Veronika Irvine | 43ce1fc | 2006-04-05 18:31:36 +0000 | [diff] [blame] | 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. |