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      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 (a] 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 (a] 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 (a] 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 (a] uunet.uu.net.
    386