read the entire book, you will probably know more about
275 JPEG than I do.
276
277 The JPEG standard itself is not available electronically; you must order a
278 paper copy through ISO. (Unless you are concerned about having a certified
279 official copy, I recommend buying the Pennebaker and Mitchell book instead;
280 it's much cheaper and includes a great deal of useful explanatory material.)
281 In the US, copies of the standard may be ordered from ANSI Sales at (212)
282 642-4900. It's not cheap: as of 1992, Part 1 is $95 and Part 2 is $47, plus
283 7% shipping/handling. The standard is divided into two parts, Part 1 being
284 the actual specification, while Part 2 covers compliance testing methods.
285 As of early 1992, Part 1 has Draft International Standard status. It is
286 titled "Digital Compression and Coding of Continuous-tone Still Images, Part
287 1: Requirements and guidelines" and has document number ISO/IEC DIS 10918-1.
288 Part 2 is still at Committee Draft status. It is titled "Digital Compression
289 and Coding of Continuous-tone Still Images, Part 2: Compliance testing" and
290 has document number ISO/IEC CD 10918-2. (NOTE: I'm told that the final
291 version of Part 2 will differ considerably from the CD draft.)
292
293 The JPEG standard does not specify all details of an interchangeable file
294 format. For the omitted details we follow the "JFIF" conventions, revision
295 1.02. A copy of the JFIF spec is available from:
296 Literature Department
297 C-Cube Microsystems, Inc.
298 399A West Trimble Road
299 San Jose, CA 95131
300 (408) 944-6300
301 A PostScript version of this document is available at ftp.uu.net, file
302 graphics/jpeg/jfif.ps.Z. It can also be obtained by e-mail from the C-Cube
303 mail server, netlib@c3.pla.ca.us. Send the message "send jfif_ps from jpeg"
304 to the server to obtain the JFIF document; send the message "help" if you have
305 trouble.
306
307 The TIFF 6.0 file format specification can be obtained by FTP from sgi.com
308 (192.48.153.1), file graphics/tiff/TIFF6.ps.Z; or you can order a printed copy
309 from Aldus Corp. at (206) 628-6593. It should be noted that the TIFF 6.0 spec
310 of 3-June-92 has a number of serious problems in its JPEG features. A
311 clarification note will probably be needed to ensure that TIFF JPEG files are
312 compatible across different implementations. The IJG does not intend to
313 support TIFF 6.0 until these problems are resolved.
314
315 If you want to understand this implementation, start by reading the
316 "architecture" documentation file. Please read "codingrules" if you want to
317 contribute any code.
318
319
320 LEGAL ISSUES
321 ============
322
323 The authors make NO WARRANTY or representation, either express or implied,
324 with respect to this software, its quality, accuracy, merchantability, or
325 fitness for a particular purpose. This software is provided "AS IS", and you,
326 its user, assume the entire risk as to its quality and accuracy.
327
328 This software is copyright (C) 1991, 1992, Thomas G. Lane.
329 All Rights Reserved except as specified below.
330
331 Permission is hereby granted to use, copy, modify, and distribute this
332 software (or portions thereof) for any purpose, without fee, subject to these
333 conditions:
334 (1) If any part of the source code for this software is distributed, then this
335 README file must be included, with this copyright and no-warranty notice
336 unaltered; and any additions, deletions, or changes to the original files
337 must be clearly indicated in accompanying documentation.
338 (2) If only executable code is distributed, then the accompanying
339 documentation must state that "this software is based in part on the work of
340 the Independent JPEG Group".
341 (3) Permission for use of this software is granted only if the user accepts
342 full responsibility for any undesirable consequences; the authors accept
343 NO LIABILITY for damages of any kind.
344
345 Permission is NOT granted for the use of any IJG author's name or company name
346 in advertising or publicity relating to this software or products derived from
347 it. This software may be referred to only as "the Independent JPEG Group's
348 software".
349
350 We specifically permit and encourage the use of this software as the basis of
351 commercial products, provided that all warranty or liability claims are
352 assumed by the product vendor.
353
354
355 ansi2knr.c is included in this distribution by permission of L. Peter Deutsch,
356 sole proprietor of its copyright holder, Aladdin Enterprises of Menlo Park, CA.
357 ansi2knr.c is NOT covered by the above copyright and conditions, but instead
358 by the usual distribution terms of the Free Software Foundation; principally,
359 that you must include source code if you redistribute it. (See the file
360 ansi2knr.c for full details.) However, since ansi2knr.c is not needed as part
361 of any program generated from the JPEG code, this does not limit you more than
362 the foregoing paragraphs do.
363
364
365 It appears that the arithmetic coding option of the JPEG spec is covered by
366 patents owned by IBM and AT&T, as well as a pending Japanese patent of
367 Mitsubishi. Hence arithmetic coding cannot legally be used without obtaining
368 one or more licenses. For this reason, support for arithmetic coding has been
369 removed from the free JPEG software. (Since arithmetic coding provides only a
370 marginal gain over the unpatented Huffman mode, it is unlikely that very many
371 implementors will support it. If you do obtain the necessary licenses,
372 contact jpeg-info@uunet.uu.net for a copy of our arithmetic coding modules.)
373 So far as we are aware, there are no patent restrictions on the remaining
374 code.
375
376
377 We are required to state that
378 "The Graphics Interchange Format(c) is the Copyright property of
379 CompuServe Incorporated. GIF(sm) is a Service Mark property of
380 CompuServe Incorporated."
381
382
383 TO DO
384 =====
385
386 The next major release will probably be a significant rewrite to allow use of
387 this code in conjunction with Sam Leffler's free TIFF library (assuming the
388 bugs in the TIFF 6.0 specification get resolved).
389
390 Many of the modules need fleshing out to provide more complete
391 implementations, or to provide faster paths for common cases.
392 Speeding things up is still high on our priority list.
393
394 We'd appreciate it if people would compile and check out the code on as wide a
395 variety of systems as possible, and report any portability problems
396 encountered (with solutions, if possible). Checks of file compatibility with
397 other JPEG implementations would also be of interest. Finally, we would
398 appreciate code profiles showing where the most time is spent, especially on
399 unusual systems.
400
401 Please send bug reports, offers of help, etc. to jpeg-info@uunet.uu.net.
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