Internet Info 1993/Internet Info CD-ROM (Walnut Creek) (1993).iso/answers/comp/compression-faq/part1

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Path: bloom-beacon.mit.edu!hookup!europa.eng.gtefsd.com!howland.reston.ans.net!EU.net!julienas!chorus!chorus.fr
From: [e-mail] (Jean-loup Gailly)
Newsgroups: comp.compression,comp.compression.research,news.answers,comp.answers
Subject: comp.compression Frequently Asked Questions (part 1/3)
Summary: *** READ THIS BEFORE POSTING ***
Keywords: data compression, FAQ
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Date: 17 Apr 94 12:28:43 GMT
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Archive-name: compression-faq/part1
Last-modified: April 17th, 1994

        "I've already explained this once, but repetition is
        the very soul of the net."        (from alt.config)

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 rtfm.mit.edu:/pub/usenet/news.answers/compression-faq/part[1-3].

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 garbo.uwasa.fi:/pc/doc-net/usenews.zip (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 rtfm.mit.edu:/pub/usenet/news.answers/killfile-faq.)
If you have corrections or suggestions for this FAQ, send them to
Jean-loup Gailly <[e-mail]>.  Thank you.

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 list of image compression hardware.

Main changes relative to the previous version:

- added .hap extension (item 2)
- fix several typos in file names (item 2)
- new version of unp (item 2)
- updated pointer to ivs (items 15 and 20)
- new ftp sites for medical images (item 55)

Contents
========

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]  The WEB 16:1 compressor.
[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 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


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


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.

There is no archive for comp.compression, the volume is too high.
(If you know an ftp site archiving all of comp.compression, tell me
at [e-mail]).

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.binaries.ibm.pc.wanted, comp.os.msdos.apps,
comp.sources.wanted, comp.sys.mac.wanted, comp.archives.msdos.d, comp.dsp,
alt.graphics.pixutils). Please post your request in comp.compression
only as a last resource.

If your question is about graphics only (no compression), please
post to comp.graphics, *after* reading the comp.graphics 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.
(oak.oakland.edu: 141.210.10.117, garbo.uwasa.fi: 128.214.87.1)
Many other sites (in particular wuarchive.wustl.edu: 128.152.135.4)
have the same programs.

To keep this list to a reasonable size, many programs are not
mentioned here. Additional information can be found in the file
ftp.cso.uiuc.edu:/doc/pcnet/compression [128.174.5.61] maintained by
David Lemson ([e-mail]). When several programs can handle
the same archive format, only one of them is given.

Sources for additional lossless data compressors can be found in
garbo.uwasa.fi:/pc/programming/lds_11.zip and
oak.oakland.edu:/pub/msdos/archivers/lz-comp2.zip.
Sources in Pascal are in garbo.uwasa.fi:/pc/turbopas/preskit2.zip.

For Macintosh programs, look on sumex-aim.stanford.edu:/info-mac [36.44.0.6].
For VM/CMS, look on vmd.cso.uiuc.edu:/public.477 [128.174.5.98].
For Atari, look on atari.archive.umich.edu [141.211.165.41]
For Amiga, look on ftp.cso.uiuc.edu:/pub/amiga [128.174.5.59]

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, it is likely that a newer
version exists in the same directory. (Tell me <[e-mail]>)

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, get the sources of the decompressor.

ext:  produced by or read by

.arc: arc, pkarc for MSDOS. (LZW algorithm)
    wuarchive.wustl.edu:/mirrors/msdos/starter/pk361.exe
    garbo.uwasa.fi:/pc/arcers/pk361.exe

      arc for Unix
        wuarchive.wustl.edu:/mirrors/misc/unix/arc521e.tar-z
    garbo.uwasa.fi:/unix/arcers/arc.tar.Z
        Contact: Howard Chu <[e-mail]>

      arc for VMS
    wuarchive.wustl.edu:/packages/compression/vax-vms/arc.exe

      arcmac for Mac
    mac.archive.umich.edu:/mac/utilities/compressionapps/arcmac.hqx

      arc for Amiga
    ftp.funet.fi:pub/amiga/fish/001-100/ff070/arc.lha

.arj: arj for MSDOS (LZ77 with hashing, plus secondary static Huffman
          encoding on a block basis)
        Contact: Robert K Jung <[e-mail]>
        wuarchive.wustl.edu:/mirrors/msdos/archivers/arj241a.exe
    garbo.uwasa.fi:/pc/arcers/arj241a.exe

      unarj for Unix. Decompresses only. (There is no arj compressor for Unix.
        Don't post a request.)
        wuarchive.wustl.edu:/mirrors/misc/unix/unarj241.tar-z
    garbo.uwasa.fi:/unix/arcers/unarj241.tar.Z

      unarj for Mac
        mac.archive.umich.edu:/mac/util/compression/unarjmac.cpt.hqx

      unarj for Amiga
    ftp.funet.fi:pub/amiga/utilities/archivers/unarj-0.5.lha

.bck: VMS BACKUP. BACKUP is *not* a compression program. Do "help backup".

.cpt: Compact Pro for Mac
        sumex-aim.stanford.edu:/info-mac/util/compact-pro-133.hqx [36.44.0.6]

      For Unix:
    sumex-aim.stanford.edu:/info-mac/unix/macutil-20b1.shar
        ftp.cwi.nl:/pub/macutil2.0b3.shar.Z

.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:
        oak.oakland.edu:/pub/msdos/execomp/unp330.zip
      To create such files:
        oak.oakland.edu:/pub/msdos/execomp/lzexe91e.zip
    nic.funet.fi:/pub/msdos/windows/util/winlite1.zip (for Windows exe)

.gif: gif files are images compressed with the LZW algorithm. See the
      comp.graphics 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 2.0 (see below); it can also extract packed and compressed files.
      For Unix, MSDOS, OS/2, VMS, Atari, Amiga, Primos:
    prep.ai.mit.edu:/pub/gnu/gzip-1.2.4.tar  (or .shar or .tar.gz : source)
    prep.ai.mit.edu:/pub/gnu/gzip-msdos-1.2.4.exe (MSDOS, lha self-extract)
    garbo.uwasa.fi:/unix/arcers/gzip-1.2.4.tar.Z  (source)
    garbo.uwasa.fi:/pc/arcers/gzip124.zip         (MSDOS exe)
    ftp.uu.net:/pub/archiving/zip/VMS/gzip123x.exe (VMS exe)
        mac.archive.umich.edu:/mac/util/compression/macgzip0.2.cpt.hqx (Mac)
        mac.archive.umich.edu:/mac/development/source/macgzip0.2src.cpt.hqx

.ha:  ha 0.98 for MSDOS (improved PPMC - 4th order Markov modeling)
    garbo.uwasa.fi:/pc/arcers/ha098.zip

.hap: Hamarsoft HAP 3.00 archiver. Contact: [e-mail]
        garbo.uwasa.fi:/pc/arcers/hap300re.zip

.hqx: Macintosh BinHex format.. (BinHex is *not* a compression program,
    it is similar to uuencode but handles multiple forks.)
       for Mac:
     mac.archive.umich.edu:/mac/utilities/compressionapps/binhex4.0.bin

       for Unix:
         sumex-aim.stanford.edu:/info-mac/cmp/mcvert-212.shar [36.44.0.6]

       for MSDOS:
         wuarchive.wustl.edu:/mirrors/msdos/xbin23.zip

.lha:
.lzh: lha for MSDOS (LZ77 with a trie data structure, plus secondary static
          Huffman coding on a block basis)
    oak.oakland.edu:/pub/msdos/archiver/lha213.exe   (exe)
    oak.oakland.edu:/pub/msdos/archiver/lha211sr.zip (sources)
    garbo.uwasa.fi:/pc/arcers/lha255b.exe

      lharc for Unix. (LZ77 with hash table and binary trees, plus secondary
          Huffman coding)
           Warning: lharc can extract .lzh files created by
           lharc 1.xx but not those created by lha. See lha for Unix below.
        wuarchive.wustl.edu:/mirrors/misc/unix/lharc102a.tar-z
    garbo.uwasa.fi:/unix/arcers/lha101u.tar.Z

      lharc for VMS. Same warning as for Unix lharc.
    wuarchive.wustl.edu:/packages/compression/vax-vms/lharc.exe

      lha for Unix. Warning: all doc is in Japanese.
    wuarchive.wustl.edu:/mirrors/misc/unix/lha101u.tar-z
    garbo.uwasa.fi:/unix/arcers/lha-1.00.tar.Z
          Contact: [e-mail] or [e-mail]

      lha for Mac
        mac.archive.umich.edu:/mac/utilities/compressionapps/maclha2.0.cpt.hqx

      lha for Amiga
    ftp.funet.fi:pub/amiga/utilities/archivers/LhA_e138.run


.pak: pak for MSDOS (LZW algorithm)
        wuarchive.wustl.edu:/mirrors/msdos/archivers/pak251.exe
    garbo.uwasa.fi:/pc/arcers/pak251.exe

.pit: PackIt (Macintosh)
       for Mac:
         sumex-aim.stanford.edu:/info-mac/util/stuffit-151.hqx  [36.44.0.6]

       for Unix:
         sumex-aim.stanford.edu:/info-mac/unix/mcvert-165.shar [36.44.0.6]

.pp: PowerPacker (Amiga)
    ftp.funet.fi:pub/amiga/fish/501-600/ff561/PPLib.lha

.sea: self-extracting archive (Macintosh)
         Run the file to extract it. The self-extraction code can be
         removed with:
      mac.archive.umich.edu:/mac/utilities/compressionapps/desea1.11.cpt.hqx

.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.

.sit: Stuffit for Macintosh
       for Mac:
         sumex-aim.stanford.edu:/info-mac/util/stuffit-lite-30.hqx [36.44.0.6]

       for Unix:
         sumex-aim.stanford.edu:/info-mac/cmp/unsit-15-unix.shar [36.44.0.6]

       for Amiga:
     ftp.funet.fi:pub/amiga/utilities/archivers/unsit-1.5c2.lha

       for MSDOS:
     garbo.uwasa.fi:/pc/arcers/unsit30.zip

.?q?: Squeeze for MSDOS (do not confuse with other 'squeeze' below).
      Static Huffman coding.
    oak.oakland.edu:/pub/msdos/starter/sqpc12a.com (squeeze)
    oak.oakland.edu:/pub/msdos/starter/nusq110.com (unsqueeze)

.sqz: Squeeze for MSDOS (do not confuse with other 'squeeze' above)
      LZ77 with hashing.
    wuarchive.wustl.edu:/mirrors/msdos/archivers/sqz1083e.exe
        garbo.uwasa.fi:/pc/arcers/sqz1083e.exe

.tar: tar is *not* a compression program. However, to be kind for you:
      for MSDOS
    wuarchive.wustl.edu:/mirrors/msdos/starter/tarread.exe
    garbo.uwasa.fi:/pc/unix/tar4dos.zoo

      for Unix
        tar (you have it already. To extract: tar xvf file.tar)

      for VMS
    wuarchive.wustl.edu:/packages/compression/vax-vms/tar.exe

      for Macintosh
        sumex-aim.stanford.edu:/info-mac/util/tar-30.hqx

      for Amiga:
    ftp.funet.fi:pub/amiga/fish/401-500/ff445/Tar.lha

.tar.Z, .tar-z, .taz: tar + compress
      For Unix:     zcat file.tar.Z | tar xvf -
      with GNU tar: tar xvzf file.tar.Z
      Other OS: first uncompress (see .Z below) then untar (see .tar above)

.tar.gz, tar.z, .tgz: tar + gzip
      For Unix: gzip -cd file.tar.gz | tar xvf -
      with GNU tar: tar xvzf file.tar.gz
      Other OS: first uncompress (see .gz above) then untar (see .tar above)

.td0: (compressed MS-DOS floppy image produced by TeleDisk)
      oak.oakland.edu:/pub/msdos/diskutil/teled212.zip

.uc2: UC2 for MSDOS and OS/2. (LZ77 with secondary static Huffman encoding on
     a block basis, and dynamic dictionaries shared among files.)
    Contact: [e-mail]
      garbo.uwasa.fi:/pc/arcers/uc2ins.exe

.z:   pack or gzip (see .gz above). pack uses static Huffman coding.
      To extract, see .gz above.

.zip: pkzip 1.10 for MSDOS. (LZ77 with hashing, plus secondary static
          Shannon-Fano encoding on whole file)
           Contact: [e-mail]
         wuarchive.wustl.edu:/mirrors/msdos/zip/pkz110eu.exe.
         garbo.uwasa.fi:/pc/goldies/pkz110eu.exe.
           Note: pkz110eu.exe is an 'export' version without encryption.

      zip 1.1 for Unix, MSDOS, VMS, OS/2, ... (compatible with pkzip 1.10.
     For corresponding unzip, see unzip 5.1 below).
     ftp.uu.net:/pub/archiving/zip/zip11.zip

      arcutil 2.0 for VM/CMS (unzip only, not yet compatible with pkzip 2.04)
         vmd.cso.uiuc.edu:/public.477/arcutil.* [128.174.5.98].

      pkzip 2.04g for MSDOS. (LZ77 with hashing, plus secondary static
           Huffman coding on a block basis)
     oak.oakland.edu:/pub/msdos/zip/pkz204g.exe
         garbo.uwasa.fi:/pc/arcers/pkz204g.exe

      zip 2.0.1 and unzip 5.1 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: [e-mail]
          (On SGI, do not confuse with the editor also named 'zip'.)

     ftp.uu.net:/pub/archiving/zip/zip201.zip          (source)
     ftp.uu.net:/pub/archiving/zip/unzip51.tar.Z       (source)
     ftp.uu.net:/pub/archiving/zip/MSDOS/zip20x.zip    (MSDOS exe)
     ftp.uu.net:/pub/archiving/zip/MSDOS/unzip51x.exe  (MSDOS exe)
     ftp.uu.net:/pub/archiving/zip/VMS/unz50p1x.exe    (Vax/VMS exe)
     ftp.uu.net:/pub/archiving/zip/VMS/zip20x-vms.zip  (Vax/VMS exe)
     ftp.uu.net:/pub/archiving/zip/AMIGA/unzip51x.*    (Amiga exe)
     ftp.uu.net:/pub/archiving/zip/AMIGA/zip201x.zip   (Amiga exe)
     ftp.uu.net:/pub/archiving/zip/OS2*/*              (OS/2 exe 16&32 bit)
     ftp.uu.net:/pub/archiving/zip/zcrypt21.zip        (encryption source)
         (Non US residents must get the crypt versions from garbo,see below)

     garbo.uwasa.fi:/unix/arcers/zip201.zip    (source)
     garbo.uwasa.fi:/unix/arcers/unzip51.tar.Z (source)
     garbo.uwasa.fi:/pc/arcers/zip20x.zip      (MSDOS exe)
     garbo.uwasa.fi:/pc/arcers/unz51x3.exe     (MSDOS exe)
         garbo.uwasa.fi:/unix/arcers/zcrypt21.zip  (encryption source)

       for Macintosh:
         mac.archive.umich.edu:/mac/util/compression/unzip2.01.cpt.hqx
     mac.archive.umich.edu:/mac/util/compression/zipit1.2.cpt.hqx
     ftp.uu.net:/pub/archiving/zip/MAC/mac-unzip-51.hqx

.zoo: zoo 2.10 for MSDOS (algorithm copied from that of lha, see lha above)
      Contact: Rahul Dhesi <[e-mail]>
     wuarchive.wustl.edu:/mirrors/msdos/zoo/zoo210.exe
     garbo.uwasa.fi:/pc/arcers/zoo210.exe

      zoo 2.10 for Unix, VMS
         oak.oakland.edu:/pub/misc/unix/zoo210.tar.Z
     garbo.uwasa.fi:/unix/arcers/zoo210.tar.Z

      zoo for Mac
      mac.archive.umich.edu:/mac/utilities/compressionapps/maczoo.sit.hqx

      zoo for Amiga
    ftp.funet.fi:pub/amiga/utilities/archivers/Zoo-2.1.lha

.F: freeze for Unix (LZ77 with hashing, plus secondary dynamic Huffman
         encoding)
     wuarchive.wustl.edu:/usenet/comp.sources.misc/volume35/freeze/part0[1-3].Z
     ftp.inria.fr:/system/arch-compr/freeze-2.5.tar.Z
     Contact: Leonid A. Broukhis <[e-mail]>

.Y: yabba for Unix, VMS, ... (Y coding, a variant of LZ78)
  wuarchive.wustl.edu:/usenet/comp.sources.unix/volume24/yabbawhap/part0[1-4].Z
  ftp.inria.fr:/system/arch-compr/yabba.tar.Z
  Contact: Dan Bernstein <[e-mail]>

.Z: compress for Unix ('the' LZW algorithm)
      It is likely that your Unix system has 'compress' already. Otherwise:
    wuarchive.wustl.edu:/packages/compression/compress-4.1.tar
        (not in .Z format to avoid chicken and egg problem)

    compress for MSDOS
    oak.oakland.edu:/pub/msdos/compress/comp430[ds].zip
    garbo.uwasa.fi:/pc/unix/comp430d.zip
    garbo.uwasa.fi:/pc/source/comp430s.zip

    compress for Macintosh
        sumex-aim.stanford.edu:/info-mac/util/maccompress-32.hqx

    compress for Amiga
    ftp.funet.fi:pub/amiga/utilities/archivers/compress-4.1.lha

    compress for Vax/VMS
    wuarchive.wustl.edu:/packages/compression/vax-vms/lzcomp.exe
    wuarchive.wustl.edu:/packages/compression/vax-vms/lzdcmp.exe

------------------------------------------------------------------------------

Subject: [3]  What is the latest PKZIP version?

The latest official version is 2.04g. Release 2.04c had serious bugs,
corrected in 2.04e and 2.04g.

Be warned that there are countless bogus PKZIP 1.20, 2.0, 2.02,
3.05 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 virii.

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.

------------------------------------------------------------------------------

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. Such programs
  are often combined with tar to create compressed archives (see
  question 50: "What is this tar compression program?").

------------------------------------------------------------------------------

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.

The only objective comparisons are speed and compression ratio. Here
is a short table comparing various programs on a 33Mhz Compaq 386.
All programs have been run on Unix SVR4, except pkzip and arj which
only run on MSDOS. (MSDOS benchmarks are available by ftp on
oak.oakland.edu:/pub/msdos/info/arctst*.zip.)

*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.

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 wuarchive.wustl.edu:/mirrors/msdos/ddjmag/ddj9102.zip
  (inner zip file nelson.zip),
- hpack is in wuarchive.wustl.edu:/mirrors/misc/unix/hpack75a.tar-z
  and garbo.uwasa.fi:/unix/arcers/hpack78src.tar.Z
- ha 0.98 is in garbo.uwasa.fi:/pc/arcers/ha098.zip
- ftp.adelaide.edu.au:/pub/compression/lzrw3-a.c  [129.127.40.3]

The 14 files used in the comparison are from the standard Calgary
Text Compression Corpus, available by ftp on ftp.cpsc.ucalgary.ca
[136.159.7.18] in /pub/text.compression.corpus/text.compression.corpus.tar.Z.

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: I did not have time to run again all benchmarks with more
recent versions of zip, freeze, arj and hpack. To be done for some
future revision of this FAQ.]

       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     35186    34950    35619    29840   26927
book1  339076    339074    318382    313566   312619   306876   237380  235733
book2  228444    228442    210521    207204   206306   208486   174085  163535
geo     68576     68574     69209     68698    68418    58976    64590   59356
news   155086    155084    146855    144954   144395   141608   128047  123335
obj1    10312     10310     10333     10307    10295    10572    10819    9799
obj2    84983     84981     82052     81213    81336    80806    85465   80381
paper1  19678     19676     18710     18519    18525    18607    16895   15675
paper2  32098     32096     30034     29566    29674    29825    25453   23956
pic     52223     52221     53578     52777    55051    51778    55461   51639
progc   13943     13941     13408     13363    13238    13475    12896   11795
progl   16916     16914     16408     16148    16175    16586    17354   15298
progp   11509     11507     11308     11214    11182    11647    11668   10498
trans   22580     22578     20046     19808    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
  wuarchive.wustl.edu:/mirrors/msdos/archivers/ar002.zip).

- 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 1991] Mark Nelson, "The Data Compression Book"
    M&T Books, Redwood City, CA, 1991.  ISBN 1-55851-216-0.
    Price $36.95 including two 5" PC-compatible disks 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

[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.


------------------------------------------------------------------------------

Subject: [8] What about patents on data compression algorithms?


[Note: the appropriate group for discussing software patents is
comp.patents (or misc.legal.computing), not comp.compression.]

All patents mentioned here are US patents, and thus probably
not applicable outside the US. 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.

- 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 algorithm previously patented by
  Eastman-Lempel-Ziv (4,464,650), and 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 patents on LZ77 with parallel lookup in
  hardware (4,841,092 and 5,003,307).

- Robert Jung, author of 'arj', has been granted patent 5,140,321
  for one variation of LZ77 with hashing.  This patent covers 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. (The US patent system allows this because of the
  'invention date' rule.)

- 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.

- 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) and in Postscript Level 2.
  (Unisys sells the license to modem manufacturers for a onetime
  $25,000 fee.) The IBM patent application was 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.)

- AP coding is patented by Storer (4,876,541). (Get the yabba package
  for source code, see question 2 above, file type .Y)


(d) arithmetic coding

- IBM holds many patents on arithmetic coding (4,286,256 4,295,125
  4,463,342 4,467,317 4,633,490 4,652,856 4,891,643 4,905,297 4,935,882).
  It has patented in particular the Q-coder implementation of arithmetic
  coding.  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 the JPEG FAQ for details.)

  AT&T has 3 patents on arithmetic coding (4,973,961, 5,023,611, 5,025,258).


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. A number of them are
taken from the list prep.ai.mit.edu:/pub/lpf/patent-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.

4,286,256
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

4,412,306
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
assignees Sperry Corporation and At&T Bell Laboratories
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.

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
The text for this patent can be ftped from rusmv1.rus.uni-stuttgart.de
(129.69.1.12) in /info/comp.patents/US4558302.Z.

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.

4,809,350
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.
granted Jan  2, 1990
assignee IBM

4,905,297
granted Feb 27, 1990
assignee IBM
Arithmetic coding encoder and decoder system.

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

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.

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 <[e-mail]> 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

5,049,881
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
Barnsley, fractal compression

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

5,155,484
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.]

Japan 2-46275
Coding system
granted Feb 26, 1990
[Patents one form of arithmetic coding.]

------------------------------------------------------------------------------

Subject: [9]  The WEB 16:1 compressor.


[WARNING: this topic has generated the greatest volume of news in the
history of comp.compression. Read this before posting on this subject.]

[WARNING 2: it is quite possible that the story is repeating itself
with a compressor called MINC by Premier Research Corporation, Ltd.
They claim a breakthrough in lossless data compression using a recursive
method, that is, applying the compressor to the compressed output of
the previous run.]

[WARNING 3: the OWS program, which also claims incredible compression
ratios, is a hoax. It just remembers the clusters which contained
the data. The data can be recovered only if those clusters are not
used again for another file. Needless to say, never trust such a
lossy program.]

(a) 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. [...]


(b) First details, by John Wallace <[e-mail]>:

I called WEB at [phone] 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 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.


(c) More information, by Rafael Ramirez <[e-mail]>:

   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. [...]


(d) The impossiblity proofs.

It is impossible for a given program to compress without loss *all*
files greater than a certain size by at least one bit. This can be
proven by a simple counting argument. (Many other proofs have been
posted on comp.compression, *please* do not post yet another one.)

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. (Stronger results
about the number of incompressible files can be obtained, but the
proofs are a little more complex.)

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
basic mathematics.

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 or a number of iterations is necessary to decompress
the data, the bits providing 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 iteration
count or file name, 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).

[See also question 73 "What is the theoretical compression limit?" in
part 2 of this FAQ.]


(e) No software version

Appeared on BIX, reposted by Bruce Hoult <[e-mail]>:

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. [...]


(f) Product cancelled

Posted by John Toebes <[e-mail]> 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.


(g) Conclusion

The last report given above should put an end to the WEB story.

[Note from the FAQ maintainer: I intended to remove this story from
the FAQ, but the recent announcement of the MINC compressor has some
similarities with the WEB story so it is useful to keep it a little
longer.]

------------------------------------------------------------------------------

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
<[e-mail]>, IEEE Micro, Oct 1992, pp. 41-53.

Short introduction, by Alejo Hausner <[e-mail]>:

The V.42bis Compression Standard was proposed by the International
Consultative Committee on Telephony and Telegraphy (CCITT) 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 escape code.  Henceforth the
data is passed as plain bytes.

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.

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 control code.  Thus the method allows the
hardware to adapt to the compressibility of the data.


The CCITT standards documents used to be available by ftp on
ftp.uu.net in /doc/standards/ccitt, but this service has been
discontinued. If you ftp to digital.resource.org, in directory pub/standards
there is a file that says that making the standards available in the
first place was just an experiment.

The documents are now on src.doc.ic.ac.uk, but the directory name
keeps changing. Check one of:
  /computing/ccitt/ccitt-standards/ccitt/
  /computing/ccitt/standards/ccitt
  /doc/ccitt-standards/ccitt
in this order. The v42bis standard is in *standards/ccitt/1992/v/v42bis.asc.Z.


A mail server for CCITT documents is available at [e-mail]
or [e-mail]. A Gopher server is also available:

Name=International Telecommunication Union (ITU)
Host=info.itu.ch
Port=70

For more information, contact Robert Shaw <[e-mail]> or
Antoinette Bautista <[e-mail]>.  Warning by John Levine
<[e-mail]> (probably obsolete by now):

  This teledoc thing is much less than meets the eye.  What it
  actually has is one-page abstracts of some but not all CCITT
  recommendations, along with junk like lists of the national
  representatives to CCITT.  If you want the actual text of a
  recommendation, you have to send large amounts of money to
  Switzerland, same as ever. However, a closer reading of the Teledoc
  announcement shows that they say they're planning to make the actual
  text of some CCITT recommendations available on-line sometime in 1993.


See also the Standards FAQ posted to news.answers or get it by ftp in
rtfm.mit.edu:/pub/usenet/news.answers/standards-faq.

------------------------------------------------------------------------------

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
  oak.oakland.edu:/pub/msdos/compress/ddjcompr.zip
  garbo.uwasa.fi:/pc/source/ddjcompr.zip [128.214.87.1]

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 available
via anonymous ftp from ftp.cpsc.ucalgary.ca (136.159.7.18) in directory
/pub/arithmetic.coding.  It only comes with a simple order-0 model but
it's set up so that adding your own more sophisticated one is
straightforward.

A low precision arithmetic coding implementation avoiding hardware
division is available on the same site (ftp.cpsc.ucalgary.ca)
in /pub/arithmetic.coding/low.precision.version/low.precision.version.shar.

Kris Popat <[e-mail]> 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 via anonymous ftp from
media-lab.media.mit.edu (18.85.0.2), in /pub/k-arith-code.

The program 'urban' in ddjcompr.zip (see item 12 above) is a high order
arithmetic coder working at the bit level. It is written by Urban Koistinen
<[e-mail]>.

------------------------------------------------------------------------------

Subject: [15] Where can I get image compression programs?


JPEG:
    Source code for most any machine:
    ftp.uu.net:/graphics/jpeg/jpegsrc.v4.tar.Z [137.39.1.9]
    nic.funet.fi:/pub/graphics/packages/jpeg/jpegsrc.v4.tar.Z [128.214.6.100]
    Contact: [e-mail] (Independent JPEG Group)

    havefun.stanford.edu:pub/jpeg/JPEGv1.2.tar.Z (supports lossless mode)
    Contact: Andy Hung <[e-mail]> (see item 20 below)

    xv, an image viewer which can read JPEG pictures, is available in
    export.lcs.mit.edu: contrib/xv-2.21.tar.Z [18.24.0.12]

MPEG:
    havefun.stanford.edu:/pub/mpeg/MPEGv1.2.alpha.tar.Z
    Contact: Andy Hung <[e-mail]> (see item 20 below)

    toe.cs.berkeley.edu:/pub/multimedia/mpeg/mpeg_play-2.0.tar.Z
    toe.cs.berkeley.edu:/pub/multimedia/mpeg/mpeg_encode-1.0.tar.Z.
    Contact: [e-mail]

    toe.cs.berkeley.edu:/pub/multimedia/mpeg/vmpeg10.zip
    decel.ecel.uwa.edu.au:/users/michael/mpegw32e.zip (for Windows and NT)

    nvr.com:/pub/NVR-software/Product-1.0.4.tar.Z   (192.82.231.50)
    (free demo copy of NVR's software toolkit for SPARCstations)
    Contact: Todd Brunhoff <[e-mail]>

H.261(P*64):
    havefun.stanford.edu:pub/p64/P64v1.2.alpha.tar.Z
    Contact: Andy Hung <[e-mail]> (see item 20 below)

    zenon.inria.fr:/rodeo/ivs/ivs3.3-src.tar.Z (Inria videoconference system)
    Contact: Thierry Turletti <[e-mail]> (see item 20 below).

JBIG:
    nic.funet.fi:/pub/graphics/misc/test-images/jbig.tar.gz.

epic: (pyramid wavelet coder, see item 72)
    whitechapel.media.mit.edu:/pub/epic.tar.Z [18.85.0.125]
    Contact: Eero P. Simoncelli <[e-mail]>
    The "Lenna" test image is available as part of the EPIC package,
    where it is named "test_image".

hcompress: (wavelet impage compression, see item 72)
    stsci.edu:/software/hcompress/hcompress.tar.Z

wavethresh: (wavelet software for the language S)
    gdr.bath.ac.uk:/pub/masgpn/wavethresh2.2.Z
    Contact: [e-mail]

rice-wlet: (wavelet software, see item 72)
    cml.rice.edu:/pub/dsp/software/rice-wlet-tools.tar.Z

compfits:
    uwila.cfht.hawaii.edu:/pub/compfits/compfits.tar.Z  [128.171.80.50]
    Contact: Jim Wright <[e-mail]>

fitspress:
    cfata4.harvard.edu:/pub/fitspress08.tar.Z [128.103.40.79]

tiff:
    For source and sample images, see question 18 below.

DCT algorithms:
    etro.vub.ac.be:/pub/DCT_ALGORITHMS/*
    Contact: Charilos Christopoulos <[e-mail]>


For fractal compression programs, see item 17 below.
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 current state-of-the-art is the JBIG algorithm.  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 a description of JBIG in ISO/IEC CD 11544, contained in
document ISO/IEC JTC1/SC2/N2285.  The only way to get it is to ask
your National Standards Body for a copy. In the USA, call ANSI at
[phone].

------------------------------------------------------------------------------

Subject: [17] What is the state of fractal compression?

It is recommended to read first item 77 in part 2 of this FAQ:
"Introduction to Fractal compression".


from Tal Kubo <[e-mail]>:

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 <[e-mail]> 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 <[e-mail]> 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 <[e-mail]> 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 <[e-mail]> 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:

A fractal image compression program is available by ftp in
lyapunov.ucsd.edu:/pub/young-fractal/unifs10.zip. (Unix users, See
item 2 above for unzip on Unix.) Note the file size before you ftp it:
1.2 MB. The package contains source for compression and decompression,
source for X-windows decompression, MSDOS executables and images.
A newer version of the program is in yuvpak20.zip.

A fractal image decompression program (note: decompression only) is
available in /pub/inls-ucsd/fractal-2.0.tar on on the same ftp site
(lyapunov.ucsd.edu). Note the file size before you ftp it: 1.3 MB.
This file also contains a paper by Yuval Fisher (see reference below),
and some executables and sample images. Reading this paper is required
to understand how the Young compression programs (see above) works.

A note from Yuval Fisher <[e-mail]>:

    Ftp to legendre.ucsd.edu and look in pub/Research/Fisher.  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 source code for the program published in the Oct 93 issue of
Byte is in ftp.uu.net:/published/byte/93oct/fractal.exe. This is
self-extractible zip file (use "unzip fractal.exe" to extract on
non MSDOS systems). The source code is for a TARGA video board.

Another fractal compression program is available by ftp in
vision.auc.dk:/pub/Limbo/Limbo*.tar.Z.

References:
  A. Jacquin, 'Fractal image coding based on a theory of iterated
    contractive image transformations', Visual Comm. and Image
    Processing, vol SPIE-1360, 1990. (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 [phone].

  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:
    The Fractal Transform,
    Michael F. Barnsley and Louisa F. Anson
    ISBN 0-86720-218-1, ca. 250 pp, $49.95

    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 [e-mail] 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: [phone] or 1-800-4FRACTL
fax: [phone]
In UK: Phone [phone], Fax [phone]

------------------------------------------------------------------------------

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 specs are available by anonymous ftp (see above answer on
V.42bis).  The T.4 and T.6 specs are on src.doc.ic.ac.uk in directory
/computing/ccitt/ccitt-standards/ccitt/1988/ascii, files 7_3_01.txt.Z and
7_3_02.txt.Z respectively.

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:

    sgi.com:/graphics/tiff/v3.2.tar.Z    [192.48.153.1]
    Contact: [e-mail]

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 TIFF6.ps.Z.
Concerning JPEG compression in TIFF 6.0, Tom Lane <[e-mail]> adds:

  The TIFF document won't do you much good unless you also have the official
  JPEG standard. You can buy it from ANSI or your national ISO member
  organization (DIN over there, I suppose). [See also the book by Pennebaker
  and Mitchell referenced in item 75 of this FAQ.]

  Worse, the TIFF 6.0 spec has serious problems in its JPEG features.  It is
  probable that section 22 (JPEG) will be rewritten from scratch.  If you are
  considering implementing TIFF/JPEG, please contact me at [e-mail] for
  the latest word.

Software for reading and writing CCITT Group 3 and 4 images is
also available in directory merry.cs.monash.edu.au:/pub/alanf/TIFF_FAX
(130.194.67.101). Contact: Alan Finlay <[e-mail]>.


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 so 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.  Standards for compressing those types
of images are being worked on by other committees, named 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 not widely 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 <[e-mail]>.  (See question
53 "Where are FAQ lists archived" if this posting has expired at your site.)

For JPEG software, see item 15 above.
For JPEG hardware, see item 85 in part 3 of this FAQ.

------------------------------------------------------------------------------

Subject: [20] I am looking for source of an H.261 codec and MPEG


The H.261 spec is available on src.doc.ic.ac.uk in
/computing/ccitt/standards/ccitt/1992/h/h261.doc.Z (or h261.rtf.Z).


For H.261 hardware, see item 85 in part 3 of this FAQ.

from Thierry TURLETTI <[e-mail]>:

     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 zenon.inria.fr 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 <[e-mail]>. If you want
     to be added or deleted to the ivs-users mailing list, please send
     e-mail to [e-mail].


from Andy Hung <[e-mail]>:

Public domain UNIX C source code to do both image and image sequence
compression and decompression is available by anonymous ftp:

MPEG-I            havefun.stanford.edu:pub/mpeg/MPEGv1.2.alpha.tar.Z
CCITT H.261(P*64)    havefun.stanford.edu:pub/p64/P64v1.2.alpha.tar.Z
JPEG            havefun.stanford.edu:pub/jpeg/JPEGv1.2.beta.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
the anonymous ftp directory havefun.stanford.edu pub/cv/CVv1.1.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
[e-mail]).  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
havefun.stanford.edu:pub/README.

------------------------------------------------------------------------------

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).
This has an extensive, though already dated, bibliography.

Here are some newer references provided by Tom Lane <[e-mail]>.
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.

The free JPEG code (jpegsrc.v4.tar.Z) has one of the fastest implementations
of the DCT code.  It's all in the files jfwddct.c and jrevdct.c (which do
the dct and idct, respectively). See item 15 for ftp locations.

------------------------------------------------------------------------------

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 by
anonymous ftp at svr-ftp.eng.cam.ac.uk:/comp.speech/sources/shorten-1.11.tar.Z.
An MSDOS executable is in shn109.exe. 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. Shorten version 1.11 also supports lossy compression.
Contact: Tony Robinson <[e-mail]>.

An MPEG audio player is available on sunsite.unc.edu in
/pub/electronic-publications/IUMA/audio_utils/mpeg_players/Workstations,
file maplay.tar.Z. The sources of the XING MPEG audio player for Windows
are also there (sunsite.unc.edu) in
/pub/electronic-publications/IUMA/audio_utils/mpeg_players/Windows/mpgaudio.zip


Copied from the comp.dsp FAQ posted by [e-mail] (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 ftp.cwi.nl
  as /pub/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.)


You can get a G.721/722/723 package by email to [e-mail], with

GET ITU-3022

as the *only* line in the body of the message.


A note on u-law from Markus Kuhn <[e-mail]>:

  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 rtfm.mit.edu:/pub/usenet/news.answers.


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

You can find the comp.dsp FAQ in comp.dsp or news.answers with subject:
"FAQ: Audio File Formats" or by ftp on rtfm.mit.edu
in /pub/usenet/news.answers/audio-fmts/part1.


CELP C code for Sun SPARCs is available for anonymous ftp at
furmint.nectar.cs.cmu.edu, in directory celp.audio.compression.
Version 3.2a is also in super.org:/pub/celp_3.2a.tar.Z.


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.


from Markus Kuhn <[e-mail]>:

  One highest quality sound compression format is called ASPEC and has
  been developped 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 frequencys 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 <[e-mail]> and Carsten Bormann
<[e-mail]>:

  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
  tub.cs.tu-berlin.de as /pub/tubmik/gsm-1.0.tar.Z.  Questions and bug
  reports should be directed to [e-mail].
  Note that the distribution is not available via E-mail (please use one
  of the ftp-via-E-mail servers).


from Bob Kimball <[e-mail]>:

  I work for Qualcomm Inc. and we are designing a digital cellular telephone
  system.  Our phone uses our variable rate vocoder (QCELP) which is designed
  for speach and compresses 64Kb/s speach to 8Kb/s through 1Kb/s with 8Kb/s
  being full rate and 1Kb/s for 1/8 rate speach.  It works great for speach.

  The QCELP process is documented in our Common Air Interface (CAI) which is
  available for anonymous ftp from lorien.qualcomm.com in /pub/cdma
  each chapter is a postscript file.  The vocoder is described in appendix A.
  The whole document is quite large.  This is the document which is currently
  going through the TIA standard committee so it is not a final version.  The
  appendix on the vocoder should be almost identical to the final version...
  whenever that comes out.


from Nicola Ferioli <[e-mail]>:

  oak.oakland.edu:/pub/msdos/sound/vocpak20.zip
    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.

------------------------------------------------------------------------------

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.

------------------------------------------------------------------------------

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 5.0p1 (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.  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 comp.binaries.ibm.pc.d.

------------------------------------------------------------------------------

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. 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. But since this
question comes up often, here is some code (by Rob Warnock <[e-mail]>).

The following C code 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;
        }
}

See also ftp.uni-erlangen.de:/pub/doc/ISO/english/async-HDLC, and the
source of all archivers, such as the file makecrc.c in the sources of
zip 2.0 (see item 2).

------------------------------------------------------------------------------

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 available by anonymous FTP from rtfm.mit.edu.
The comp.compression FAQ that you are reading is in directory
   /pub/usenet/news.answers/compression-faq

If you don't have FTP access, you can access the archives by mail
server.  Send an email message to [e-mail]
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 [e-mail]

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Subject: [54] I need specs for graphics formats


Have a look in directory /pub/graphics.formats on zamenhof.cs.rice.edu.
It contains descriptions of gif, tiff, fits, etc...

See also the FAQ list for comp.graphics. See item 53 for an ftp site.

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Subject: [55] Where can I find Lenna and other images?


A bunch of standard images (lenna, baboon, cameraman, crowd, moon
etc..) are on ftp site eedsp.gatech.edu (130.207.226.2) in directory
/database/images. The images are in 256-level grayshades (256x256
pixels, 256 "colors").

[Note: the site ipl.rpi.edu 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 ipl.rpi.edu (128.113.14.50) has standard images in two
directories:
   ipl.rpi.edu:/pub/image/still/usc
   ipl.rpi.edu:/pub/image/still/canon

(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 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
   ipl.rpi.edu:/pub/image/still/usc/color/lena  # 24 bit RGB
   ipl.rpi.edu:/pub/image/still/usc/bgr/lena    # 24 bit BGR
   ipl.rpi.edu:/pub/image/still/usc/gray/lena   # 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 ftp.ee.lbl.gov:/pbmplus*.tar.Z].
Questions about the ipl archive should be sent to [e-mail].


There are few gray-scale still images and some raw data of test results
available in directory nic.funet.fi:/pub/graphics/misc/test-images.

Medical images can be found in decaf.stanford.edu:/pub/images/medical/mri,
eedsp.gatech.edu:/database/images/wchung/medical, and
omicron.cs.unc.edu:/pub/softlab/CHVRTD.


Rodney Peck <[e-mail]> 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
[e-mail].


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.

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 Lenna Sjooblom's photo is being widely infringed.
It sounds as if you will have to get permission from Playboy to
publish it in the future.

The CCITT test images are available on nic.funet.fi in directory
pub/graphics/misc/test-images, files ccitt1.tif to ccitt8.tif.

Note on the CCITT test images, by Robert Estes <[e-mail]>:

The ccitt files are in ipl.rpi.edu:/image-archive/bitmap/ccitt
(128.113.14.50). [Note from FAQ maintainer: this directory has
now disappeared; ipl.rpi.edu is a very volatile ftp site :-).]
They are named ccitt-n.ras.Z where n goes from 1 to 8.
Each file has an accompanying doc file called ccitt-n.ras.doc which
describes the image file. Here's the doc file for ccitt-1.ras:

Name ccitt-1.ras
Size 1728 x 2376 x 1
Type 1 bit standard format sun rasterfile
Keywords binary standard image 1 bit fax
Description
One of eight images from the standard binary CCITT test image set.

This set is commonly used to compare binary image compression
techniques. The images are are 1728x2376 pixels.

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Subject: [56] I am looking for a message digest algorithm


Look on the ftp site rsa.com, in directory /pub. MD4 and MD5 are there.
This question would be more appropriate on sci.crypt.



          End of part 1 of the comp.compression faq.