Internet Surfer 2.0/Internet Surfer 2.0 (Wayzata Technology) (1996).iso/pc/text/mac/faqs.206

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Frequently Asked Questions (FAQS);faqs.206



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 are available by ftp on src.doc.ic.ac.uk,
in directory doc/ccitt-standards/ccitt. The v42bis standard is in
/doc/ccitt-standards/ccitt/1992/v/v42bis.asc.Z.
(ccitt standards used to be on ftp.uu.net in /doc/standards/ccitt
but this directory is now empty -- Aug 11th.)

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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
  wsmr-simtel20.army.mil in pd1:<msdos.compress>ddjcompr.zip. [192.88.110.2]
  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.

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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 fsa.cpsc.ucalgary.ca (136.159.2.1) 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 (fsa.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]>.

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


JPEG:
      Source code for most any machine:
      ftp.uu.net:/graphics/jpeg/jpegsrc.v3.tar.Z [137.39.1.9]
      nic.funet.fi:/pub/graphics/programs/jpeg/jpegsrc.v3.tar.Z [128.214.6.100]
      Contact: [e-mail] (Independent JPEG Group)

      xv, an image viewer which can read JPEG pictures, is available in
      ftp.cicb.fr:/pub/X11R5/contrib/xv-2.20.tar.Z  [129.20.128.2]

epic:
      whitechapel.media.mit.edu:/pub/epic.tar.Z [18.85.0.125]
      The "Lenna" test image is available as part of the EPIC package,
      where it is named "test_image".

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.

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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].

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Subject: [17] What is the state of 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.


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 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 program (see above) works.


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).

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

Barnsley's company is:

Iterated Systems, Inc.
5550A Peachtree Parkway, Suite 545
Norcross, GA  30092
404-840-0728
404-840-0029 (fax)

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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 spec is in ccitt/1988/ascii/7_3_01.txt.Z, the T.6 spec
is in 7_3_02.txt.Z.

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/v3.0beta.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 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.  That, you have to buy from ANSI or your national ISO member
organization (DIN over there, I suppose).

Worse, the TIFF 6.0 spec has a number of serious problems in its JPEG
features.  A clarification note will probably be needed to ensure that TIFF
JPEG files are compatible across different implementations.  I can't in good
faith recommend that anyone use TIFF-JPEG until these problems are resolved.


See also question 54 below.

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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.
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.)

A good introduction to JPEG 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.)

See also the book "JPEG Still Image Data Compression Standard" by
William B. Pennebaker and Joan L. Mitchell. Published by Van Nostrand
Reinhold (phone [phone]), ISBN 0-442-01272-1. 650 Pages, $59.95.
Review by Tom Lane: "This is by far the most complete exposition of
JPEG in existence.  It's written by two people who know what they are
talking about: both serve on the ISO JPEG standards committee.  If you
want to know how JPEG works or why it works that way, this is the book
to have."

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


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


from Thierry TURLETTI <[e-mail]>:

We have implemented a software version of H.261 codec.
It runs on top of UNIX and X-Windows. The coder uses the simple video capture
board "VideoPix" provided by SUN for the SparcStation. The output is directed
towards a standard TCP connection, instead of the leased lines or switched
circuits for which regular H.261 codecs are designed. This enable us to test
video conferences over regular internet connections.
We have to polish it a bit, but the first release is now available by anonymous
ftp from avahi.inria.fr, in "/pub/h261.tar.Z".

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Subject: [25] Fast DCT (Discrete Cosine Transform) algorithms


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

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.

The Rao & Yip book cited in the jpeg v3 DCT code (see item 15 above) is a
good overall introduction, with an extensive (though now dated) bibliography.

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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:/pub/misc/shorten-0.4.shar. It works
by using Huffman coding of prediction residuals.  Compression is generally
better than that obtained by applying general purpose compression utilities
to audio files.


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 that U-LAW and silence detection can also be considered
compression schemes.)


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 3.2 (CELP 3.2)
- 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 pit-manager.mit.edu
in /pub/usenet/news.answers/audio-fmts/part1.


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

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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.

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Subject: [31] pkunzip reports a CRC error!


The portable zip 1.0 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 1.10 or unzip 5.0 (see question
2 above for ftp sites).

Immediately after zip 1.0 was released, a new undocumented feature
of pkunzip was discovered, which causes CRC errors even with pkunzip 1.10
on rare occasions. A patch is available on valeria.cs.ucla.edu in
/pub/zip10.patch.

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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.

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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.

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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;
        }
}

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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.

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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 (18.72.1.58).
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
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.

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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").

The site ftp.ipl.rpi.edu also has standard images, in two directories:
   ftp.ipl.rpi.edu:/pub/image/still/usc
   ftp.ipl.rpi.edu:/pub/image/still/canon

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
   ftp.ipl.rpi.edu:/pub/image/still/usc/color/lena  # 24 bit RGB
   ftp.ipl.rpi.edu:/pub/image/still/usc/bgr/lena    # 24 bit BGR
   ftp.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].

The archive maintainer at ftp.ipl.rpi.edu is interested in some method
of establishing a canonical ftp database of images and could volunteer
the ipl to be 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.