Search the FAQ Archives

3 - A - B - C - D - E - F - G - H - I - J - K - L - M
N - O - P - Q - R - S - T - U - V - W - X - Y - Z - Internet FAQ Archives

comp.compression Frequently Asked Questions (part 1/3)

( Part1 - Part2 - Part3 - MultiPage )
[ Usenet FAQs | Web FAQs | Documents | RFC Index | Zip codes ]
Archive-name: compression-faq/part1
Last-modified: Sep 5th, 1999

See reader questions & answers on this topic! - Help others by sharing your knowledge
                            It is insufficiently considered that 
                            men more often require to be reminded 
                            than informed.
                                                  Samuel Johnson

This file is part 1 of a set of Frequently Asked Questions (FAQ) for
the groups comp.compression and comp.compression.research.  If you
can't find part 2 or 3, see item 53 below. A copy of this FAQ is available
by ftp in
files part1 to part3. This FAQ is also accessible in the World Wide Web at or

Certain questions get asked time and again, and this is an attempt to
reduce the bandwidth taken up by these posts and their associated
replies.  If you have a question, *please* check this file before you
post.  It may save a lot of peoples time.

If you have not already read the overall Usenet introductory material
posted to "news.announce.newusers", please do. It is also available by
ftp in (see item 2 below
about .zip).

If you don't want to see this FAQ regularly, please add the subject
line to your kill file.  If you don't know what a kill file is, get by
ftp the file
If you have corrections or suggestions for this FAQ, send them to
Jean-loup Gailly <jloup at>. (Replace " at " with "@". This is
a protection against junk mail. Sorry for the inconvenience.)

Part 1 is oriented towards practical usage of compression programs.
Part 2 is more intended for people who want to know how compression works.
Part 3 is a long (but somewhat obsolete) list of image compression hardware.

Main changes relative to the previous version:

- LZO page moved [item 2]
- new url for the File Format Collection [items 2 & 54]
- new url for ACE [item 2]
- new pkzip version [item 3]
- fixed url for arithmetic coder [item 13]
- add url for DjVu, an image compression library [item 15]
- remove obsolete link [item 15]
- new url for DCT algorithms [item 15]
- add url for lossless image compression benchmarks [item 16]
- added urls for the MP3 audio compression standard, and for benchmarks
  of lossless audio compression programs [item 26]


General questions:

[1]  What are these newsgroups about?
[2]  What is this .xxx file type?
     Where can I find the corresponding compression program?
[3]  What is the latest pkzip version?
[4]  What is an archiver?
[5]  What is the best general purpose compression program?
[7]  Which books should I read?
[8]  What about patents on data compression algorithms?
[9]  Compression of random data (WEB, Gilbert and others)
[10] Fake compression programs (OWS, WIC)
[11] What is the V.42bis standard?
[12] I need source for the winners of the Dr Dobbs compression contest
[13] I need source for arithmetic coding

Image and audio compression:

[15] Where can I get image compression programs?
[16] What is the state of the art in lossless image compression?
[17] What is the state of fractal compression?
[18] I need specs and source for TIFF and CCITT group 4 Fax.
[19] What is JPEG?
[20] I am looking for source of an H.261/H.263 codec and MPEG
[25] Fast DCT (Discrete Cosine Transform) algorithms
[26] Are there algorithms and standards for audio compression?

Common problems:

[30] My archive is corrupted!
[31] pkunzip reports a CRC error!
[32] VMS zip is not compatible with pkzip!
[33] I have a problem with Stacker or DoubleSpace!

Questions which do not really belong to comp.compression:

[50] What is this 'tar' compression program?
[51] I need a CRC algorithm
[52] What about those people who continue to ask frequently asked questions?
[53] Where are FAQ lists archived?
[54] I need specs for graphics formats
[55] Where can I find Lenna and other images?
[56] I am looking for a message digest algorithm
[57] I have lost my password on a .zip file

Part 2: (Long) introductions to data compression techniques

[70] Introduction to data compression (long)
       Huffman and Related Compression Techniques
       Arithmetic Coding
       Substitutional Compressors
          The LZ78 family of compressors
          The LZ77 family of compressors

[71] Introduction to MPEG (long)
       What is MPEG?
       Does it have anything to do with JPEG?
       Then what's JBIG and MHEG?
       What has MPEG accomplished?
       So how does MPEG I work?
       What about the audio compression?
       So how much does it compress?
       What's phase II?
       When will all this be finished?
       How do I join MPEG?
       How do I get the documents, like the MPEG I draft?

[72] What is wavelet theory?
[73] What is the theoretical compression limit?
[74] Introduction to JBIG
[75] Introduction to JPEG
[76] What is Vector Quantization?
[77] Introduction to Fractal compression
[78] The Burrows-Wheeler block sorting algorithm

Part 3: (Long) list of image compression hardware

[85] Image compression hardware
[99] Acknowledgments

Search for "Subject: [#]" to get to question number # quickly. Some news
readers can also take advantage of the message digest format used here.

If you know very little about data compression, read question 70 in
part 2 first.

Subject: [1] What are these newsgroups about? comp.compression is the place to discuss about data compression, both lossless (for text or data) and lossy (for images, sound, etc..). comp.compression.research was created later to provide a forum for current research on data compression and data compression algorithms; this group is now moderated. If you are not experienced in data compression, please post in comp.compression only. An archive of this newsgroup since Oct 1993 is available in Excellent collections of compression based information are provided at and If you only want to find a particular compression program for a particular operating system, please read first this FAQ and the article "How to find sources" which is regularly posted in news.answers. If you can't resist posting such a request, other groups are probably more appropriate (, comp.os.msdos.apps, comp.sources.wanted, comp.sys.mac.wanted, comp.archives.msdos.d, comp.dsp, Please post your request in comp.compression only as a last resource. If your question is about graphics only (no compression), please post to, *after* reading the FAQ (see item 54 below). For some unknown reason, many questions about graphics are incorrectly posted to comp.compression. For questions related to audio compression, check also comp.dsp. Please do not post any program in binary form to comp.compression. Very short sources can be posted, but long sources should be be posted to the specialized source groups, such as comp.sources.* or alt.sources. If the program is already available by ftp, just give the name of the ftp site and the full path name of the file. As for any newsgroups, do not post the same message separately to comp.compression and comp.compression.research.
Subject: [2] What is this .xxx file type? Where can I find the corresponding compression program? All the programs mentioned in this section are lossless. For most programs, one US and one European ftp site are given. ( and Many other sites (in particular have the same programs. To keep this list to a reasonable size, many programs are not mentioned here. When several programs can handle the same archive format, only one of them is given. If you don't find a particular archiver here, look also in: Sources for additional lossless data compressors can be found in (sources in Pascal) (Splay tree compression) For Macintosh programs, look on or in For VM/CMS, look on For Atari, look on For Amiga, look on A general purpose lossless data compression library is available in or; see for more information. Another library favoring speed over compression ratio is available at If you don't know how to use ftp or don't have ftp access, read the article "How to find sources" which is regularly posted in news.answers. If you can't find a program given below, a newer version probably exists in the same directory. Tell me at <jloup at> A very short description of the compression algorithm is given for most programs. For the meaning of LZ77, LZ78 and LZW, see question 70 in part 2 of the FAQ. If you are looking for the file format of a specific compression program, look at "The File Format Collection" in and/or get the sources of the decompressor. For the format of uuencode, do "man 5 uuencode" on a Unix box. ext: produced by or read by .ace: ACE .arc, .ark: arc, pkarc for MSDOS. (LZW algorithm) arc for Unix Contact: Howard Chu <> arc for VMS for Mac arc for Amiga .arj: arj for MSDOS (LZ77 with hashing, plus secondary static Huffman encoding on a block basis) Contact: Robert K Jung <> unarj for Unix. Decompresses only. (There is no arj compressor for Unix. Don't post a request.) unarj for Mac unarj for Amiga base64 (MIME encoding): This is *not* a compression issue but it keeps coming as a question on comp.compression. So: (source) (MSDOS exe) (Mac) .bck: VMS BACKUP. BACKUP is *not* a compression program. Do "help backup". .bz2: bzip2 by Julian Seward (Burrows-Wheeler block sorting, see item 78) .cab: Microsoft Cabinets .cpt: Compact Pro for Mac and Power PC For Unix: For DOS: .ddi: files made by DiskDupe (Pro) .exe: self-extracting MSDOS executable (creates files on disk when run) Run the file, or try unzip, lha or arj on it. .exe: compressed MSDOS executable (decompresses itself in memory then runs the decompressed code). To get the original uncompressed .exe: To create such files: (for Windows) .gif: gif files are images compressed with the LZW algorithm. See the FAQ list for programs manipulating .gif files. See suffix .Z below for source of LZW. .gz, .z: gzip (or pack, see .z below). gzip uses the same algorithm as zip 2.0x (see below); it can also extract packed and compressed files. Contact: Jean-loup Gailly <support at> For Unix, MSDOS, OS/2, VMS, Atari, Amiga, Primos: (.shar or .tar.gz: source) (MSDOS self-extract) (MSDOS) (source) (MSDOS exe) (WIN95 & NT) (OS/2) (VMS exe) (Solaris 2) (MVS exe) For Mac: (MacGzip page) .ha: ha 0.99 (improved PPMC - 4th order Markov modeling) Contact: Harri Hirvola <> .hap: Hamarsoft HAP archiver (Markov modeling + arithmetic coding) Contact: or .hpk: hpack (archiver with strong encryption) Contact: Peter Gutmann <> .hqx: Macintosh BinHex format.. (BinHex is *not* a compression program, it is similar to uuencode but handles multiple forks.) for Mac: for Unix: for MSDOS: .jam: JAM real-time compressor for MSDOS .lha: .lzh: lha for MSDOS (LZ77 with a trie data structure, plus secondary static Huffman coding on a block basis) lharc for Unix. (LZ77 with hash table and binary trees, plus secondary Huffman coding). See lha for Unix below. Warning: lharc can extract .lzh files created by lharc 1.xx but not those created by lha. lharc for VMS. Same warning as for Unix lharc. lha for Unix. Contact: or lha for Mac lha for Amiga lha for OS/2: MIME: see base64 above .pak: pak for MSDOS (LZW algorithm) .pit: PackIt (Macintosh) for Mac: for Unix: .pp: PowerPacker (Amiga) .rar: RAR Contact: Eugene Roshal <> or Andrey Spasibozhko <> MSDOS: rar*.exe rar*.exe rar*.exe *rar2*.exe Unix: rar*.exe Mac Amiga (Unrar): .sea: self-extracting archive (Macintosh) Run the file to extract it. The self-extraction code can be removed with: (MS Windows, .sit only) .sdn: used by the Shareware Distribution Network. Try the decompressors for .pak or .arj (see above) .shar: Shell archive. This is not a compression program. Use "sh foo.shar" to extract on Unix. (MSDOS) (Mac) .sit: Stuffit for Macintosh for Mac: for Amiga: for MSDOS: .?q?: Squeeze for MSDOS (do not confuse with other 'squeeze' below). Static Huffman coding. (squeeze) (unsqueeze) .sqz: Squeeze for MSDOS (do not confuse with other 'squeeze' above) LZ77 with hashing. .tar: tar is *not* a compression program. However, to be kind for you: for MSDOS for Unix tar (you have it already. To extract: tar xvf file.tar) for VMS for Macintosh for Amiga: .tar.Z, .tar-z, .taz: tar + compress For Unix: zcat file.tar.Z | tar xvf - with GNU tar: tar xvzf file.tar.Z for MSDOS: (MSDOS exe)* (sources)* (MSDOS exe) Other OS: first uncompress (see .Z below) then untar (see .tar above) .tar.gz, .tgz, .tar-gz, .tar.z: tar + gzip For Unix: gzip -cd file.tar.gz | tar xvf - with GNU tar: tar xvzf file.tar.gz for MSDOS: for MSDOS, Windows 95, NT & OS/2: or for Windows 95 & Windows NT: or Other OS: first uncompress (see .gz above) then untar (see .tar above) .td0: (compressed MS-DOS floppy image produced by TeleDisk) .uc2: UC2 for MSDOS and OS/2. (LZ77 with secondary static Huffman encoding on a block basis, and dynamic dictionaries shared among files.) Contact: (or uc2pro.exe) .z: pack or gzip (see .gz above). pack uses static Huffman coding. To extract, see .gz above. .zip: pkzip 2.04g for MSDOS. (LZ77 with hashing, plus secondary static Huffman coding on a block basis). Contact: or (WIN95) arcutil 2.0 for VM/CMS (unzip only, not yet compatible with pkzip 2.04)* zip 1.1 for Unix, MSDOS, VMS, OS/2, ... (compatible with pkzip 1.10. For corresponding unzip, see unzip 5.32 below). zip 2.2 and unzip 5.32 for Unix, MSDOS, VMS, OS/2, Amiga, ... Compatible with pkzip 2.04g (LZ77 with hashing, plus secondary static Huffman coding on a block basis). Contact: See also (On SGI, do not confuse with the editor also named 'zip'.) (source) (source) (MSDOS exe) (MSDOS exe) (Win95 & NT) (Win95 & NT) [The Win95 version supports long file names; MSDOS version doesn't] (OS/2 exe 16&32 bit) See also AMIGA, ATARI, MAC, UNIX, RISCOS, VMS... subdirectories. (encryption source) for Macintosh: WinZip by Nico Mak <> (uses Info-ZIP compress. code): or (MS Windows) .zoo: zoo 2.10 for MSDOS (algorithm copied from that of lha, see lha above) zoo 2.10 for Unix, VMS zoo for Mac zoo for Amiga .??_: Microsoft compress.exe and expand.exe. compress.exe is available in the Windows SDK (Software Development Kit) and in .F: freeze for Unix (LZ77 with hashing, plus secondary dynamic Huffman encoding) Contact: Leonid A. Broukhis <> .Y: yabba for Unix, VMS, ... (Y coding, a variant of LZ78) Contact: Dan Bernstein <> .Z: compress for Unix ('the' LZW algorithm) It is likely that your Unix system has 'compress' already. Otherwise: (not in .Z format to avoid chicken and egg problem) compress for MSDOS compress for Macintosh compress for Amiga compress for VAX/VMS
Subject: [3] What is the latest PKZIP version? The latest official DOS version is 2.04g. The latest command line version for Windows 95 is 2.50. The latest Windows 95 version is 2.70. See for more information. Be warned that there are countless bogus PKZIP 1.20, 2.0, 2.02, 3.00B, 3.05, 4.1 and whatever scams floating around. They usually are hacks of PKZIP 1.93A beta test version. Some of them are trojans and / or carry computer viruses. Note about pkzip 2.06 from a PKware employee: Version 2.06 was released as an INTERNAL use only IBM version. It is identical to 2.04G, but it has IBM names in the help screens and such. That release is meant for IBM only. If pkunzip indicates that you need version 2.8 to extract an archive, your archive has been corrupted by a transfer not made in binary mode (see item 30 below).
Subject: [4] What is an archiver? There is a distinction between archivers and other compression programs: - an archiver takes several input files, compresses them and produces a single archive file. Examples are arc, arj, lha, zip, zoo. - other compression programs create one compressed file for each input file. Examples are freeze, yabba, compress, gzip. Such programs are often combined with tar to create compressed archives (see question 50: "What is this tar compression program?"). For a comparison of zip and gzip, see the gzip README file. (In short: zip is an archiver, gzip is not; only zip is compatible with pkzip.)
Subject: [5] What is the best general purpose compression program? The answer is: it depends. (You did not expect a definitive answer, did you?) It depends whether you favor speed, compression ratio, a standard and widely used archive format, the number of features, etc... Just as for text editors, personal taste plays an important role. compress has 4 options, arj 2.30 has about 130 options; different people like different programs. *Please* do not start or continue flame wars on such matters of taste. Several benchmarks of MSDOS archivers are available: -*.zip and by Jeff Gilchrist <> - by Jonathan Burt <> Please do not post your own benchmarks made on your own files that nobody else can access. If you think that you must absolutely post yet another benchmark, make sure that your test files are available by anonymous ftp. Since all other benchmarks are for MSDOS only, here is one mainly for Unix, on a 33Mhz Compaq 386. All programs have been run on Unix SVR4, except pkzip and arj which only run on MSDOS. The programs compared here were chosen because they are the most popular or because they run on Unix and source is available. For ftp information, see above. Three programs (hpack, comp-2 and ha) have been added because they achieve better compression (at the expense of speed) and one program (lzrw3-a) has been added because it favors speed at the expense of compression: - comp-2 is in (inner zip file, - hpack is in - ha 0.98 is in - lzrw3-a is in The 14 files used in the comparison are from the standard Calgary Text Compression Corpus, available in The whole corpus includes 18 files, but the 4 files paper[3-6] are generally omitted in benchmarks. It contains several kinds of file (ascii, binary, image, etc...) but has a bias towards large files. You may well get different ratings on the typical mix of files that you use daily, so keep in mind that the comparisons given below are only indicative. The programs are ordered by decreasing total compressed size. For a fair comparison between archivers and other programs, this size is only the size of the compressed data, not the archive size. The programs were run on an idle machine, so the elapsed time is significant and can be used to compare Unix and MSDOS programs. [Note: These benchmarks are now *very* old. I have to do them again on more recent hardware with the latest programs. For recent results on MSDOS, check ] size lzrw3a compress lharc yabba pkzip freeze version: 4.0 1.02 1.0 1.10 2.3.5 options: -m300000 ------ ----- ------ ------ ------ ------ ------ bib 111261 49040 46528 46502 40456 41354 41515 book1 768771 416131 332056 369479 306813 350560 344793 book2 610856 274371 250759 252540 229851 232589 230861 geo 102400 84214 77777 70955 76695 76172 68626 news 377109 191291 182121 166048 168287 157326 155783 obj1 21504 12647 14048 10748 13859 10546 10453 obj2 246814 108040 128659 90848 114323 90130 85500 paper1 53161 24522 25077 21748 22453 20041 20021 paper2 82199 39479 36161 35275 32733 32867 32693 pic 513216 111000 62215 61394 65377 63805 53291 progc 39611 17919 19143 15399 17064 14164 14143 progl 71646 24358 27148 18760 23512 17255 17064 progp 49379 16801 19209 12792 16617 11877 11686 trans 93695 30292 38240 28092 31300 23135 22861 3,141,622 1,400,105 1,259,141 1,200,580 1,159,340 1,141,821 1,109,290 real 0m35s 0m59s 5m03s 2m40s 5m27s user 0m25s 0m29s 4m29s 1m46s 4m58s sys 0m05s 0m10s 0m07s 0m18s 0m08s MSDOS: 1m39s zoo lha arj pkzip zip hpack comp-2 ha 2.10 1.0(Unix) 2.30 2.04g 1.9 0.75a 0.98 ah 2.13(MSDOS) -jm -ex -6 a2 ------ ------ ------ ------ ------- ------ ------ ------ bib 40742 40740 36090 35126 34950 35619 29840 26927 book1 339076 339074 318382 312490 312619 306876 237380 235733 book2 228444 228442 210521 206513 206306 208486 174085 163535 geo 68576 68574 69209 68706 68418 58976 64590 59356 news 155086 155084 146855 144545 144395 141608 128047 123335 obj1 10312 10310 10333 10306 10295 10572 10819 9799 obj2 84983 84981 82052 81132 81336 80806 85465 80381 paper1 19678 19676 18710 18531 18525 18607 16895 15675 paper2 32098 32096 30034 29568 29674 29825 25453 23956 pic 52223 52221 53578 52409 55051 51778 55461 51639 progc 13943 13941 13408 13341 13238 13475 12896 11795 progl 16916 16914 16408 16122 16175 16586 17354 15298 progp 11509 11507 11308 11200 11182 11647 11668 10498 trans 22580 22578 20046 19462 18879 20506 21023 17927 1,096,166 1,096,138 1,036,934 1,019,451 1,021,043 1,005,367 890,976 845,854 real 4m07s 6m03s 1m49s 1h22m17s 27m05s user 3m47s 4m23s 1m43s 1h20m46s 19m27s sys 0m04s 0m08s 0m02s 0m12s 2m03s MSDOS: 1m49s 2m41s 1m43s 14m43s Notes: - the compressed data for 'zoo ah' is always two bytes longer than for lha. This is simply because both programs are derived from the same source (ar002, written by Haruhiko Okumura, available by ftp in - hpack 0.75a gives slightly different results on SunOS. (To be checked with latest version of hpack). - the MSDOS versions are all optimized with assembler code and were run on a RAM disk. So it is not surprising that they often go faster than their Unix equivalent.
Subject: [7] Which books should I read? [BWC 1989] Bell, T.C, Cleary, J.G. and Witten, I.H, "Text Compression", Prentice-Hall 1989. ISBN: 0-13-911991-4. Price: approx. US$60 The reference on text data compression. [Nel 1996] Mark Nelson & Jean-loup Gailly, "The Data Compression Book", 2nd edition. M&T Books, New York, NY 1996. ISBN 1-55851-434-1 541 pages. List price in the US is $39.95 including one PC-compatible disk bearing all the source code printed in the book. A practical introduction to data compression. The book is targeted at a person who is comfortable reading C code but doesn't know anything about data compression. Its stated goal is to get you up to the point where you are competent to program standard compression algorithms. [Will 1990] Williams, R. "Adaptive Data Compression", Kluwer Books, 1990. ISBN: 0-7923-9085-7. Price: US$75. Reviews the field of text data compression and then addresses the problem of compressing rapidly changing data streams. [Stor 1988] Storer, J.A. "Data Compression: Methods and Theory", Computer Science Press, Rockville, MD. ISBN: 0-88175-161-8. A survey of various compression techniques, mainly statistical non-arithmetic compression and LZSS compression. Includes complete Pascal code for a series of LZ78 variants. [Stor 1992] Storer, J.A. "Image and Text Compression", Kluwer Academic Publishers, 1992, ISBN 0-7923-9243-4 [Say 1996] Sayood, Khalid. "Introduction to Data Compression", San Francisco: Morgan Kaufmann Publishers, 1996. ISBN 1-55860-346-8; US&Canada $64.95. More info in The book covers both lossy and lossless compression techniques and their applications to image, speech, text, audio, and video compression. [HHJ 1997] Darrel Hankerson, Greg A. Harris, and Peter D. Johnson Jr. "Introduction to Information Theory and Data Compression", 1997. ISBN 0-8493-3985-5 [DS 1997] David Salomon, "Data Compression: The Complete Reference" Springer, 1997, ISBN 0-387-98280-9. Price: US$ 39.95. [BK 95] Bhaskaran V. and Konstantinides K., "Image and Video Compression Standards: Algorithms and Architectures", Kluwer Academic Publishers, 1995. ISBN 0-7923-9591-3 [ACG 1991] Advances in Speech Coding, edited by Atal, Cuperman, and Gersho, Kluwer Academic Press, 1991. [GG 1991] Vector Quantization and Signal Compression, by Gersho and Gray, Kluwer Acad. Press, 1991, ISBN 0-7923-9181-0. [CT 1991] Elements of Information Theory, by T.M.Cover and J.A.Thomas John Wiley & Sons, 1991. ISBN 0-471-06259-6. Review papers: [BWC 1989] Bell, T.C, Witten, I.H, and Cleary, J.G. "Modeling for Text Compression", ACM Computing Surveys, Vol.21, No.4 (December 1989), p.557 A good general overview of compression techniques (as well as modeling for text compression); the condensed version of "Text Compression". [Lele 1987] Lelewer, D.A, and Hirschberg, D.S. "Data Compression", ACM Computing Surveys, Vol.19, No.3 (September 1987), p.261. A survey of data compression techniques which concentrates on Huffman compression and makes only passing mention of other techniques. Bibliographies for Image Compression are available at
Subject: [8] What about patents on data compression algorithms? [Note: the appropriate group for discussing software patents is comp.patents or, not comp.compression.] Only a very small subset of all patents on data compression are mentioned here; there are several hundred patents on lossless data compression alone. All patents mentioned here are US patents, and thus probably not applicable outside the US. The abstracts and claims of all recent US patents can be obtained from See item 70, "Introduction to data compression" for the meaning of LZ77, LZ78 or LZW. (a) Run length encoding - Tsukiyama has two patents on run length encoding: 4,586,027 and 4,872,009 granted in 1986 and 1989 respectively. The first one covers run length encoding in its most primitive form: a length byte followed by the repeated byte. The second patent covers the 'invention' of limiting the run length to 16 bytes and thus the encoding of the length on 4 bits. Here is the start of claim 1 of patent 4,872,009, just for pleasure: 1. A method of transforming an input data string comprising a plurality of data bytes, said plurality including portions of a plurality of consecutive data bytes identical to one another, wherein said data bytes may be of a plurality of types, each type representing different information, said method comprising the steps of: [...] - O'Brien has patented (4,988,998) run length encoding followed by LZ77. (b) LZ77 - Waterworth patented (4,701,745) the algorithm now known as LZRW1, because Ross Williams reinvented it later and posted it on comp.compression on April 22, 1991. (See item 5 for the ftp site with all LZRW derivatives.) The *same* algorithm has later been patented by Gibson & Graybill (see below). The patent office failed to recognize that the same algorithm was patented twice, even though the wording used in the two patents is very similar. The Waterworth patent is now owned by Stac Inc, which won a lawsuit against Microsoft, concerning the compression feature of MSDOS 6.0. Damages awarded were $120 million. (Microsoft and Stac later settled out of court.) - Fiala and Greene obtained in 1990 a patent (4,906,991) on all implementations of LZ77 using a tree data structure. Claim 1 of the patent is much broader than the algorithms published by Fiala and Greene in Comm.ACM, April 89. The patent covers the algorithm published by Rodeh and Pratt in 1981 (J. of the ACM, vol 28, no 1, pp 16-24). It also covers the algorithms used in lharc, lha and zoo. - Notenboom (from Microsoft) 4,955,066 uses three levels of compression, starting with run length encoding. - The Gibson & Graybill patent 5,049,881 covers the LZRW1 algorithm previously patented by Waterworth and reinvented by Ross Williams. Claims 4 and 12 are very general and could be interpreted as applying to any LZ algorithm using hashing (including all variants of LZ78): 4. A compression method for compressing a stream of input data into a compressed stream of output data based on a minimum number of characters in each input data string to be compressed, said compression method comprising the creation of a hash table, hashing each occurrence of a string of input data and subsequently searching for identical strings of input data and if such an identical string of input data is located whose string size is at least equal to the minimum compression size selected, compressing the second and all subsequent occurrences of such identical string of data, if a string of data is located which does not match to a previously compressed string of data, storing such data as uncompressed data, and for each input strings after each hash is used to find a possible previous match location of the string, the location of the string is stored in the hash table, thereby using the previously processed data to act as a compression dictionary. Claim 12 is identical, with 'method' replaced with 'apparatus'. Since the 'minimal compression size' can be as small as 2, the claim could cover any dictionary technique of the LZ family. However the text of the patent and the other claims make clear that the patent should cover the LZRW1 algorithm only. (In any case the Gibson & Graybill patent is likely to be invalid because of the prior art in the Waterworth patent.) - Phil Katz, author of pkzip, also has a patent on LZ77 (5,051,745) but the claims only apply to sorted hash tables, and when the hash table is substantially smaller than the window size. - IBM patented (5,001,478) the idea of combining a history buffer (the LZ77 technique) and a lexicon (as in LZ78). - Stac Inc patented (5,016,009 and 5,126,739) yet another variation of LZ77 with hashing. The '009 patent was used in the lawsuit against Microsoft (see above). Stac also has a patent on LZ77 with parallel lookup in hardware (5,003,307). - Robert Jung, author of 'arj', has been granted patent 5,140,321 for one variation of LZ77 with hashing. This patent is very close to the LZRW3-A algorithm, also previously discovered by Ross Williams. LZRW3-A was posted on comp.compression on July 15, 1991. The patent was filed two months later on Sept 4, 1991. Microsoft has patented a similar idea (two level table with pseudo-LRU managment of slots inside the level-2 table) in 5,455,577 (filed in 1993). - Chambers 5,155,484 is yet another variation of LZ77 with hashing. The hash function is just the juxtaposition of two input bytes, this is the 'invention' being patented. The hash table is named 'direct lookup table'. (c) LZ78 - One form of the original LZ78 algorithm was patented (4,464,650) by its authors Lempel, Ziv, Cohn and Eastman. This patent is owned by Unisys. - The LZW algorithm used in 'compress' is patented by IBM (4,814,746) and Unisys (4,558,302). It is also used in the V.42bis compression standard (see question 11 on V.42bis below), in Postscript Level 2, in GIF and TIFF. Unisys sells the license to modem manufacturers for a onetime fee (contact: Welch Patent Desk, Unisys Corp., P.O. Box 500, Bluebell, PA 19424 Mailcode C SW 19). CompuServe is licensing the usage of LZW in GIF products for 1.5% of the product price, of which 1% goes to Unisys; usage of LZW in non-GIF products must be licensed directly from Unisys. For more information, see or email to The IBM patent application was first filed three weeks before that of Unisys, but the US patent office failed to recognize that they covered the same algorithm. (The IBM patent is more general, but its claim 7 is exactly LZW.) - Klaus Holtz also claims that patent 4,366,551 for his "autosophy" data compression method covers LZ78 and LZW. According to Holtz, most of the largest V.42bis modem manufacturers have paid for patent licenses. - AP coding is patented by Storer (4,876,541). (Get the yabba package for source code, see question 2 above, file type .Y) Storer also claims that his patent covers V.42bis. (d) arithmetic coding - IBM holds many patents on arithmetic coding (4,122,440 4,286,256 4,295,125 4,463,342 4,467,317 4,633,490 4,652,856 4,792,954 4,891,643 4,901,363 4,905,297 4,933,883 4,935,882 5,045,852 5,099,440 5,142,283 5,210,536 5,414,423 5,546,080). It has patented in particular the Q-coder implementation of arithmetic coding. The JBIG standard, and the arithmetic coding option of the JPEG standard requires use of the patented algorithm. No JPEG-compatible method is possible without infringing the patent, because what IBM actually claims rights to is the underlying probability model (the heart of an arithmetic coder). (See item 75 for details.) See also below details on many other patents on arithmetic coding (4,973,961 4,989,000 5,023,611 5,025,258 5,272,478 5,307,062 5,309,381 5,311,177 5,363,099 5,404,140 5,406,282 5,418,532). The list is not exhaustive. (e) predictor - The 'predictor' algorithm was first described in the paper Raita, T. and Teuhola, J. (1987), "Predictive text compression by hashing", ACM Conference on Information Retrieval This algorithm has been patented (5,229,768) by K. Thomas in 1993. It is used in the Internet Draft "PPP Predictor Compression Protocol" (see (f) compression of random data - The US patent office no longer grants patents on perpetual motion machines, but has recently granted a patent on a mathematically impossible process (compression of truly random data): 5,533,051 "Method for Data Compression". See item 9.5 of this FAQ for details. As can be seen from the above list, some of the most popular compression programs (compress, pkzip, zoo, lha, arj) are now covered by patents. (This says nothing about the validity of these patents.) Here are some references on data compression patents. Some of them are taken from the list 3,914,586 Data compression method and apparatus filed 10/25/73, granted 10/21/75 General Motors Corporation, Detroit MI Duane E. McIntosh, Santa Ynez CA Data compression apparatus is disclosed is operable in either a bit pair coding mode of a word coding mode depending on the degree of redundancy of the data to be encoded. 3,976,844 Data communication system for transmitting data in compressed form filed Apr. 4, 1975, granted Aug. 24, 1976 inventor Bernard K. Betz, assignee Honeywell Information Systems, Inc. [encode differences with previous line] 4,021,782 Data compaction system and apparatus inventor Hoerning filed 04/30/1975, granted 05/03/1977 [A primitive form of LZ77 with implicit offsets (compare with previous record)] 4,054,951 Data expansion apparatus inventor R.D. Jackson, assignee IBM filed Jun. 30, 1976, granted Oct. 18, 1977 [Covers only decompression of data compressed with a variant of LZ77.] 4,087,788 Data compression system filed 1/14/77, granted 5/2/78 NCR Canada LTD - NCR Canada Ltee, Mississauga CA Brian J. Johannesson, Waterloo CA A data compression system is disclosed in which the left hand boundary of a character is developed in the form of a sequence of Freeman direction codes, the codes being stored in digital form within a processor. 4,122,440 Method and means for arithmetic string coding assignee IBM filed 1977/03/04, granted 1978/10/24 [This is the basic idea of arithmetic coding. Note that the patent is expired now.] 4,286,256 Method and means for arithmetic coding using a reduced number of operations. granted Aug 25, 1981 assignee IBM 4,295,125 A method and means for pipeline decoding of the high to low order pairwise combined digits of a decodable set of relatively shifted finite number of strings granted Oct 13, 1981 assignee IBM 4,366,551 Associative Memory Search System filed June 16, 1975, granted Dec. 28, 1982. inventor Klaus Holtz, assignee Omni Dimensional Networks. 4,412,306 System for minimizing space requirements for storage and transmission of digital signals filed May 14, 1981, granted Oct. 25, 1983 inventor Edward W. Moll 4,463,342 A method and means for carry-over control in a high order to low order combining of digits of a decodable set of relatively shifted finite number strings. granted Jul 31, 1984 assignee IBM 4,491,934 Data compression process filed May 12, 1982, granted Jan. 1, 1985 inventor Karl E. Heinz 4,464,650 Apparatus and method for compressing data signals and restoring the compressed data signals inventors Lempel, Ziv, Cohn, Eastman assignee Sperry Corporation (now Unisys) filed 8/10/81, granted 8/7/84 A compressor parses the input data stream into segments where each segment comprises a prefix and the next symbol in the data stream following the prefix. [This is the original LZ78 algorithm.] 4,467,317 High-speed arithmetic compression using using concurrent value updating. granted Aug 21, 1984 assignee IBM 4,494,108 Adaptive source modeling for data file compression within bounded memory filed Jun. 5, 1984, granted Jan. 15, 1985 invntors Glen G. Langdon, Jorma J. Rissanen assignee IBM order 1 Markov modeling 4,558,302 High speed data compression and decompression apparatus and method inventor Welch assignee Sperry Corporation (now Unisys) filed 6/20/83, granted 12/10/85 re-examined: filed 12/14/92, granted 4/1/94. The text of the original 1985 patent can be ftped from There is also a European Patent 0,129,439 1/2/89 for DE, FR, GB, IT and patent pending for Japan. 4,560,976 Data compression filed 6/5/84, granted 12/24/85 Codex Corporation, Mansfield MA Steven G. Finn, Framingham, MA A stream of source characters, which occur with varying relative frequencies, is encoded into a compressed stream of codewords, each having one, two or three subwords, by ranking the source characters by their current frequency of appearance, encoding the source characters having ranks no higher than a first number as one subword codewords, source characters having ranks higher than the first number but no higher than a second number as two subword codewords, and the remaining source characters as three subword codewords. 4,586,027 Method and system for data compression and restoration inventor Tsukimaya et al. assignee Hitachi filed 08/07/84, granted 04/29/86 patents run length encoding 4,597,057 System for compressed storate of 8-bit ascii bytes using coded strings of 4-bit nibbles. inventor Snow, assignee System Development corporation. filed 12/31/1981, granted 06/24/1986. Compression using static dictionary of common words, prefixes and suffixes. 4,612,532 Data compression apparatus and method inventor Bacon, assignee Telebyte Corportion filed Jun. 19, 1984, granted Sep. 16, 1986 [Uses followsets as in the pkzip 0.92 'reduce' algorithm, but the followsets are dynamically updated. This is in effect a sort of order-1 Markov modeling.] 4,622,545 Method and apparatus for image compression and Manipulation inventor William D. Atkinson assignee Apple computer Inc. filed 9/30/82 granted 11/11/86 4,633,490 Symmetrical adaptive data compression/decompression system. granted Dec 30, 1985 assignee IBM 4,652,856 A multiplication-free multi-alphabet arithmetic code. granted Feb 4, 1986 assignee IBM 4,667,649 Data receiving apparatus filed 4/18/84, granted 6/30/87 inventors Kunishi et al. assignee Canon Kabushiki Kaisha, Tokyo Japan compression of Fax images. 4,682,150 Data compression method and apparatus inventors Mathes and Protheroe, assignee NCR Corporation, Dayton OH A system and apparatus for compressing redundant and nonredundant binary data generated as part of an operation of a time and attendance terminal in which the data represents the time an employee is present during working hours. 4,701,745 Data compression system inventor Waterworth John R assignee Ferranti PLC GB, patent rights now acquired by Stac Inc. filed 03/03/1986 (03/06/1985 in GB), granted 10/20/1987 Algorithm now known as LZRW1 (see above) I claim: 1. A data compression system comprising an input store for receiving and storing a plurality of bytes of uncompressed data from an outside source, and data processing means for processing successive bytes of data from the input store; the data processing means including circuit means operable to check whether a sequence of successive bytes to be processed identical with a sequence of bytes already processed, and including hash generating means responsive to the application of a predetermined number of bytes in sequence to derive a hash code appropriate to those bytes, a temporary store in which the hash code may represent the address of a storage location, and a pointer counter operable to store in the temporary store at said address a pointer indicative of the position in the input store of one of the predetermined number of bytes; output means operable to apply to a transfer medium each byte of data not forming part of such an identical sequence; and encoding means responsive to the identification of such a sequence to apply to the transfer medium an identification signal which identifies both the location in the input store of the previous occurrence of the sequence of bytes and the number of bytes contained in the sequence. 4,730,348 Adaptive data compression system inventor MacCrisken, assignee Adaptive Computer Technologies filed Sep. 19, 1986, granted Mar. 8, 1988 [order-1 Markov modeling + Huffman coding + LZ77] 4,758,899 Data compression control device inventor Tsukiyama, assignee Hitachi filed 11/20/1985, granted 07/19/1988 Limits compression to ensure that tape drive stays busy. 4,792,954 Concurrent detection of errors in arithmetic data compression coding assignee IBM filed 1986/10/31, granted 1988/12/20 4,809,350 Data compression system filed Jan. 30, 1987, granted Feb. 28, 1989 inventor Yair Shimoni & Ron Niv assignee Elscint Ltd., Haifa, Israel [Image compression via variable length encoding of differences with predicted data.] 4,814,746 Data compression method inventors Victor S. Miller, Mark N. Wegman assignee IBM filed 8/11/86, granted 3/21/89 A previous application was filed on 6/1/83, three weeks before the application by Welch (4,558,302) Communications between a Host Computing System and a number of remote terminals is enhanced by a data compression method which modifies the data compression method of Lempel and Ziv by addition of new character and new string extensions to improve the compression ratio, and deletion of a least recently used routine to limit the encoding tables to a fixed size to significantly improve data transmission efficiency. 4,841,092 continued in 5,003,307 4,853,696 Code converter for data compression/decompression filed 4/13/87, granted 8/1/89 inventor Amar Mukherjee, Maitland FL assignee University of Central Florida, Orlando FL Another hardware Huffman encoder: A code converter has a network of logic circuits connected in reverse binary tree fashion with logic paths between leaf nodes and a common root node. 4,872,009 Method and apparatus for data compression and restoration inventor Tsukimaya et al. assignee Hitachi filed 12/07/87, granted 10/03/89 This patent on run length encoding covers the 'invention' of limiting the run length to 16 bytes and thus the encoding of the length on 4 bits. 4,876,541 Stem [sic] for dynamically compressing and decompressing electronic data filed 10/15/87, granted 10/24/89 inventor James A. Storer assignee Data Compression Corporation A data compression system for encoding and decoding textual data, including an encoder for encoding the data and for a decoder for decoding the encoded data. 4,891,643 Arithmetic coding data compression/de-compression by selectively employed, diverse arithmetic encoders and decoders. file 1986/09/15, granted 1990/01/02 assignee IBM 4,901,363 System for compressing bi-level data assignee IBM [arithmetic coding] 4,905,297 Arithmetic coding encoder and decoder system. granted Feb 27, 1990 assignee IBM 4,906,991 Textual substitution data compression with finite length search window filed 4/29/1988, granted 3/6/1990 inventors Fiala,E.R., and Greene,D.H. assignee Xerox Corporation extended in 5,058,144 4,933,883 Probability adaptation for arithmetic coders. granted Jun 12, 1990 assignee IBM 4,935,882 Probability adaptation for arithmetic coders. granted Jun 19, 1990 assignee IBM 4,941,193 Barnsley, fractal compression. 4,943,869 Compression Method for Dot Image Data filed 1988-05-04, granted 1990-07-24 assignee Fuji Photo Film Co. Lossy and lossless image compression schemes. 4,955,066 Compressing and Decompressing Text Files filed 10/13/89, granted 09/04/90 inventor Notenboom, L.A. assignee Microsoft Now extended as 5,109,433 [Noted in signon screen of Word 5.5 and on the outside of the MS-DOS 5.0 Upgrade.] A method of compressing a text file in digital form is disclosed. A full text file having characters formed into phrases is provided by an author. The characters are digitally represented by bytes. A first pass compression is sequentially followed by a second pass compression of the text which has previously been compressed. A third or fourth level of compression is serially performed on the compressed text. For example, in a first pass, the text is run-length compressed. In a second pass, the compressed text is further compressed with key phrase compression. In a third pass, the compressed text is further compressed with Huffman compression. The compressed text is stored in a text file having a Huffman decode tree, a key phrase table, and a topic index. The data is decompressed in a single pass and provided one line at a time as an output. Sequential compressing of the text minimizes the storage space required for the file. Decompressing of the text is performed in a single pass. As a complete line is decompressed, it is output rapidly, providing full text to the user. 4,973,961 Method and apparatus for carry-over control in arithmetic coding. granted Nov 27, 1990 assignee AT&T 4,988,998 Data compression system for successively applying at least two data compression methods to an input data stream. inventor O'Brien assignee Storage Technology Corporation, Louisville, Colorado filed Sep 5, 1989, granted Jan 29, 1991. Run length encoding followed by LZ77. 4,989,000 Data string compression using arithmetic encoding with simplified probability subinterval estimation filed 1989/06/19, granted 1991/01/29] [shift & add instead of multiply] 5,001,478 Method of Encoding Compressed Data filed 12/28/89, granted 03/19/91 inventor Michael E. Nagy assignee IBM 1. A method of encoding a compressed data stream made up of a sequence of literal references, lexicon references and history references, which comprises the steps of: assigning to each literal reference a literal identifier; assigning to each history reference a history identifier; assigning to each lexicon reference a lexicon identifier; and emitting a data stream with said identifiers assigned to said references. Gordon Irlam <> says: The invention can probably be best understood by considering the decompressor. It consists of a history buffer, and a lexicon buffer, both of which are initially empty. The history buffer contains the last n symbols emitted. Whenever a history buffer reference is to be output the string so referenced is subsequently moved to the lexicon buffer for future reference. Thus the history buffer keeps track of strings that may be repeated on a very short term basis, while the lexicon buffer stores items for a longer time. Furthermore a history reference involves specifying both the offset and length within the history buffer, whereas a lexicon reference simply specifies a number denoting the string. Both buffers have a finite size. 5,003,307 Data compression apparatus with shift register search means filed Oct. 6, 1989, granted Mar. 26, 1991 inventors George Glen A, Ivey Glen E, Whiting Douglas L assignee Stac Inc continuation of 4,841,092 5,016,009 Data compression apparatus and method filed 01/13/1989, granted 05/14/1991 inventors George Glen A, Ivey Glen E, Whiting Douglas L assignee Stac Inc LZ77 with offset hash table (extended in 5,126,739) 5,023,611 Entropy encoder/decoder including a context extractor. granted Jun 11, 1991 assignee AT&T 5,025,258 Adaptive probability estimator for entropy encoder/decoder. granted Jun 18, 1991 assignee AT&T 5,045,852 Dynamic model selection during data compression assignee IBM [arithmetic coding] 5,049,881 Apparatus and method for very high data rate-compression incorporating lossless data compression and expansion utilizing a hashing technique inventors Dean K. Gibson, Mark D. Graybill assignee Intersecting Concepts, Inc. filed 6/18/90, granted 9/17/91 [covers lzrw1, almost identical with Waterworth 4,701,745] 5,051,745 String searcher, and compressor using same filed 8/21/90, granted 9/24/91 inventor Phillip W. Katz (author of pkzip) In the string search method and apparatus pointers to the string to be searched are indexed via a hashing function and organized according to the hashing values of the string elements pointed to. The hashing function is also run on the string desired to be found, and the resulting hashing value is used to access the index. If the resulting hashing value is not in the index, it is known that the target string does not appear in the string being searched. Otherwise the index is used to determine the pointers which correspond to the target hashing value, these pointers pointing to likely candidates for matching the target string. The pointers are then used to sequentially compare each of the locations in the string being searched to the target string, to determine whether each location contains a match to the target string. In the method and apparatus for compressing a stream of data symbols, a fixed length search window, comprising a predetermined contiguous portion of the symbol stream, is selected as the string to be searched by the string searcher. If a string to be compressed is found in the symbol stream, a code is output designating the location within the search window of the matching string and the length of the matching string. 5,065,447 (continued in 5,347,600) Method and apparatus for processing digital data filed Jul. 5, 1989, granted Nov. 12, 1991 inventors Michael F. Barnsley and Alan D. Sloan [Patents image compression with the "Fractal Transform"] 5,099,440 Probability adaptation for arithmetic coders 5,109,433 Compressing and decompressing text files inventor Notenboom assignee Microsoft extension of 4,955,066 5,126,739 Data Compression Apparatus and Method filed Nov. 27, 1990, granted June 30, 1992. inventor Whiting et. al assignee Stac Inc LZ77 with offset hash table (extension of 5,016,009) 5,140,321 Data compression/decompression method and apparatus filed 9/4/91, granted 8/18/92 inventor Robert Jung assignee Prime Computer 5,142,283 Arithmetic compression coding using interpolation for ambiguous symbols filed 1990/07/10, granted 1992/08/25 assignee IBM 5,155,484 Fast data compressor with direct lookup table indexing into history buffer filed 9/13/1991, granted 10/13/1992 inventor Chambers, IV, Lloyd L., Menlo Park, California assignee Salient Software, Inc., Palo Alto, California (02) Uses a 64K hash table indexed by the first two characters of the input string. Includes several claims on the LZ77 file format (literal or pair offset,length). 5,179,378 file Jul. 30, 1991, granted Jan. 12, 1993 inventor Ranganathan assignee University of South Florida Method and apparatus for the compression and decompression of data using Lempel-Ziv based techniques. [This covers LZ77 hardware compression with a systolic array of processors working in parallel.] 5,210,536 Data compression/coding method and device for implementing said method assignee IBM [PPM + arithmetic coding] 5,229,768 Adaptive data compression system granted Jul. 20, 1993 inventor Kasman E. Thomas assignee Traveling Software, Inc. A system for data compression and decompression is disclosed. A series of fixed length overlapping segments, called hash strings, are formed from an input data sequence. A retrieved character is the next character in the input data sequence after a particular hash string. A hash function relates a particular hash string to a unique address in a look-up table (LUT). An associated character for the particular hash string is stored in the LUT at the address. When a particular hash string is considered, the content of the LUT address associated with the hash string is checked to determine whether the associated character matches the retrieved character following the hash string. If there is a match, a Boolean TRUE is output; if there is no match, a Boolean FALSE along with the retrieved character is output. Furthermore, if there is no match, then the LUT is updated by replacing the associated character in the LUT with the retrieved character. [...] [This algorithm is used in the Internet draft "PPP Predictor Compression Protocol".] 5,272,478 Method and apparatus for entropy coding assignee Ricoh [arithmetic coding with finite state machine] 5,307,062 Coding system filed 1992/12/15, granted 1994/04/26 assignee Mitsubishi [binary arithmetic coding, see also 5,404,140] 5,309,381 Probability estimation table apparatus filed 1992/04/08, granted 1994/05/03 assignee Ricoh [arithmetic coding] 5,311,177 Code transmitting apparatus with limited carry propagation filed 1992/06/19, granted 1994/05/10 assignee Mitsubishi [arithmetic coding] 5,347,600 (continuation of 5,065,447) Method and apparatus for compression and decompression of digital image filed 10/23/1991, granted 09/13/1994 inventors Barnsley and Sloan 5,363,099 Method and apparatus for entropy coding [arithmetic coding with state machine] 5,384,867 (continued in 5,430,812) filed 10/23/1991, granted 01/24/1995 Fractal transform compression board inventors Barnsley et al. 5,404,140 Coding system filed 1994/01/13, granted 1995/04/04 assignee Mitsubishi [binary arithmetic coding, see also 5,307,062] 5,406,282 Data coding and decoding with improved efficiency assignee Ricoh [PPM & arithmedic coding] 5,414,423 Stabilization of probability estimates by conditioning on prior decisions of a given context assignee IBM arithmetic coding] 5,416,856 Method of encoding a digital image using iterated image transformations to form an eventually contractive map filed 1992/03/30, granted 1995/05/16 inventors Jacobs, Boss and Fisher 5,418,532 Method and system for efficient, multiplication-free arithmetic coding filed 1993/05/13, granted 1995/05/23. inventors Lei & Shaw-Min assignee Bell Communications Research, Inc. (Livingston, NJ). 5,430,812 (continuation of 5,384,867) Fractal transform compression board filed 1994/05/18, granted 1995/07/04 inventors Barnsley et al. 5,455,577 Method and system for data compression filed 1993/03/12, granted 1995/10/03 inventors Slivka & Rashid, assignee Microsoft LZ77 with two-level search data structure 5,533,051 Method for Data Compression filed 1993/03/12, granted 1996/07/02 inventor David C. James, assignee The James Group This is a patent on compression of random data, see item 9.5 below. Japan 2-46275 Coding system granted Feb 26, 1990 [Patents one form of arithmetic coding.]
Subject: [9] Compression of random data (WEB, Gilbert and others) [Note from the FAQ maintainer: this topic has generated and is still generating the greatest volume of news in the history of comp.compression. Read this before posting on this subject. I intended to remove the WEB story from the FAQ, but similar affairs come up regularly on comp.compression. The advertized revolutionary methods have all in common their supposed ability to compress random or already compressed data. I will keep this item in the FAQ to encourage people to take such claims with great precautions.] 9.1 Introduction It is mathematically impossible to create a program compressing without loss *all* files by at least one bit (see below and also item 73 in part 2 of this FAQ). Yet from time to time some people claim to have invented a new algorithm for doing so. Such algorithms are claimed to compress random data and to be applicable recursively, that is, applying the compressor to the compressed output of the previous run, possibly multiple times. Fantastic compression ratios of over 100:1 on random data are claimed to be actually obtained. Such claims inevitably generate a lot of activity on comp.compression, which can last for several months. Large bursts of activity were generated by WEB Technologies and by Jules Gilbert. Premier Research Corporation (with a compressor called MINC) made only a brief appearance but came back later with a Web page at The Hyper Space method invented by David C. James is another contender with a patent obtained in July 96. Another large burst occured in Dec 97 and Jan 98: Matthew Burch <> applied for a patent in Dec 97, but publicly admitted a few days later that his method was flawed; he then posted several dozen messages in a few days about another magic method based on primes, and again ended up admitting that his new method was flawed. (Usually people disappear from comp.compression and appear again 6 months or a year later, rather than admitting their error.) Other people have also claimed incredible compression ratios, but the programs (OWS, WIC) were quickly shown to be fake (not compressing at all). This topic is covered in item 10 of this FAQ. 9.2 The counting argument [This section should probably be called "The counting theorem" because some people think that "argument" implies that it is only an hypothesis, not a proven mathematical fact. The "counting argument" is actually the proof of the theorem.] The WEB compressor (see details in section 9.3 below) was claimed to compress without loss *all* files of greater than 64KB in size to about 1/16th their original length. A very simple counting argument shows that this is impossible, regardless of the compression method. It is even impossible to guarantee lossless compression of all files by at least one bit. (Many other proofs have been posted on comp.compression, please do not post yet another one.) Theorem: No program can compress without loss *all* files of size >= N bits, for any given integer N >= 0. Proof: Assume that the program can compress without loss all files of size >= N bits. Compress with this program all the 2^N files which have exactly N bits. All compressed files have at most N-1 bits, so there are at most (2^N)-1 different compressed files [2^(N-1) files of size N-1, 2^(N-2) of size N-2, and so on, down to 1 file of size 0]. So at least two different input files must compress to the same output file. Hence the compression program cannot be lossless. The proof is called the "counting argument". It uses the so-called pigeon-hole principle: you can't put 16 pigeons into 15 holes without using one of the holes twice. Much stronger results about the number of incompressible files can be obtained, but the proofs are a little more complex. (The MINC page uses one file of strictly negative size to obtain 2^N instead of (2^N)-1 distinct files of size <= N-1 .) This argument applies of course to WEB's case (take N = 64K*8 bits). Note that no assumption is made about the compression algorithm. The proof applies to *any* algorithm, including those using an external dictionary, or repeated application of another algorithm, or combination of different algorithms, or representation of the data as formulas, etc... All schemes are subject to the counting argument. There is no need to use information theory to provide a proof, just very basic mathematics. [People interested in more elaborate proofs can consult ] In short, the counting argument says that if a lossless compression program compresses some files, it must expand others, *regardless* of the compression method, because otherwise there are simply not enough bits to enumerate all possible output files. Despite the extreme simplicity of this theorem and its proof, some people still fail to grasp it and waste a lot of time trying to find a counter-example. This assumes of course that the information available to the decompressor is only the bit sequence of the compressed data. If external information such as a file name, a number of iterations, or a bit length is necessary to decompress the data, the bits necessary to provide the extra information must be included in the bit count of the compressed data. Otherwise, it would be sufficient to consider any input data as a number, use this as the file name, iteration count or bit length, and pretend that the compressed size is zero. For an example of storing information in the file name, see the program lmfjyh in the 1993 International Obfuscated C Code Contest, available on all comp.sources.misc archives (Volume 39, Issue 104). A common flaw in the algorithms claimed to compress all files is to assume that arbitrary bit strings can be sent to the decompressor without actually transmitting their bit length. If the decompressor needs such bit lengths to decode the data (when the bit strings do not form a prefix code), the number of bits needed to encode those lengths must be taken into account in the total size of the compressed data. Another common (but still incorrect) argument is to assume that for any file, some still to be discovered algorithm might find a seed for a pseudo-random number generator which would actually generate the whole sequence of bytes contained in the file. However this idea still fails to take into account the counting argument. For example, if the seed is limited to 64 bits, this algorithm can generate at most 2^64 different files, and thus is unable to compress *all* files longer than 8 bytes. For more details about this "magic function theory", see Yet another popular idea is to split the input bit stream into a sequence of large numbers, and factorize those numbers. Unfortunately, the number of bits required to encode the factors and their exponents is on average not smaller than the number of bits of the original bit stream, so this scheme too cannot compress all data. Another idea also related to primes is to encode each number as an index into a table of primes and an offset relative to the indexed prime; this idea doesn't work either because the number of bits required to encode the index, the offset and the separation between index and offset is on average not smaller than the number of bits of the original bit stream. Steve Tate <> suggests a good challenge for programs that are claimed to compress any data by a significant amount: Here's a wager for you: First, send me the DEcompression algorithm. Then I will send you a file of whatever size you want, but at least 100k. If you can send me back a compressed version that is even 20% shorter (80k if the input is 100k) I'll send you $100. Of course, the file must be able to be decompressed with the program you previously sent me, and must match exactly my original file. Now what are you going to provide when... er... if you can't demonstrate your compression in such a way? So far no one has accepted this challenge (for good reasons). Mike Goldman <> makes another offer: I will attach a prize of $5,000 to anyone who successfully meets this challenge. First, the contestant will tell me HOW LONG of a data file to generate. Second, I will generate the data file, and send it to the contestant. Last, the contestant will send me a decompressor and a compressed file, which will together total in size less than the original data file, and which will be able to restore the compressed file to the original state. With this offer, you can tune your algorithm to my data. You tell me the parameters of size in advance. All I get to do is arrange the bits within my file according to the dictates of my whim. As a processing fee, I will require an advance deposit of $100 from any contestant. This deposit is 100% refundable if you meet the challenge. 9.3 The WEB 16:1 compressor 9.3.1 What the press says April 20, 1992 Byte Week Vol 4. No. 25: "In an announcement that has generated high interest - and more than a bit of skepticism - WEB Technologies (Smyrna, GA) says it has developed a utility that will compress files of greater than 64KB in size to about 1/16th their original length. Furthermore, WEB says its DataFiles/16 program can shrink files it has already compressed." [...] "A week after our preliminary test, WEB showed us the program successfully compressing a file without losing any data. But we have not been able to test this latest beta release ourselves." [...] "WEB, in fact, says that virtually any amount of data can be squeezed to under 1024 bytes by using DataFiles/16 to compress its own output multiple times." June 1992 Byte, Vol 17 No 6: [...] According to Earl Bradley, WEB Technologies' vice president of sales and marketing, the compression algorithm used by DataFiles/16 is not subject to the laws of information theory. [...] ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 9.3.2 First details, by John Wallace <> I called WEB at (404)514-8000 and they sent me some product literature as well as chatting for a few minutes with me on the phone. Their product is called DataFiles/16, and their claims for it are roughly those heard on the net. According to their flier: "DataFiles/16 will compress all types of binary files to approximately one-sixteenth of their original size ... regardless of the type of file (word processing document, spreadsheet file, image file, executable file, etc.), NO DATA WILL BE LOST by DataFiles/16." (Their capitalizations; 16:1 compression only promised for files >64K bytes in length.) "Performed on a 386/25 machine, the program can complete a compression/decompression cycle on one megabyte of data in less than thirty seconds" "The compressed output file created by DataFiles/16 can be used as the input file to subsequent executions of the program. This feature of the utility is known as recursive or iterative compression, and will enable you to compress your data files to a tiny fraction of the original size. In fact, virtually any amount of computer data can be compressed to under 1024 bytes using DataFiles/16 to compress its own output files muliple times. Then, by repeating in reverse the steps taken to perform the recusive compression, all original data can be decompressed to its original form without the loss of a single bit." Their flier also claims: "Constant levels of compression across ALL TYPES of FILES" "Convenient, single floppy DATA TRANSPORTATION" From my telephone conversation, I was assured that this is an actual compression program. Decompression is done by using only the data in the compressed file; there are no hidden or extra files. 9.3.3 More information, by Rafael Ramirez <>: Today (Tuesday, 28th) I got a call from Earl Bradley of Web who now says that they have put off releasing a software version of the algorithm because they are close to signing a major contract with a big company to put the algorithm in silicon. He said he could not name the company due to non-disclosure agreements, but that they had run extensive independent tests of their own and verified that the algorithm works. [...] He said the algorithm is so simple that he doesn't want anybody getting their hands on it and copying it even though he said they have filed a patent on it. [...] Mr. Bradley said the silicon version would hold up much better to patent enforcement and be harder to copy. He claimed that the algorithm takes up about 4K of code, uses only integer math, and the current software implementation only uses a 65K buffer. He said the silicon version would likely use a parallel version and work in real-time. [...] 9.3.4 No software version Appeared on BIX, reposted by Bruce Hoult <>: tojerry/chaos #673, from abailey, 562 chars, Tue Jun 16 20:40:34 1992 Comment(s). ---------- TITLE: WEB Technology I promised everyone a report when I finally got the poop on WEB's 16:1 data compression. After talking back and forth for a year and being put off for the past month by un-returned phone calls, I finally got hold of Marc Spindler who is their sales manager. _No_ software product is forth coming, period! He began talking about hardware they are designing for delivery at the end of the year. [...] 9.3.5 Product cancelled Posted by John Toebes <> on Aug 10th, 1992: [Long story omitted, confirming the reports made above about the original WEB claims.] 10JUL92 - Called to Check Status. Was told that testing had uncovered a new problem where 'four numbers in a matrix were the same value' and that the programmers were off attempting to code a preprocessor to eliminate this rare case. I indicated that he had told me this story before. He told me that the programmers were still working on the problem. 31JUL92 - Final Call to Check Status. Called Earl in the morning and was told that he still had not heard from the programmers. [...] Stated that if they could not resolve the problem then there would probably not be a product. 03AUG92 - Final Call. Earl claims that the programmers are unable to resolve the problem. I asked if this meant that there would not be a product as a result and he said yes. 9.3.6 Byte's final report Extract from the Nov. 95 issue of Byte, page 42: Not suprisingly, the beta version of DataFiles/16 that reporter Russ Schnapp tested didn't work. DataFiles/16 compressed files, but when decompressed, those files bore no resemblance to their originals. WEB said it would send us a version of the program that worked, but we never received it. When we attempted to follow up on the story about three months later, the company's phone had been disconnected. Attempts to reach company officers were also unsuccessful. [...] 9.4 Jules Gilbert As opposed to WEB Technologies, Jules Gilbert <> does not claim to compress *all* files, but only "random or random-appearing" files. Here are some quotes from a few of Mr Gilbert's articles, which can be helpful to get a better idea of his claims. No comments or conclusions are given; if you need more information contact Mr. Gilbert directly. From: (Jules Gilbert) Newsgroups: comp.compression Subject: Re: No Magic Compressors, No Factoring Compressors, Jules Gilbert is a liar Date: 14 May 1996 03:13:31 -0400 Message-ID: <4n9bqr$> [...] I will, in front of several Boston area computer scientists ('monitors'), people I choose but generally known to be fair and competent, under conditions which are sufficient to prevent disclosure of the method and fully protect the algorithm and other aspects of the underlying method from untoward discovery, use two computers, (which I am permitted to examine but not alter) with both machine's running Linux, and with the file-systems and Linux OS freshly restored from commercial CD-ROM's do the following: On one machine (the 'src-CPU') will be loaded a copy of the CALGARY-CORPUS. (Or other agreed on '.ZIP' or '.ARJ' file.) I will compress the CALGARY-CORPUS for transfer from the src-CPU onto 3.5" disks and transfer it (by sneaker-net) to the other machine for decompression and produce a perfect copy of the CORPUS file on the 'dst-CPU'. The CORPUS archive contents will not be 'cracked', ie', the original CORPUS can be encrypted and the password kept from me. All I care about is that the input file is highly random-aprearing. I claim that I can perform this process several times, and each iteration will reduce the overall file by at least 50%, ie., a ratio of 2:1. An 'iteration' will constitute copying, using compression, from the src-CPU to the dst-CPU, and then reversing the direction to achieve another iteration. For example, for say a 4M input file, it is reasonable to expect an approximately 1M output file, after two complete iterations. [...] ONLY RANDOM OR RANDOM-APPEARING DATA INPUT CAN BE COMPRESSED BY MY METHOD. [...] If one iteration (of the compression 'sandwich') consists of two parts, say an LZ phase followed by a JG phase, the LZ method will compression by perhaps a ration of 2:1 (at the first iteration), perhaps much better if the input is text, and the JG phase will do 3-4:1, but slowly!! During subsequent iterations, the LZ phase will do perhaps 1.25:1 and the JG phase will continue to do about 3-4:1. Experimentally, I have achieved compression results of nearly 150:1, overall, ^^^^^^^^^^^^^^ ^^^^^ for a 60M file. (I started with a '.arj' archive of a large DOS partition.) [...] ---------------------------------------------------------------------------- From: (Jules Gilbert) Newsgroups: comp.compression Subject: Re: Explanation: that uh, alg thing... Date: 15 May 1996 16:38:18 -0400 Message-ID: <4ndfbq$> [...] One more thing, I am preparing a short technical note to deal with the reason most programmers' and computer scientists' think it's impossible to (further) compress random input. (Many people think that because you can't get more than 2^N messages from a N-bit compressed msg, that it means that you can't compress random input. (Lot's of folks have told me that.) The short story is: I agree that you can not get more than 2^N messages from N bits. No question about it. BUT THAT STATMENT HAS NOTHING TO DO WHATSOEVER WITH THE INTERPRETATION OF WHAT THOSE BITS 'MEAN'. [...] ---------------------------------------------------------------------------- From: (Jules Gilbert) Newsgroups: comp.compression Subject: Seeing is believing! Date: 9 Jun 1996 03:20:52 -0400 Message-ID: <4pdu0k$> [...] If your firm needs industrial-strength compression, contact '' and ask us for an on-site demonstration of our MR2 compressors. Each can compress large files of 'random-appearing' information, whether RSA-encrypted blocks, or files already compressed using LZ-techniques. Our demonstration will give you the opportunity to observe compression of 'random-appearing' files of at least 100MB by at least 3:1 per iteration. Usually, several iterations are possible. (These are minimum figures easily exceeded.) [...] ---------------------------------------------------------------------------- From: (Jules Gilbert) Newsgroups: comp.compression Subject: Re: My remarks on Jules Gilbert Date: 24 Jul 1996 18:05:44 -0400 Message-ID: <4t66no$> [...] My claims can not possibly be true IF I'M PLAYING BY THE 'RULES' THAT YOU ASSUME APPLY TO ME. (Sorry to shout). Clearly, anyone sending a signal (in the Shannon context), is constrained by limits which make it impossible to compress RAD ('random-appearing data') input. [...] 1) I can't compress bits any better than the next guy. Maybe not as well, in fact. 2) I have designed an engine that accepts RAD input and emits far too little data to reconstitute the original data, based on conventional assumptions. Okay! I know this. 3) But, I none-the-less reconstitute the original data. [...] ---------------------------------------------------------------------------- From: (Jules Gilbert) Newsgroups: comp.compression Subject: Re: Jules Gilbert's New Compresssion Technology Date: 12 Aug 1996 08:11:10 -0400 Message-ID: <4un70u$> I have multiple methods for compressing RAD. Watch carefully: MR1 does 3:1, on large buffers and is repeatable until the volume of input data falls below 128k or so. (This figure is under user control, but compreesion quality will suffer as the buffer size is decreased). Recent changes make this method about as fast as any conventional compressor. MR2 does at least 6:1, with a minimum buffer size of perhaps 32k. It is also repeatable. MR2 does not actually compress, though. Instead, it translates an input buffer into an output buffer of roughly equivalent size. This output buffer contains mostly constants, and other things, such as simple sequences: 28,29,31,32,33,35,40,41,42,43,44,45. (An actual sequence of bytes). Obviously, this kind of information is readily compressed, and that is why I claim that MR2 can achieve a minimum of 6:1. Again, like MR1, this process can be re-applied over it's own output. When, I've said, "No, it's impossible to compress by 100:1" I was trying to get this audience to see this as realistic. But I can compress RAD files 100:1 if allowed to re-process the output through the same process. I first actually achieved a 100:1 compression level in March of this year using tools ^^^^^^^^^^^^^^^^^^^^^^^^^ designed for experimenting in RAD issues. But now I have C programs which have been written to be easy to understand and are intended to be part of my technology transfer process for clients. [...] So, can someone compress by 100:1 or even 1000:1? Yes! But ONLY if the input file is sufficiently large. A 1000:1 compression ratio would require a very large input file, and, at least for PC users, archive files of this size are almost never produced. ---------------------------------------------------------------------------- From: (Jules Gilbert) Newsgroups: comp.compression Subject: Re: Gilbert's RAD compression product Date: 18 Aug 1996 08:40:28 -0400 Message-ID: <4v72vs$> [...] (In my original remarks), I am quoted above as claiming that a 3,152,896 byte 'tar 'file (conventionally compressed to 1,029,790 bytes) can be compressed to 50*1024 bytes. It's an accurate quote. Now how can that be possible? If a gzip compressed version of the Corpus requires roughly a 1MB, what do I do with the 950k bytes I don't store in the compressed intermediate file? Well, that's certainly a puzzler! For now, all I will say is that it does not go into the compressed intermediate file. And because it doesn't, Shannons' channel capacity axioms apply only to the 50k component. ---------------------------------------------------------------------------- From: (Jules Gilbert) Newsgroups: comp.compression Subject: Some answers about MR1 Date: 22 Aug 1996 23:45:54 -0400 Message-ID: <4vj9hi$> [...] However, arrangements are being made to do another demo in September at MIT. One of the files compressed and decompressed will be the Corpus, after it's already been compressed using ARJ, a good quality conventional compressor. (It should be about a 1MB at that point). My program has made the corpus as small as 6k, although that requires SEVERAL separate physical passes. ^^^^^^^^^^^^^^ Because we will only have a few minutes to spend on this single file, I'll likely stop at 250k or so. Under Linux, the total size of the compressor and decompressor load modules is about 50k bytes. And under DOS, using the Intel C compiler (a great product, but sadly, not sold anymore), the same files total about 300k bytes. MR1 contains code that is highly dependent on the particularities of a host computer's floating point processor, or more correctly, architectural differ- ences existing between the source machine and the target machine would likely cause failure to de-compress. [...] 9.5 Patents on compression of random data or recursive compression 9.5.1 David C. James On July 2, 1996, David C. James was granted patent 5,533,051 "Method for data compression" for a method claimed to be effective even on random data. From: (Peter J. Cranstone) Newsgroups: comp.compression Subject: Re: Jules Gilbert's Compression Technology Date: Sun Aug 18 12:48:11 EDT 1996 Wh have just been issued a patent (US. #5,533,051) and have several more pending on a new method for data compression. It will compess all types of data, including "random", and data containing a uniform distribution of "0's" and "1's". [...] The first line of the patent abstract is: Methods for compressing data including methods for compressing highly randomized data are disclosed. Page 3, line 34 of the patent states: A second aspect of the present invention which further enhances its ability to achieve high compression percentages, is its ability to be applied to data recursively. Specifically, the methods of the present invention are able to make multiple passes over a file, each time further compressing the file. Thus, a series of recursions are repeated until the desired compression level is achieved. Page 27, line 18 of the patent states that the claimed method can compress without loss *all* files by at least one bit: the direct bit encode method of the present invention is effective for reducing an input string by one bit regardless of the bit pattern of the input string. The counting argument shows that this is mathematically impossible (see section 9.2) above. If the method were indeed able to shrink any file by at least one bit, applying it recursively would shrink gigabytes down to a few bits. The patent contains evasive arguments to justify the impossible claims: Page 12, line 22: Of course, this does not take into account any overhead registers or other "house-keeping" type information which must be tracked. However such overhead tends to be negligible when processing the large quantities of data typically encountered in data compression applications. Page 27, line 17: Thus, one skilled in the art can see that by keeping the appropriate counters, the direct bit encode method of the present invention is effective for reducing an input string by one bit regardless of the bit pattern of the input string. Although a certain amount of "loss" is necessary in keeping and maintaining various counters and registers, for files which are sufficiently large, this overhead is insignificant compared to the savings obtained by the direct bit encode method. The flaw in these arguments is that the the "house-keeping" type information is *not* negligible. If it is properly taken it into account, it cancels any gains made elsewhere when attempting to compress random data. The patent contains even more evasive arguments: Page 22, line 31: It is commonly stated that perfectly entropic data streams cannot be compressed. This misbelief is in part based on the sobering fact that for a large set of entropic data, calculating the number of possible bit pattern combinations is unfathomable. For example, if 100 ones and 100 zeros are randomly distributed in a block 200 bits long, there are 200C100 = 9.055 10^58 combinations possible. The numbers are clearly unmanageable and hence the inception that perfectly entropic data streams cannot be compressed. The key to the present compression method under discussion is that it makes no attempt to deal with such large amounts of data and simply operates on smaller portions. The actual claims of the patent are harmless since they only describe methods which cannot work (they actually expand random data instead of compressing it). For example, claims 6 and 7 are: 6. A method of compressing a stream of binary data, comprising the steps of: A) parsing n-bits from said stream of binary data; B) determining the value of said parsed n-bits; C) based on the results of step B, coding said values of said n-bits in at least one of a first, second, and third target string, wherein coding said value includes generating a plurality of code strings and correlating said value with one of said code strings and dividing said correlated code string variable length codes and dividing at least some of said into at least first and second segments, and assigning at least one of said correlated code string segments to at least one of said first, second, and third target strings, wherein at least one of said plurality of codes is not greater than n-1 bits long. 7. The method of compressing a stream of binary data of claim 6, wherein n=2. Making abstraction of the legalese, claim 7 says in short that you can compress an arbitrary sequence of two bits down to one bit. 9.5.2 Michael L. Cole Patent 5,488,364 "Recursive data compression", granted Jan. 30, 1996, also claims that recursive compression of random data is possible. See for the text and a short analysis of this patent. 9.5.3 John F. Remillard Patent 5,486,826 "Method and apparatus for iterative compression of digital data" uses methods very similar to those of the "magic function theory" (see section 9.2 above). The patent is available at See also from the same person patent 5,594,435 "Permutation-based data compression" The assignee for this patent is Philosophers' Stone LLC. (The Philosopher's Stone is the key to all the riches in the universe; an LLC is a Limited Liability Corporation.)
Subject: [10] Fake compression programs (OWS, WIC) Some programs claimed to achieve incredible compression ratios are completely fake: they do not compress at all but just stored the uncompressed data in hidden files on the hard disk or keep it in unused clusters. Needless to say, such programs are dangerous and should never be used because there is a significant risk of losing all the data. The OWS program just remembers which clusters contained the data on the hard disk. The data can be recovered only if those clusters are not used again for another file. The WIC program searches for the first directory in drive C: and creates a hidden file called WINFILE.DLL containing a copy of all the original files. If you copy the compressed file to another computer (which doesn't have the file WINFILE.DLL), WIC reports a CRC error.
Subject: [11] What is the V.42bis standard? A description of the V.42bis standard is given in "The V.42bis standard for data-compressing modems," by Clark Thomborson <>, IEEE Micro, Oct 1992, pp. 41-53. If you are looking for freeware source of V.42bis, please read the note below by Peter Gutman explaining why there is no such source code. Short introduction, by Alejo Hausner <>: The V.42bis Compression Standard was proposed by the International Consultative Committee on Telephony and Telegraphy (CCITT, now ITU-T) as an addition to the v.42 error-correction protocol for modems. Its purpose is to increase data throughput, and uses a variant of the Lempel-Ziv-Welch (LZW) compression method. It is meant to be implemented in the modem hardware, but can also be built into the software that interfaces to an ordinary non-compressing modem. V.42bis can send data compressed or not, depending on the data. There are some types of data that cannot be compressed. For example, if a file was compressed first, and then sent through a V.42bis modem, the modem would not likely reduce the number of bits sent. Indeed it is likely that the amount of data would increase somewhat. To avoid this problem, the algorithm constantly monitors the compressibility of the data, and if it finds fewer bits would be necessary to send it uncompressed, it switches to transparent mode. The sender informs the receiver of this transition through a reserved code word. Henceforth the data is passed as plain bytes. While transmitting in transparent mode, the sender maintains the LZW trees of strings, and expects the receiver to do likewise. If it finds an advantage in returning to compressed mode, it will do so, first informing the receiver by a special escape code. Thus the method allows the hardware to adapt to the compressibility of the data. The choice of escape code is clever. Initially, it is a zero byte. Any occurrence of the escape code is replaced, as is customary, by two escape codes. In order to prevent a string of escape codes from temporarily cutting throughput in half, the escape code is redefined by adding 51 mod 256 each time it is used. A note from Peter Gutman <> about V.42bis implementations: V.42bis is covered by patents, and the licensing terms are rather complex because you need to license it from multiple organisations. At one point British Telecom were charging something like 30,000 pounds for a license (this was a few years ago, things may have changed since then). Because of this, noone has ever implemented a freely-available version of V.42bis as you'd find in a modem. There is a Unix implementation (called "compact") of a V.42bis-like algorithm which comes with a great many disclaimers that it can only be used for research purposes. [Note from FAQ maintainer: "compact" is available in The 'shrink' method of zip 1.1 (see item 2 above) is also similar to V.42bis] If you've ever wondered why noone other than modem manufacturers ever use V.42bis for anything, this is it. Some CCITT (ITU-T) standards documents are available by ftp in A mail server for CCITT (ITU-T) documents is available at or A Gopher server is also available at gopher:// The V42bis standard is also in For ISO documents, try See also item 20 below for other sites with standards documents.
Subject: [12] I need source for the winners of the Dr Dobbs compression contest The source of the top 6 programs of the Feb 91 Dr Dobbs data compression contest are available by ftp on The sources are in MSDOS end-of-line format, one directory per program. Unix or VMS users, use "unzip -a ddjcompr" to get correct end-of-lines (add -d to recreate the directory structure if you are using an obsolete version of unzip such as 4.1). Three of the 6 programs are not portable and only run on MSDOS. compact and urban work on Unix, sixpack only requires minor modifications.
Subject: [13] I need source for arithmetic coding (See question 70 for an introduction to arithmetic coding.) The source for the arithmetic coder described in Chap.5 of Bell, Cleary, and Witten's book "Text Compression" (see question 7 above) (or, equivalently, in: Witten, Neal, and Cleary's article "Arithmetic Coding for data Compression" from Communications of the Association for Computing Machinery, 30 (6), pp.520-540, June, 1987) is in It only comes with a simple order-0 model but it's set up so that adding your own more sophisticated one is straightforward. Look also in C language source code for adaptive arithmetic coding based on the Witten-Neal-Cleary source code is in This version uses structures for the coder, decoder and data model instead of global variables. This object oriented approach allows you to simultaneously use several arithmetic coders, each streaming bits to a different file and to have several adaptive models for each coder, each with a different number of symbols and frequncy table. Written by Fred Wheeler <> A low precision arithmetic coding implementation avoiding hardware division is available on the same site in file low.precision.version.shar Kris Popat <> has worked on "Scalar Quantization with Arithmetic Coding." It describes an arithmetic coding technique which is quite general and computationally inexpensive. The documentation and example C code are available in The program 'urban' in (see item 12 above) is a high order arithmetic coder working at the bit level. It is written by Urban Koistinen <>. The DMC program is available in It implements the algorithm described in "Data Compression using Dynamic Markov Modelling", by Gordon Cormack and Nigel Horspool, Computer Journal 30:6 (December 1987). This program uses Guazzo's version of arithmetic coding. An implementation of Moffat's arithmetic coder is available in Michael Schindler <> provides a page on Range Coding which includes an article and source code. According to Michael Schindler, the performance of that coder is a lot faster than the A. Moffat et al. CACAM coder or even the Moffat Data Compression Conference 1995 coder, at least for machines supporting a fast integer multipy and divide.
Subject: [15] Where can I get image compression programs? JPEG: Source code for most any machine (Independent JPEG Group): Contact: (has lossless mode) Contact: Andy Hung <> (see item 20 below) (lossless jpeg) (lossless jpeg by David Clunie) xv, an image viewer which can read JPEG pictures, is available in JPEG-LS (LOCO-I): MPEG: If you don't find here what you are looking for, check also and Contact: Andy Hung <> (see item 20 below) mpeg_play-2.3-patched-src.tar.gz Contact: (MPEG-I Multi-Stream System Layer encoder/player; includes an enhanced version of mpeg_play) Contact: Jim Boucher <> or Ziv Yaar <> [MPEG library] Contact: Gregory Ward <> (free demo copy of NVR's software toolkit for SPARCstations) Contact: Todd Brunhoff <> Contact: Andy Hung <> (see item 20 below) mpeg_play-2.3-patched-src.tar.gz Contact: (MPEG-I Multi-Stream System Layer encoder/player; includes an enhanced version of mpeg_play) Contact: Jim Boucher <> or Ziv Yaar <> [MPEG library] Contact: Gregory Ward <> (free demo copy of NVR's software toolkit for SPARCstations) Contact: Todd Brunhoff <> or Contacts: MPEG Software Simulation Group <> Concerning VMPEG: Stefan Eckart <> (MPEGTV Software MPEG Video Player for Unix) Contact: Tristan Savatier <> H.261(P*64): Contact: Andy Hung <> (see item 20 below) (Inria videoconference system) Contact: Thierry Turletti <> (see item 20 below). H.261 player for Windows95. Contact: H.263: (Telenor Research) (Roalt Aalmoes) JBIG: Contact: Markus Kuhn <> PNG: For code and sample images, see: mg: (the MG system for compressing and indexing text and images, see item 16) Contact: Stuart Inglis <> BTPC: Binary Tree Predictive Coding Contact: John Robinson <> epic: (Efficient Pyramid Wavelet Coder, see item 72) Contact: Eero P. Simoncelli <> C source code provided. The "Lenna" test image is available as part of the EPIC package, where it is named "test_image". hcompress: (wavelet image compression, see item 72) wavethresh: (wavelet software for the language S) Contact: rice-wlet: (wavelet software, see item 72) Wavelet Transform Coder Construction Kit: Contact: Geoff Davis <> scalable: (2 & 3 dimensional subband transformation) Contact: SPIHT: (Set Partitioning in Hierarchical Trees, wavelet-based) DjVu: Image compression library from AT&T Labs (includes a wavelet-based image compressor) Contact: compfits: Contact: Jim Wright <> fitspress: tiff: For source and sample images, see question 18 below. DCT algorithms used to be in: Contact: Charilos Christopoulos <> for the sources xanim: (X11 animation viewer, supports Quicktime and several other formats) A demo of image compression using neural networks is available in For fractal compression programs, see item 17 below. For Vector Quantization software, see item 76 in part 2 of this FAQ. For image compression hardware, see item 85 in part 3 of this FAQ.
Subject: [16] What is the state of the art in lossless image compression? The JBIG algorithm is one of the best available for lossless image compression. For an introduction to JBIG, see question 74 in part 2. JBIG works best on bi-level images (like faxes) and also works well on Gray-coded grey scale images up to about six or so bits per pixel. You just apply JBIG to the bit planes individually. For more bits/pixel, lossless JPEG provides better performance, sometimes. (For JPEG, see question 19 below.) You can find the specification of JBIG in International Standard ISO/IEC 11544 or in ITU-T Recommendation T.82. You can order a copy directly from ISO ( or ITU ( or from your National Standards Body. In the USA, call ANSI at (212) 642-4900. See also the MG system containing an implementation of the 'FELICS' algorithm of P.G. Howard and J.S. Vitter. FELICS usually gives better and faster compression than lossless JPEG, at least for 8-bit grayscale images. (See item 15 above for ftp location). From the MG README file: The MG system is a suite of programs for compressing and indexing text and images. Most of the functionality implemented in the suite is as described in the book ``Managing Gigabytes: Compressing and Indexing Documents and Images'', I.H. Witten, A. Moffat, and T.C. Bell; Van Nostrand Reinhold, New York, 1994, ISBN 0-442-01863-0; US $54.95; call 1 (800) 544-0550 to order. These features include: -- text compression using a Huffman-coded semi-static word-based scheme -- two-level context-based compression of bi-level images -- FELICS lossless compression of gray-scale images -- combined lossy/lossless compression for textual images -- indexing algorithms for large volumes of text in limited main memory -- index compression -- a retrieval system that processes Boolean and ranked queries -- an X windows interface to the retrieval system Paul Howard's PhD thesis, which among other things describes FELICS, is available in JPEG-LS is the emerging ISO standard for lossless/near-lossless compression of continuous-tone images. Marcelo Weinberger <> says about it: JPEG-LS is being developed by ISO/IEC JTC1/SC29/WG1 (final committee draft document FCD14495-1 as of July 1997), and it is based on HP's LOCO-I algorithm (reference: M. Weinberger, G. Seroussi, G. Sapiro, "LOCO-I: A Low Complexity, Context-Based, Lossless Image Compression Algorithm," Proc. IEEE Data Compression Conference, Snowbird, Utah, March-April 1996). [...] The main feature of JPEG-LS is its superior placement in the compression/complexity trade-off curve. Tested over a wide variety of image types, it was shown to be, on the average, within about 4% of the best available lossless image compression at a fraction of the complexity. In particular, JPEG-LS significantly outperforms FELICS and lossless JPEG Huffman at similar levels of complexity (it also outperforms lossless JPEG arithmetic, which is of significantly higher complexity). [...] A software implementation of JPEG-LS, is now available at: There, the DCC'96 paper on LOCO-I is also available. The standard draft is also available through a link to the official JPEG Web site. Benchmarks of some lossless image compression programs are in
Subject: [17] What is the state of fractal compression? You may want to read first item 77 in part 2 of this FAQ: "Introduction to Fractal compression". from Tal Kubo <>: According to Barnsley's book 'Fractals Everywhere', this method is based on a measure of deviation between a given image and its approximation by an IFS code. The Collage Theorem states that there is a convergent process to minimize this deviation. Unfortunately, according to an article Barnsley wrote for BYTE a few years ago, this convergence was rather slow, about 100 hours on a Cray, unless assisted by a person. Barnsley et al are not divulging any technical information beyond the meager bit in 'Fractals Everywhere'. The book explains the idea of IFS codes at length, but is vague about the application of the Collage theorem to specific compression problems. There is reason to believe that Barnsley's company has *no algorithm* which takes a given reasonable image and achieves the compression ratios initially claimed for their fractal methods. The 1000-to-1 compression advertised was achieved only for a 'rigged' class of images, with human assistance. The best unaided performance I've heard of is good lossy compression of about 80-1. Steve Tate <> confirms: Compression ratios (unzoomed) seem to range from 20:1 to 60:1... The quality is considerably worse than wavelets or JPEG on most of the non-contrived images I have seen. But Yuval Fisher <> disagrees: Their performance has improved dramatically beyond what they were talking about in BYTE a few years ago. Human assistance to the compression is no longer needed and the compression time is reasonable, although the more time and compute power you throw at the compression, the smaller the resulting file for the same level of quality. Geoffrey A Stephenson <> adds: Iterated systems are shipping a general purpose compressor at about 300 Pounds in the UK that claims "640x480 24 bit colour compression of about 1 min at 922k -> 10k on a 486/50 software only, decomp. to 8 bits in 3 secs, etc." At a recent multimedia conference in London they handed out free demo disks that show the decomp. in action. The package runs under both DOS anf WIN (DLLs provided for use in applications). They also sell a board to speed up compression and offer versions supporting full motion video (but not apparently at all SVGA sizes like the static picture version). I have not yet got my hands on a full version to test different types of pictures, but friends have a and claim it looks good. Thomas W. Colthurst <> clarifies the distinction between IFS and the Fractal Transform: It is time, once and for all, to put to death the Barnsley myth that IFSs are good for image compression. They are not. Various algorithms have been proposed for this "inverse problem" ranging from the trendy (genetic algorithms) to the deep (moment methods) to the ad hoc (the hungry algorithm) to the absurd (the so-called "graduate student algorithm", consisting of locking up a grad student in a tiny office with a SGI workstation and not letting them out until they come up with a good IFS for your image). They are all useless for practical image compression. In fact, there are even good theoretical reasons for believing that IFSs will never be useful for image compression. For example, even if you have an IFS for object A and an IFS for object B, there is no way to combine these IFSs to get an IFS for object A union B or object A intersect B. Even Barnsley himself admits, in his latest book, that he doesn't use IFS image compression. Instead, he uses the so-called "fractal transform," which is really just a variant of vector quantization where you use the image itself, sampled at a higher scale, as the VQ codebook. To be fair, the fractal transform can be analyzed using local IFSs, but local IFSs are immensely more complicated and general than normal IFSs, to the point where one feels suspect even using the word "IFS" to describe them. It should be emphasized that the fractal transform is a real, working method that performs about as well as other existing methods like VQ or the discrete cosine transform. The fractal transform will probably never beat vector quantization (VQ) as for size of the compressed image, but does have the advantage that you don't need to carry your codebook around. The latest results have it slightly winning over the discrete cosine transform; only time and more research will tell if this advantage persists. Just like VQ, the fractal transform takes a while to compress, but is quick at decompression (Barnsley's company has hardware to do this in realtime). In short, IFSs are good for just about everything fractals are (and more!), but are absolutely horrid for image compression. Programs: Check for pointers to some fractal compression programs and lots of papers on fractal compression. The Waterloo BragZone ( or ) compares the results of various image compression schemes against a 32 element test suite. Numerous rate-distortion graphs, data tables, and sample images are available. A fractal image compression program is available by ftp in ; it contains source for compression and decompression, source for X-windows decompression, MSDOS executables and images. [Note from FAQ maintainer: Fisher's program (see below) implements the same algorithm but is more general; see for the source code.] A fractal image decompression program (note: decompression only) is available in In the same directory, is the paper "Fractal image compression" by Yuval Fisher, Siggraph 92. Reading this paper is required to understand how the Young compression programs (see above) works. A note from Yuval Fisher <>: Connect to . There is information there on my new book of contributed articles on fractal image compression, as well as the book's table of contents, some C code to encode and decode raw byte files of any size using a quadtree method, a manual explaining the use of the code, a fractal image compression bibliography (not guaranteed to be complete or close to it), some better executable code with sample encodings, and the SIGGRAPH '92 course notes on fractal image compression (these are based on appendix A of Chaos and Fractals by Peitgen et al., Springer Verlag). [The C code is also available in ] Another fractal compression program is available by ftp in It is also based on quadtrees, as yuvpak20 and frac_comp. The source code for the program published in the Oct 93 issue of Byte is in This is a self-extractible arc file (must be run on MSDOS for extraction). The source code is for a TARGA video board. [Note from FAQ maintainer: this code is taken from Barnsley's book "Fractal Image Compression"; it implements the brute force method and is thus very slow.] Iterated Systems have released a beta version of their fractal imager. It will let you view a number of formats including JPG and do conversions to their fractal format. The program can be downloaded from "The Data Compression Book" (see [NEL 1996] in item 7 above) contains a chapter on fractal compression; it includes source code for a simple fractal compression program. The source is also available at Several fractal compression programs, including a volume coder, are available at Several papers on fractal image compression are available on in directory /documents/papers/fractal . A biliography is in References: A. Jacquin, 'Fractal image coding based on a theory of iterated contractive image transformations', Proc. SPIE Visual Communications and Image Processing, 1990, pages 227-239. (The best paper that explains the concept in a simple way.) A. Jacquin, "A Fractal Theory of Iterated Markov Operators with Applications to Digital Image Coding", PhD Thesis, Georgia Tech, 1989. It can be obtained from university microfilms for $35, phone 1-800-521-0600. M. Barnsley, L. Anson, "Graphics Compression Technology, SunWorld, October 1991, pp. 42-52. M.F. Barnsley, A. Jacquin, F. Malassenet, L. Reuter & A.D. Sloan, 'Harnessing chaos for image synthesis', Computer Graphics, vol 22 no 4 pp 131-140, 1988. M.F. Barnsley, A.E. Jacquin, 'Application of recurrent iterated function systems to images', Visual Comm. and Image Processing, vol SPIE-1001, 1988. A. Jacquin, "Image Coding Based on a Fractal Theory of Iterated Contractive Image Transformations" p.18, January 1992 (Vol 1 Issue 1) of IEEE Trans on Image Processing. A.E. Jacquin, 'A novel fractal block-coding technique for digital images', Proc. ICASSP 1990. G.E. Oien, S. Lepsoy & T.A. Ramstad, 'An inner product space approach to image coding by contractive transformations', Proc. ICASSP 1991, pp 2773-2776. D.S. Mazel, Fractal Modeling of Time-Series Data, PhD Thesis, Georgia Tech, 1991. (One dimensional, not pictures) S. A. Hollatz, "Digital image compression with two-dimensional affine fractal interpolation functions", Department of Mathematics and Statistics, University of Minnesota-Duluth, Technical Report 91-2. (a nuts-and-bolts how-to-do-it paper on the technique) Stark, J., "Iterated function systems as neural networks", Neural Networks, Vol 4, pp 679-690, Pergamon Press, 1991. Monro D M and Dudbridge F, "Fractal block coding of images", Electronics Letters 28(11):1053-1054 (1992) Beaumont J M, "Image data compression using fractal techniques", British Telecom Technological Journal 9(4):93-108 (1991) Fisher Y, "Fractal image compression", Siggraph 92 Graf S, "Barnsley's Scheme for the Fractal Encoding of Images", Journal Of Complexity, V8, 72-78 (1992). Monro D.M. 'A hybrid fractal transform', Proc ICASSP 93, pp. V: 169-72 Monro D.M. & Dudbridge F. 'Fractal approximation of image blocks', Proc ICASSP 92, pp. III: 485-488 Monro D.M., Wilson D., Nicholls J.A. 'High speed image coding with the Bath Fractal Transform', IEEE International Symposium on Multimedia Technologies Southampton, April 1993 Jacobs, E.W., Y. Fisher and R.D. Boss. "Image Compression: A study of the Iterated Transform Method." _Signal Processing 29_ (1992) 25-263 Vrscay, Edward R. "Iterated Function Systems: Theory, Applications, and the Inverse Problem." _Fractal Geometry and Analysis_, J. Belair and S. Dubuc (eds.) Kluwer Academic, 1991. 405-468. Books: Fractal Image Compression: Theory and Application, Yuval Fisher (ed.), Springer Verlag, New York, 1995. To order the book, call 1-800-SPRINGER and ask for the book with ISBN number 0-387-94211-4 or check Fractal Image Compression Michael F. Barnsley and Lyman P. Hurd ISBN 0-86720-457-5, ca. 250 pp., $49.95 Copies can be ordered directly from the publisher by sending a message to with name, address and a Mastercard or Visa card number with expiration date. Barnsley's company is: Iterated Systems, Inc. 5550A Peachtree Parkway, Suite 650 Norcross, GA 30092 tel: 404-840-0310 or 1-800-4FRACTL fax: 404-840-0806 In UK: Phone (0734) 880261, Fax (0734) 880360
Subject: [18] I need specs and source for TIFF and CCITT group 4 Fax Specs for Group 3 and 4 image coding (group 3 is very similar to group 4) are in CCITT (1988) volume VII fascicle VII.3. They are recommendations T.4 and T.6 respectively. There is also an updated spec contained in 1992 recommendations T.1 to T.6. CCITT (now ITU-T) specs are available by anonymous ftp (see above answer on V.42bis). The T.4 and T.6 specs are in The following paper covers T.4, T.6 and JBIG: "Review of standards for electronic imaging for facsimile systems" in Journal of Electronic Imaging, Vol. 1, No. 1, pp. 5-21, January 1992. Source code can be obtained as part of a TIFF toolkit - TIFF image compression techniques for binary images include CCITT T.4 and T.6: Contact: There is also a companion compressed tar file (v3.0pics.tar.Z) that has sample TIFF image files. A draft of TIFF 6.0 is in Concerning JPEG compression in TIFF 6.0, Tom Lane <> adds: TIFF 6.0's scheme for incorporating JPEG compression (spec section 22) has a bunch of serious deficiencies. Don't use it. A revised design is given by TIFF Technical Note #2, The revised design will replace section 22 in TIFF 7.0, and is implemented in Sam Leffler's libtiff. See also item 75 of this FAQ for more JPEG info. Software for reading and writing CCITT Group 3 and 4 images is also available in directory Contact: Alan Finlay <>. See also question 54 below.
Subject: [19] What is JPEG? JPEG (pronounced "jay-peg") is a standardized image compression mechanism. JPEG stands for Joint Photographic Experts Group, the original name of the committee that wrote the standard. JPEG is designed for compressing either full-color or gray-scale digital images of "natural", real-world scenes. It does not work very well on non-realistic images, such as cartoons or line drawings. JPEG does not handle black-and-white (1-bit-per-pixel) images, nor does it handle motion picture compression. Related standards for compressing those types of images exist, and are called JBIG and MPEG respectively. Regular JPEG is "lossy", meaning that the image you get out of decompression isn't quite identical to what you originally put in. The algorithm achieves much of its compression by exploiting known limitations of the human eye, notably the fact that small color details aren't perceived as well as small details of light-and-dark. Thus, JPEG is intended for compressing images that will be looked at by humans. If you plan to machine-analyze your images, the small errors introduced by JPEG may be a problem for you, even if they are invisible to the eye. The JPEG standard includes a separate lossless mode, but it is rarely used and does not give nearly as much compression as the lossy mode. Question 75 "Introduction to JPEG" (in part 2 of this FAQ) gives an overview of how JPEG works and provides references for further reading. Also see the JPEG FAQ article, which covers JPEG software and usage hints. The JPEG FAQ is posted regularly in news.answers by Tom Lane <>. (See also question 53 "Where are FAQ lists archived".) For JPEG software, see item 15 above. For JPEG hardware, see item 85 in part 3 of this FAQ. The ISO JPEG standards committee's home page is
Subject: [20] I am looking for source of an H.261/H.263 codec and MPEG Many standards and draft recommendations (including H.261, H.263, H.320, H.324), are available in The H.261 spec is available in For H.261 hardware, see item 85 in part 3 of this FAQ. Current drafts of H.324 and related recommendations including H.263 are available in Telenor Research have made available a complete simulation of H.263. See An H.263 encoder library is available at from Thierry TURLETTI <>: IVS (INRIA VIDEOCONFERENCING SYSTEM) - X11-based videoconferencing tool for SPARC, HP, DEC and Silicon Graphic workstations. ivs allows users to conduct multi-host audio and video conferences over the Internet. ivs requires a workstation with a screen with 1, 4, 8 or 24 bits depth. Multi-host conferences require that the kernel support multicast IP extensions (RFC 1112). On video input, video frames are grabbed by the VideoPix, SunVideo or Parallax boards for SparcStations or Raster Rops board for HP stations or the IndigoVideo board for SGI IRIS Indigo workstations. or the VIDEOTX board for DEC stations. No special hardware apart from the workstation's build-in audio hardware is required for audio conference. Video encoding is done according to the H.261 standard. The video stream can be encoded in either Super CIF (704x576 pixels) format or CIF (352x288 pixels) format or QCIF (176x144 pixels). Default format is CIF. Sources, binaries & manuals are freely available by anonymous ftp from in the rodeo/ivs directory. An INRIA report describing this application is also available in the same directory. If you ftp & use this package, please send all remarks or modifications made to <>. If you want to be added or deleted to the ivs-users mailing list, please send e-mail to from Andy Hung <>: Public domain UNIX C source code to do both image and image sequence compression and decompression is available by anonymous ftp: MPEG-I CCITT H.261(P*64)*.tar.Z JPEG*.tar.Z These codecs operate on raw raster scanned images. A software program to display raw raster-scanned YUV images and image sequences on X grayscale or color monitors is provided by a program in*.tar.Z If you are using the codecs above, we recommend that you ftp this file over as well. The source code has been compiled on DEC and SUN workstations. Caution: the P64 codec has not been tested compliant (any available p64 video streams would be much appreciated - please let us know at The other codecs have been tested with streams from other encoders. We also have some IPB MPEG-I video coded streams in pub/mpeg/*.mpg; and P64 video streams in pub/p64/*.p64 that we have generated using our codecs. For a more complete description see the file
Subject: [25] Fast DCT (Discrete Cosine Transform) algorithms Many image compression methods, including the JPEG, MPEG, and H.261 standards, are based on the discrete cosine transform. A good overall introduction to DCT is the book "Discrete Cosine Transform---Algorithms, Advantages, Applications" by K.R. Rao and P. Yip (Academic Press, London, 1990), ISBN 0-12-580203-X. This has an extensive, though already dated, bibliography. Here are some references mostly provided by Tom Lane <>. (This list is now rather dated.) Most of these are in IEEE journals or conference proceedings, notably ICASSP = IEEE Intl. Conf. on Acoustics, Speech, and Signal Processing. ICCAS = IEEE Intl. Conf. on Circuits and Systems. DCC = Data Compression Conference. Polynomial Transform Computation of the 2-D DCT, Duhamel & Guillemot, ICASSP '90 p. 1515. A Forward-Mapping Realization of the Inverse DCT, McMillan & Westover, DCC '92 p. 219. A Fast Algorithm for 2-D DCT, Cho, Yun & Lee, ICASSP '91 p. 2197. Fast Algorithm and Implementation of 2-D DCT, Cho & Lee, Tr. CAS v38 p. 297. A DCT Chip based on a new Structured and Computationally Efficient DCT Algorithm, Duhamel, Guillemot & Carlach, ICCAS '90 p. 77. Trade-offs in the Computation of Mono- and Multi-dimensional DCTs, Vetterli, Duhamel & Guillemot, ICASSP '89 p. 999. Practical Fast 1-D DCT Algorithms with 11 Multiplications, Loeffler, Ligtenberg & Moschytz, ICASSP '89 p. 988. New Scaled DCT Algorithms for Fused Multiply/Add Architectures, Linzer & Feig, ICASSP '91 p. 2201. Fast Algorithms for the 2-D Discrete Cosine Transform, Kamangar & Rao, IEEE Tr. Computers, v C-31 p. 899. Fast 2-D Discrete Cosine Transform, Vetterli, ICASSP '85 p. 1538. A Two-Dimensional Fast Cosine Transform, Haque, Tr. ASSP v ASSP-33 p. 1532. Real-Time Parallel and Fully Pipelined 2-D DCT Lattice Structures with Application to HDTV Systems, Chiu & Liu, Tr. CAS for Video Tech, v 2 p. 25. J.F. Blinn, "What's the Deal with the DCT", IEEE Computer Graphics and Applications, July 1993, pp.78-83. A C Hung and TH-Y Meng, "A Comparison of fast DCT algorithms, Multimedia Systems", No. 5 Vol. 2, Dec 1994 For actual implementations, try the JPEG and MPEG software listed in item 15.
Subject: [26] Are there algorithms and standards for audio compression? Yes. See the introduction to MPEG given in part 2 of this FAQ. A lossless compressor for 8bit and 16bit audio data (.au) is available in or Shorten works by using Huffman coding of prediction residuals. Compression is generally better than that obtained by applying general purpose compression utilities to audio files. Also supports lossy compression. Contact: Tony Robinson <>. Benchmarks of shorten and other lossless audio compression programs are in Audio software is available in subdirectories of : - An MPEG audio player is in mpeg_players/Workstations/maplay1_2.tar.Z. - The sources of the XING MPEG audio player for Windows is in mpeg_players/Windows/ - An encoder/decoder is in converters/source/mpegaudio.tar.Z. MSDOS audio software is available in In particular, MPEG-2 audio software is in and MPEG audio files are available in and The site is dedicated to the MP3 audio compression standard. It has information about the MP3 standard, audio compression techniques, tests, sources, etc... Copied from the comp.dsp FAQ posted by (Guido van Rossum): Strange though it seems, audio data is remarkably hard to compress effectively. For 8-bit data, a Huffman encoding of the deltas between successive samples is relatively successful. For 16-bit data, companies like Sony and Philips have spent millions to develop proprietary schemes. Public standards for voice compression are slowly gaining popularity, e.g. CCITT G.721 and G.723 (ADPCM at 32 and 24 kbits/sec). (ADPCM == Adaptive Delta Pulse Code Modulation.) Free source code for a *fast* 32 kbits/sec ADPCM (lossy) algorithm is available by ftp from as /pub/audio/adpcm.shar. (** NOTE: if you are using v1.0, you should get v1.1, released 17-Dec-1992, which fixes a serious bug -- the quality of v1.1 is claimed to be better than uLAW **) (Note that U-LAW and silence detection can also be considered compression schemes.) Information and source code for adpcm are available in Source for Sun's free implementation of CCITT compression types G.711, G.721 and G.723 is in You can get a G.721/722/723 package by email to, with GET ITU-3022 as the *only* line in the body of the message. A note on u-law from Markus Kuhn <>: u-law (more precisely (greek mu)-law or 5-law if you have an 8-bit ISO terminal) is more an encoding then a compression method, although a 12 to 8 bit reduction is normally part of the encoding. The official definition is CCITT recommendation G.711. If you want to know how to get CCITT documents, check the Standards FAQ posted to news.answers or get the file standards-faq by ftp in directory See also the comp.dsp FAQ for more information on: - The U.S. DoD's Federal-Standard-1016 based 4800 bps code excited linear prediction voice coder version 3.2a (CELP 3.2a) - The U.S. DoD's Federal-Standard-1015/NATO-STANAG-4198 based 2400 bps linear prediction coder version 53 (LPC-10e v53) - Realtime DSP code and hardware for FS-1015 and FS-1016 The comp.dsp FAQ is in comp.dsp with subject "FAQ: Audio File Formats" and in CELP C code for Sun SPARCs is in An LPC10 speech coder is in ; a derived version is available from Source code for ITU-T (CCITT) G.728 Low Delay CELP speech compression is in Recommended reading: Digital Coding of Waveforms: Principles and Applications to Speech and Video. N. S. Jayant and Peter Noll. Prentice-Hall, 1984, ISBN 0-13-211913-7. Information on GSM sound compression is available at from Markus Kuhn <>: One highest quality sound compression format is called ASPEC and has been developed by a team at the Frauenhofer Institut in Erlangen (Germany) and others. ASPEC produces CD like quality and offers several bitrates, one is 128 kbit/s. It is a lossy algorithm that throws away frequencies that aren't registered in the human cochlea in addition to sophisticated entropy coding. The 64 kbit/s ASPEC variant might soon bring hifi quality ISDN phone connections. It has been implemented on standard DSPs. The Layer 3 MPEG audio compression standard now contains what is officially called the best parts of the ASPEC and MUSICAM algorithms. A reference is: K.Brandenburg, G.Stoll, Y.F.Dehery, J.D.Johnston, L.v.d.Kerkhof, E.F.Schroeder: "The ISO/MPEG-Audio Codec: A Generic Standard for Coding of High Quality Digital Audio", 92nd. AES-convention, Vienna 1992, preprint 3336 from Jutta Degener <> and Carsten Bormann <>: GSM 06.10 13 kbit/s RPE/LTP speech compression available -------------------------------------------------------- The Communications and Operating Systems Research Group (KBS) at the Technische Universitaet Berlin is currently working on a set of UNIX-based tools for computer-mediated telecooperation that will be made freely available. As part of this effort we are publishing an implementation of the European GSM 06.10 provisional standard for full-rate speech transcoding, prI-ETS 300 036, which uses RPE/LTP (residual pulse excitation/long term prediction) coding at 13 kbit/s. GSM 06.10 compresses frames of 160 13-bit samples (8 kHz sampling rate, i.e. a frame rate of 50 Hz) into 260 bits; for compatibility with typical UNIX applications, our implementation turns frames of 160 16-bit linear samples into 33-byte frames (1650 Bytes/s). The quality of the algorithm is good enough for reliable speaker recognition; even music often survives transcoding in recognizable form (given the bandwidth limitations of 8 kHz sampling rate). Version 1.0 of the implementation is available per anonymous ftp from in the directory /pub/local/kbs/tubmik/gsm/ ; more information about the library can be found on the World-Wide Web at . Questions and bug reports should be directed to and . from Nicola Ferioli <>: Lossless 8-bit sound file compressor VOCPACK is a compressor/decompressor for 8-bit digital sound using a lossless algorithm; it is useful to save disk space without degrading sound quality. It can compress signed and unsigned data, sampled at any rate, mono or stereo. Since the method used is not lossy, it isn't necessary to strip file headers before compressing. VOCPACK was developed for use with .VOC (SoundBlaster) and .WAV (Windows) files, but any 8-bit sound can be compressed since the program takes no assumptions about the file structure. The typical compression ratio obtained goes from 0,8 for files sampled at 11 KHz to 0,4 for 44 Khz files. The best results are obtained with 44 KHz sounds (mono or stereo): general-purpose archivers create files that can be twice longer than the output of VOCPACK. You can obtain smaller values using lossy compressors but if your goal is to keep the sound quality unaltered you should use a lossless program like VOCPACK. from Harald Popp <>: new version 1.0 of ISO/MPEG1 Audio Layer 3 Shareware available major improvements of the new version: - encoder works twice as fast - improved file handling for encoder including .WAV files You may download the shareware from ( from the directory /pub/layer3 The source code for the MPEG1 audio decoder layer 1, 2 and 3 is now available on ( in /pub/layer3/public_c. There are two files: mpeg1_iis.tar.Z (Unix: lines seperated by line feed only) (PC: lines seperated by carriage return and line feed) For more information about this product and MPEG Audio Layer 3, see the document "Informations about MPEG Audio Layer-3" maintained by Juergen Zeller <>, available in from Monty <>: OggSquish is a compression package designed to reduce the file size of digitized 8 and 16 bit audio samples (or samples of any periodic data). OggSquish will operate on files sampled at any speed, but it is designed to work with very high quality samples, for example, CD quality samples. [OggSquish is now at or or ] from Dmitrij V. Schmunk <>: Take a look at This compressor gives you about 2-3 times better compression for 44.1kHz stereo sound then MPEG layer-3. from Dennis Lee <> WA incorporates lossless audio codecs (similar to SHORTEN and OggSquish) into an easy to use archiver program. WA only supports compression of the popular ".WAV" audio format. To the author's knowledge, WA can compress waveform data better than any existing software. With default settings, WA also compresses faster than PKZIP, so it is convenient to use. This software can be found at from Jack Berlin <>: Pegasus has a new lossless sound compressor based on our patent pending arithmetic coder. Currently beats Shorten in all cases. Trial app for Windows: ($39 to register)
Subject: [30] My archive is corrupted! The two most common reasons for this are (1) failing to use the magic word "tenex" (when connected to SIMTEL20 and other TOPS20 systems) or "binary" (when connected to UNIX systems) when transferring the file from an ftp site to your host machine. The reasons for this are technical and boring. A synonym for "tenex" is "type L 8", in case your ftp doesn't know what "tenex" means. (2) failing to use an eight-bit binary transfer protocol when transferring the file from the host to your PC. Make sure to set the transfer type to "binary" on both your host machine and your PC. gopher is also known to corrupt binary files. In particular, if gzip complains about a multi-part file, it's likely that the .gz file has been corrupted by gopher. Use ftp in binary mode instead.
Subject: [31] pkunzip reports a CRC error! The portable zip 1.1 contains many workarounds for undocumented restrictions in pkunzip. Compatibility is ensured for pkunzip 1.10 only. All previous versions (pkunzip 1.0x) have too many bugs and cannot be supported. This includes Borland unzip. So if your pkunzip reports a CRC error, check that you are not using an obsolete version. Get either pkzip 2.04g or unzip 5.12 (see question 2 above for ftp sites). To generate zip files compatible with pkunzip 1.10, use zip 1.1 (see item 2 above for ftp site).
Subject: [32] VMS zip is not compatible with pkzip! The problem is most likely in the file transfer program. Many use kermit to transfer zipped files between PC and VMS VAX. The following VMS kermit settings make VMS-ZIP compatible with PKZIP: VMS kermit PC kermit --------------- -------------- Uploading PKZIPped file to be UNZIPped: set fi ty fixed set fi ty bi Downloading ZIPped file to be PKUNZIPped: set fi ty block set fi ty bi If you are not using kermit, transfer a file created by pkzip on MSDOS to VMS, transfer it back to your PC and check that pkunzip can extract it.
Subject: [33] I have a problem with Stacker or DoubleSpace! The newsgroup comp.compression is *not* the appropriate place to discuss about one specific program on one specific operating system. Since you have bought a legal copy of Stacker or MSDOS 6.x, you have the documentation of your product; please read it. If you can't find the answer in the documentation, please report the problem to the Stac or Microsoft customer support. (For Stac, use one of, or If you really feel that the net has to know about your problem, please post in one of the MSDOS newsgroups, such as comp.os.msdos.apps or
Subject: [50] What is this 'tar' compression program? tar is not a compression program. It just combines several files into one, without compressing them. tar file are often compressed with 'compress', resulting in a .tar.Z file. See question 2, file type .tar.Z. GNU tar has the capability to (de)compress files as well. When you have to archive a lot of very small files, it is often preferable to create a single .tar file and compress it, than to compress the individual files separately as in a zip archive. The compression program can thus take advantage of redundancy between separate files. The disadvantage is that you must uncompress the whole .tar file to extract any member. You can also improve compression by grouping files by type, as in: tar cvf - `ls | sort -t. +1` | gzip > file.tar.gz
Subject: [51] I need a CRC algorithm As its name implies (Cyclic Redundancy Check) a crc adds redundancy whereas the topic of this group is to remove it. Yet this question comes up often in comp.compression. The file is a pretty comprehensive description of the whole CRC concept, including a C program. See also: - Schwaderer W.D., "CRC Calculation", April 85 PC Tech Journal, pp.118-133. - "Calculating CRCs by Bits and Bytes", BYTE Magazine, September 1986 - Ramabadran T.V., Gaitonde S.S., "A tutorial on CRC computations", IEEE Micro, Aug 1988. - the source of all archivers, such as the file makecrc.c in the Info-ZIP sources (see extension .zip in item 2) For the related topic of Reed-Solomon encoding, see the Error Correcting Codes Home Page The following C code (by Rob Warnock <>) does CRC-32 in BigEndian/BigEndian byte/bit order. That is, the data is sent most significant byte first, and each of the bits within a byte is sent most significant bit first, as in FDDI. You will need to twiddle with it to do Ethernet CRC, i.e., BigEndian/LittleEndian byte/bit order. [Left as an exercise for the reader.] The CRCs this code generates agree with the vendor-supplied Verilog models of several of the popular FDDI "MAC" chips. u_long crc32_table[256]; /* Initialized first time "crc32()" is called. If you prefer, you can * statically initialize it at compile time. [Another exercise.] */ u_long crc32(u_char *buf, int len) { u_char *p; u_long crc; if (!crc32_table[1]) /* if not already done, */ init_crc32(); /* build table */ crc = 0xffffffff; /* preload shift register, per CRC-32 spec */ for (p = buf; len > 0; ++p, --len) crc = (crc << 8) ^ crc32_table[(crc >> 24) ^ *p]; return ~crc; /* transmit complement, per CRC-32 spec */ } /* * Build auxiliary table for parallel byte-at-a-time CRC-32. */ #define CRC32_POLY 0x04c11db7 /* AUTODIN II, Ethernet, & FDDI */ init_crc32() { int i, j; u_long c; for (i = 0; i < 256; ++i) { for (c = i << 24, j = 8; j > 0; --j) c = c & 0x80000000 ? (c << 1) ^ CRC32_POLY : (c << 1); crc32_table[i] = c; } }
Subject: [52] What about those people who continue to ask frequently asked questions in spite of the frequently asked questions document? Just send them a polite mail message, referring them to this document. There is no need to flame them on comp.compression. That would just add more noise to this group. Posted answers that are in the FAQ are just as annoying as posted questions that are in the FAQ.
Subject: [53] Where are FAQ lists archived? Many are crossposted to news.answers. That newsgroup should have a long expiry time at your site; if not, talk to your sysadmin. FAQ lists are in and The comp.compression FAQ that you are reading is in If you don't have FTP access, you can access the archives by mail server. Send an email message to containing the commands send usenet/news.answers/compression-faq/part1 send usenet/news.answers/compression-faq/part2 send usenet/news.answers/compression-faq/part3 For instructions, send an email message to the same address with the words "help" and "index" (no quotes) on separate lines. If you don't get a reply, check your return address, or add a line such as path
Subject: [54] I need specs for graphics formats Get the book by Murray & vanRyper "Encyclopedia of graphics file formats", O'Reilly & associates, ISBN 1-56592-058-9. See also the FAQ and the Graphics Formats FAQ. The latter is in Check also "The File Format Collection" in and the File Formats Encyclopedia 2.0
Subject: [55] Where can I find Lenna and other images? The Waterloo BragZone ( or ) compares the results of various image compression schemes against a 32 element test suite. Sample images are available. The Computer Vision Home Page has many links to test images in A bunch of standard images (lenna, baboon, cameraman, crowd, moon etc..) used to be in . The images are in 256-level grayshades (256x256 pixels, 256 "colors"). [Note: the site mentioned below keeps changing. Images stay there for a while then disappear. They are again available at the time of writing (27 Dec 93).] The site ( has standard images in two directories: (The directory /pub/image/sequence was taken offline because of possible copyright problems, but has come back again. In particular, Miss America is in subdirectories of /pub/image/sequence/missa.) In each of those directories are (usually) the following directories: bgr - 24 bit blue, green, red color - 24 bit red, green, blue gray - 8 bit grayscale uniform weighted gray601 - 8 bit grayscale CCIR-601 weighted And in these directories are the actual images. For example, the popular lena image is in # 24 bit BGR # 8 bit gray All of the images are in Sun rasterfile format. You can use the pbm utilities to convert them to whatever format is most convenient. [pbm is available in*.tar.Z ]. Questions about the ipl archive should be sent to There are few gray-scale still images and some raw data of test results available in directory There are lots of .gif images in Medical images can be found in: The WWW address for the National Library of Medicine is A list of health and medical related Internet resources is available ftp://in Rodney Peck <> is interested in some method of establishing a canonical ftp database of images but does not have the resources to provide an ftp site for that database. Send suggestions to Beware: the same image often comes in many different forms, at different resolutions, etc... The original lenna image is 512 wide, 512 high, 8 bits per pel, red, green and blue fields. Gray-scale versions of Lenna have been obtained in two different ways from the original: (1) Using the green field as a gray-scale image, and (2) Doing an RGB->YUV transformation and saving the Y component. Method (1) makes it easier to compare different people's results since everyone's version should be the same using that method. Method (2) produces a more correct image. For the curious: 'lena' or 'lenna' is a digitized Playboy centerfold, from November 1972. (Lenna is the spelling in Playboy, Lena is the Swedish spelling of the name.) Lena Soderberg (ne Sjooblom) was last reported living in her native Sweden, happily married with three kids and a job with the state liquor monopoly. In 1988, she was interviewed by some Swedish computer related publication, and she was pleasantly amused by what had happened to her picture. That was the first she knew of the use of that picture in the computer business. A scan of the original Lenna from Playboy is available at The editorial in the January 1992 issue of Optical Engineering (v. 31 no. 1) details how Playboy has finally caught on to the fact that their copyright on Lena Sjooblom's photo is being widely infringed. However Wired says in "Although Playboy is notorious for cracking down on illegal uses of its images, it has decided to overlook the widespread distribution of this particular centerfold". The CCITT (ITU-T) test images are in and [The images in*.tif are corrupted.] This set is commonly used to compare binary image compression techniques. The images are 1728x2376 pixels.
Subject: [56] I am looking for a message digest algorithm Look in . MD4 and MD5 are there. See also This question would be more appropriate on sci.crypt.
Subject: [57] I have lost my password on a .zip file This question would be more appropriate on sci.crypt. See Here are a few programs to break pkzip encryption: These are brute force crackers. A known plaintext attack is also possible, see or End of part 1 of the comp.compression faq.

User Contributions:

The Major Advantages of uniform dating

online dating services is not a newest platform but it has taken a sudden rise after the advancement in the technologies. This has revolutionised the way singles meet. With many struggles of dating in today sphere, More people these days are turning their heads towards the internet.

Even for arranged your marriage, Older dating and stuff like that, People are changing themselves and adopting the platform with all open heart and mind. Beside several advantages these platforms possess, Location and religion are the factors that set it apart. You can find your life partner from everywhere and out of any religion. These religions and castes barely matters when it comes to online dating sites.

really fast, Easy and convenient: At the first, Dating platforms might seem to be a daunting process, [url=]online dating ukraine[/url] But in real it is a simple and efficient process to register and connect with people all around the globe. The only thing you need to do is creating an eye catchy and appealing profile mentioning all needed details about you, Your hobbies and your fascinates. Its speedy and convenient access makes it a must have platform for those busy corporate out there.

Less pressing: This is one of the best platforms especially for the people who are shy or nervous as they can connect with people they find interesting via chats unless they get familiar enough with them to either have a verbal talk or start with dating in person. It gives you relaxed atmosphere, Where you can take out efficient time to think to understand and what you want to say to proceed ahead with the conversation.

Meet lots more people: This platform gives user a numerous choices to select from and also it is possible that you can connect with many people altogether and you can find sometimes a person who can be a great friend and a person who can be eligible to be your partner in the future time. This platform will allow you to pick the best one out of many with whom you think you can share your interests with.

add on a Deeper Level: This online site help you know a person back to front. The only appearance you have of your mate is his profile picture, Else you have to know it with the help of the chat you have with him. this can help you evaluate a person behind his face, numerous experts judge who these persons are truly are. Such dating platforms leave you unbiased to be love someone you share similar interests with.

Full Disclosure: The online dating sites allow you to specify whatever is your expectation and intention, Right right from the start so that you can find people looking for the same things and interests as of yours. The major benefit of all such platform is that it helps preventing misunderstandings and disappointments.

fee: Last but not the cheapest, Cost saving is the most appealing benefit of online dating because real life dates are expensive. You need to hangout with your partner every weekend and you further have to spend money either on food or night-life or both.

These platforms therefore gives you enable you to get to know the person well in advance through these online portals and thereafter you should spend money on real dates. different, On the very first meet even you must spend money with no surety that you will like the person as a partner or not.
Hey ... Im looking a man..
I love sex. Here are my erotic photos -
Hello !! Im looking a lover...
I dream of hard sex! Write me -
Hey baby.. my name is Kelly...
I love oral sex! Write me -
Fill out the form and win a FREE $500 or $1000 voucher! -
Hello dear!!! Im looking a man!
I dream of hard sex! Write me -
Hi baby... my name Diane!!!
I love sex. Here are my erotic photos -
Hey dear.. Im looking a man...
I want sex! Write me -
hallo baby!! Im looking a lover!!
I dream of hard sex! Write me -
Oct 27, 2021 @ 3:15 pm
Hello dear! my name is Evelyn..
If you want to meet me, I'm here -
Nov 5, 2021 @ 9:09 am
hey baby. my name Lauren!!!
I love oral sex! Write me -
Nov 14, 2021 @ 10:22 pm
hallo baby.. my name Mary!
Do you want to see a beautiful female body? Here are my erotic photos -
Dec 19, 2021 @ 11:23 pm
Hi dear!! Im looking a man!
I want sex! Here are my photos -
Dec 27, 2021 @ 2:02 am
hallo !! my name Megan!
I love oral sex! Write me -
Jan 12, 2022 @ 10:10 am
Hey baby!!! Im looking a man!
I love sex. Write me -
Jan 22, 2022 @ 1:13 pm
I can agree with Judyhb in this topic. Her name may really be Evelyn, but she choosed another nickname out of shame. Kind of sad, Evelyn is a nice name..
Also, I want sex! Don't have any photos though :/
Sep 3, 2022 @ 7:19 pm
Good write ups. With thanks! degree thesis
May 7, 2023 @ 4:16 pm
How Are You

Hello, I wish for to subscribe for this webpage to get most recent updates, thus where can i do it please help out.

Best Regards
Sep 22, 2023 @ 11:11 am
hallo baby! Im looking a lover!
If you want to meet me, I'm here -
Pan Afuki
Dec 25, 2023 @ 12:12 pm
Matt Mahoney is the trashcan matt or trashcan man. "MY LIFE FOR YOU FLAGG!". We will do the FAGGY PANTS and follow it up with RLE. A close second is Albert Reddit's GAY GZIP, or GAY ZIP, or simply ARZ. We do the faggy pants and it go like this and like that and do the RLE and there we are.

Comment about this article, ask questions, or add new information about this topic:

Part1 - Part2 - Part3 - MultiPage

[ Usenet FAQs | Web FAQs | Documents | RFC Index ]

Send corrections/additions to the FAQ Maintainer:

Last Update March 27 2014 @ 02:11 PM