SGI Developer Toolbox 6.1/SGI Developer Toolbox 6.1 - Disc 4.iso/FAQs/netfaqs/jpeg-faq

Text File | 1994-08-01 | 62KB | 1,130 lines

Newsgroups: comp.graphics,alt.graphics.pixutils,alt.binaries.pictures.utilities,alt.binaries.pictures.d,alt.binaries.pictures.erotica.d,comp.answers,alt.answers,news.answers
From: [e-mail] (Tom Lane)
Subject: JPEG image compression: Frequently Asked Questions
Followup-To: alt.binaries.pictures.d
Summary: Useful info about JPEG (JPG) image files and programs
Keywords: JPEG, image compression, FAQ
Supersedes: <[e-mail]>
Reply-To: [e-mail]
Organization: Independent JPEG Group
Date: Sun, 17 Apr 1994 20:56:39 GMT
Approved: [e-mail]
Expires: Sun, 15 May 1994 20:56:35 GMT
Lines: 1113
Xref: bloom-beacon.mit.edu comp.graphics:23553 alt.graphics.pixutils:3382 alt.binaries.pictures.utilities:13578 alt.binaries.pictures.d:8039 alt.binaries.pictures.erotica.d:19712 comp.answers:4943 alt.answers:2456 news.answers:18181

Archive-name: jpeg-faq
Last-modified: 17 April 1994

This article discusses JPEG image compression.  Suggestions for additions
and clarifications are welcome.

New since version of 4 April 1994:
  * New versions of ImageMagick (2.3.7), QPEG (1.3n), PMJPEG (1.63), and
    Viewtek (2.1).
  * Added entry for Paint Shop Pro.


This article includes the following sections:

[1]  What is JPEG?
[2]  Why use JPEG?
[3]  When should I use JPEG, and when should I stick with GIF?
[4]  How well does JPEG compress images?
[5]  What are good "quality" settings for JPEG?
[6]  Where can I get JPEG software?
    [6A] viewers, application programs, etc.
    [6B] source code
[7]  What's all this hoopla about color quantization?
[8]  What are some rules of thumb for converting GIF images to JPEG?
[9]  Does loss accumulate with repeated compression/decompression?
[10]  Why all the argument about file formats?
[11]  How do I recognize which file format I have, and what do I do about it?
[12]  How does JPEG work?
[13]  Isn't there a lossless JPEG?
[14]  What about arithmetic coding?
[15]  Could an FPU speed up JPEG?  How about a DSP chip?

Sections 1-6 are basic info that every JPEG user needs to know;
sections 7-15 are more advanced info.

This article is posted every 2 weeks.  You can always find the latest
version in the news.answers archive at rtfm.mit.edu (18.70.0.209).  By FTP,
fetch rtfm.mit.edu:/pub/usenet/news.answers/jpeg-faq.  If you don't have
FTP, send e-mail to [e-mail] containing the line
    send usenet/news.answers/jpeg-faq
(If you don't get a reply, the server may be misreading your return address;
add a line such as "path myname@mysite" to specify your correct e-mail
address to reply to.)  Many other FAQ articles are available in the
news.answers archive, which is also accessible via WAIS, WWW, and Gopher
services.  For more information about the archive, retrieve the file
rtfm.mit.edu:/pub/usenet/news.answers/news-answers/introduction.


----------


[1]  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 images
of natural, real-world scenes.  It works well on photographs, naturalistic
artwork, and similar material; not so well on lettering, simple cartoons,
or line drawings.  JPEG handles only still images, but there is a related
standard called MPEG for motion pictures.

JPEG is "lossy," meaning that the decompressed image isn't quite the same as
the one you started with.  (There are lossless image compression algorithms,
but JPEG achieves much greater compression than is possible with lossless
methods.)  JPEG is designed to exploit known limitations of the human eye,
notably the fact that small color changes are perceived less accurately than
small changes in brightness.  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.

A useful property of JPEG is that the degree of lossiness can be varied by
adjusting compression parameters.  This means that the image maker can trade
off file size against output image quality.  You can make *extremely* small
files if you don't mind poor quality; this is useful for applications like
indexing image archives.  Conversely, if you aren't happy with the output
quality at the default compression setting, you can jack up the quality
until you are satisfied, and accept lesser compression.

Another important aspect of JPEG is that decoders can trade off decoding
speed against image quality, by using fast but inaccurate approximations to
the required calculations.  Until recently, most publicly available JPEG
code has adopted a best-possible-quality philosophy, but we are now starting
to see the appearance of viewers that give up some image quality in order to
obtain significant speedups.


[2]  Why use JPEG?

There are two good reasons: to make your image files smaller, and to store
24-bit-per-pixel color data instead of 8-bit-per-pixel data.

Making image files smaller is a big win for transmitting files across
networks and for archiving libraries of images.  Being able to compress a
2 Mbyte full-color file down to 100 Kbytes or so makes a big difference in
disk space and transmission time!  (If you are comparing GIF and JPEG, the
size ratio is more like four to one.  More details in section 4.)

If your viewing software doesn't support JPEG directly, you'll have to
convert JPEG to some other format for viewing or manipulating images.  Even
with a JPEG-capable viewer, it takes longer to decode and view a JPEG image
than to view an image of a simpler format such as GIF.  Thus, using JPEG is
essentially a time/space tradeoff: you give up some time in order to store
or transmit an image more cheaply.

It's worth noting that when network or phone transmission is involved, the
time savings from transferring a shorter file can be greater than the extra
time needed to decompress the file.

The second fundamental advantage of JPEG is that it stores full color
information: 24 bits/pixel (16 million colors).  GIF, the other image format
widely used on Usenet, can only store 8 bits/pixel (256 or fewer colors).
GIF is reasonably well matched to inexpensive computer displays --- most
run-of-the-mill PCs can't display more than 256 distinct colors at once.
But full-color hardware is getting cheaper all the time, and JPEG images
look *much* better than GIFs on such hardware.  Within a couple of years,
8-bit GIF will seem as obsolete as black-and-white MacPaint format does
today.  Furthermore, for reasons detailed in section 7, JPEG is far more
useful than GIF for exchanging images among people with widely varying
display hardware.  Hence JPEG is considerably more appropriate than GIF for
use as a Usenet posting standard.

A lot of people are scared off by the term "lossy compression".  But when
it comes to representing real-world scenes, *no* digital image format can
retain all the information that impinges on your eyeball.  By comparison
with the real-world scene, JPEG loses far less information than GIF.  The
technical meaning of "lossy" has nothing to do with this, though; it refers
to loss of information over repeated compression cycles, a problem that you
may or may not care about.  (If you do, see section 9.)


[3]  When should I use JPEG, and when should I stick with GIF?

JPEG is *not* going to displace GIF entirely; for some types of images,
GIF is superior in image quality, file size, or both.  One of the first
things to learn about JPEG is which kinds of images to apply it to.

Generally speaking, JPEG is superior to GIF for storing full-color or
gray-scale images of "realistic" scenes; that means scanned photographs and
similar material.  Any continuous variation in color, such as occurs in
highlighted or shaded areas, will be represented more faithfully and in less
space by JPEG than by GIF.

GIF does significantly better on images with only a few distinct colors,
such as line drawings and simple cartoons.  Not only is GIF lossless for
such images, but it often compresses them more than JPEG can.  For example,
large areas of pixels that are all *exactly* the same color are compressed
very efficiently indeed by GIF.  JPEG can't squeeze such data as much as GIF
does without introducing visible defects.  (One implication of this is that
large single-color borders are quite cheap in GIF files, while they are best
avoided in JPEG files.)

Computer-drawn images (ray-traced scenes, for instance) usually fall between
photographs and cartoons in terms of complexity.  The more complex and
subtly rendered the image, the more likely that JPEG will do well on it.
The same goes for semi-realistic artwork (fantasy drawings and such).

JPEG has a hard time with very sharp edges: a row of pure-black pixels
adjacent to a row of pure-white pixels, for example.  Sharp edges tend to
come out blurred unless you use a very high quality setting.  Edges this
sharp are rare in scanned photographs, but are fairly common in GIF files:
borders, overlaid text, etc.  The blurriness is particularly objectionable
with text that's only a few pixels high.  If you have a GIF with a lot of
small-size overlaid text, don't JPEG it.

Plain black-and-white (two level) images should never be converted to JPEG;
they violate all of the conditions given above.  You need at least about
16 gray levels before JPEG is useful for gray-scale images.  It should also
be noted that GIF is lossless for gray-scale images of up to 256 levels,
while JPEG is not.

If you have a large library of GIF images, you may want to save space by
converting the GIFs to JPEG.  This is trickier than it may seem --- even
when the GIFs contain photographic images, they are actually very poor
source material for JPEG, because the images have been color-reduced.
Non-photographic images should generally be left in GIF form.  Good-quality
photographic GIFs can often be converted with no visible quality loss, but
only if you know what you are doing and you take the time to work on each
image individually.  Otherwise you're likely to lose a lot of image quality
or waste a lot of disk space ... quite possibly both.  Read sections 7 and 8
if you want to convert GIFs to JPEG.


[4]  How well does JPEG compress images?

Very well indeed, when working with its intended type of image (photographs
and suchlike).  For full-color images, the uncompressed data is normally 24
bits/pixel.  The best known lossless compression methods can compress such
data about 2:1 on average.  JPEG can typically achieve 10:1 to 20:1
compression without visible loss, bringing the effective storage requirement
down to 1 to 2 bits/pixel.  30:1 to 50:1 compression is possible with small
to moderate defects, while for very-low-quality purposes such as previews or
archive indexes, 100:1 compression is quite feasible.  An image compressed
100:1 with JPEG takes up the same space as a full-color one-tenth-scale
thumbnail image, yet it retains much more detail than such a thumbnail.

For comparison, a GIF version of the same image would start out by
sacrificing most of the color information to reduce the image to 256 colors
(8 bits/pixel).  This provides 3:1 compression.  GIF has additional "LZW"
compression built in, but LZW doesn't work very well on typical photographic
data; at most you may get 5:1 compression overall, and it's not at all
uncommon for LZW to be a net loss (less than 3:1 overall compression).
When a JPEG file is made from full-color data, using a quality setting just
high enough to prevent visible loss, the JPEG will typically be a factor of
four or five smaller than a GIF file made from the same data.

Gray-scale images do not compress by such large factors.  Because the human
eye is much more sensitive to brightness variations than to hue variations,
JPEG can compress hue data more heavily than brightness (gray-scale) data.
A gray-scale JPEG file is generally only about 10%-25% smaller than a
full-color JPEG file of similar visual quality.  But the uncompressed
gray-scale data is only 8 bits/pixel, or one-third the size of the color
data, so the calculated compression ratio is much lower.  The threshold of
visible loss is often around 5:1 compression for gray-scale images.

The exact threshold at which errors become visible depends on your viewing
conditions.  The smaller an individual pixel, the harder it is to see an
error; so errors are more visible on a computer screen (at maybe 70
dots/inch) than on a high-quality color printout (300 or more dots/inch).
Thus a higher-resolution image can tolerate more compression ... which is
fortunate considering it's much bigger to start with.  The numbers quoted
above are typical for screen viewing.  Also note that the threshold of
visible error varies considerably across images.


[5]  What are good "quality" settings for JPEG?

Most JPEG compressors let you pick a file size vs. image quality tradeoff by
selecting a quality setting.  There seems to be widespread confusion about
the meaning of these settings.  "Quality 95" does NOT mean "keep 95% of the
information", as some have claimed.  The quality scale is purely arbitrary;
it's not a percentage of anything.

In fact, quality scales aren't even standardized across JPEG programs.  The
quality settings discussed in this article apply to the free JPEG software
described in section 6B, and to many programs based on it.  Other JPEG
implementations, notably Apple's and HSI's, use completely different quality
scales; for instance, Apple's scale covers 0-4, not 0-100.  Some programs
don't even provide a numeric scale, just "high"/"medium"/"low"-style
choices.  (Fortunately, this doesn't prevent different implementations from
exchanging compressed files.)

In most cases the user's goal is to pick the lowest quality setting, or
smallest file size, that decompresses into an image indistinguishable from
the original.  This setting will vary from one image to another and from one
observer to another, but here are some rules of thumb.

For good-quality, full-color source images, the default quality setting
(Q 75) is very often the best choice.  This setting is about the lowest you
can go without expecting to see defects in a typical image.  Try Q 75 first;
if you see defects, then go up.

If the image was less than perfect quality to begin with, you might be able
to drop down to Q 50 without objectionable degradation.  On the other hand,
you might need to go to a *higher* quality setting to avoid further loss.
Q 85 to 95 is often best for converting GIFs (see section 8 for more info).

Except for experimental purposes, never go above about Q 95; using Q 100
will produce a file two or three times as large as Q 95, but of hardly any
better quality.  If you see a file made with Q 100, it's a pretty sure sign
that the maker didn't know what he/she was doing.

If you want a very small file (say for preview or indexing purposes) and are
prepared to tolerate large defects, a Q setting in the range of 5 to 10 is
about right.  Q 2 or so may be amusing as "op art".


[6]  Where can I get JPEG software?

Most of the programs described in this section are available by FTP.
If you don't know how to use FTP, see the "Anonymous FTP FAQ List" article.
(If you don't have direct access to FTP, read about ftpmail servers in the
same article.)  That article appears regularly in news.answers, or you can
get it by sending e-mail to [e-mail] with the single line
"send usenet/news.answers/ftp-list/faq" in the body.

NOTE: this list changes frequently.  If you have a copy more than a couple
months old, get the latest JPEG FAQ from the news.answers archive.


[6A]  If you are looking for viewers, application programs, etc:

This section covers programs for the following kinds of systems:
    X Windows, MS-DOS, Microsoft Windows, OS/2, Macintosh,
    Amiga, Atari ST, Acorn Archimedes, NeXT.
If you don't see what you want for your machine, check out the free JPEG
source code described in section 6B.  Assuming you have a C compiler and at
least a little knowledge of compiling C programs, you should be able to
prepare JPEG conversion programs from the source code.  You'll also need a
viewer program.  If your display is 8 bits or less, any GIF viewer will do
fine; if you have a display with more color capability, try to find a viewer
that can read Targa or PPM 24-bit image files.

Note that this list concentrates on free and shareware programs that you can
obtain over Internet; but a few commercial programs are listed too.  If you
choose a commercial JPEG product, make sure that it can handle the Usenet-
standard JFIF file format, or you won't be able to exchange images with
anyone else.  (See section 10 if you want to know more about file formats.)


X Windows:

XV (shareware, $25) is an excellent viewer for JPEG, GIF, and many other
image formats.  It can also do format conversion and some simple image
manipulations.  It's available for FTP from ftp.cis.upenn.edu (130.91.6.8),
file pub/xv/xv-3.00a.tar.Z.  Version 3.00 is a major upgrade with support
for 24-bit displays and many other improvements; however, it is brand new
and still has some bugs lurking.  If you prefer not to be on the bleeding
edge, stick with version 2.21, available from the same place.  Note that
version 2.21 is not a good choice if you have a 24-bit display (you'll get
only 8-bit color), nor is it good for converting 24-bit images to JPEG.
But 2.21 works fine for converting GIF and other 8-bit images to JPEG.
CAUTIONS:
* with version 3.0, if you have an 8-bit display then you need to "lock
  8-bit mode" to get decent display of JPEG images.  (But do NOT do this
  if you intend to resave the image, because it'll be written from the
  8-bit version, thus costing you image quality.)
* with version 2.21, you need to check the "save at normal size" checkbox
  when saving a JPEG file, or the file will be blurry.
Both of these workarounds can be set up in your .Xdefaults file.

Another good choice for X Windows is John Cristy's free ImageMagick package,
available from ftp.x.org (198.112.44.100), file contrib/ImageMagick.tar.gz.
This package handles many image processing and conversion tasks.  The
ImageMagick viewer handles 24-bit displays correctly; for colormapped
displays, it does better (though slower) color quantization than XV or the
basic free JPEG software.  The current version is 2.3.7.

Both of the above are large, complex packages.  If you just want a simple
image viewer, try xloadimage or xli.  xloadimage views and converts many
image file types including JPEG.  Version 4.1 has better JPEG support than
prior versions and is easier to install.  xloadimage is free and available
from ftp.x.org, file contrib/xloadimage.4.1.tar.Z.  xli is a variant version
of xloadimage; xli is slightly better as an interactive viewer, but it can't
be used as a converter, and it supports fewer file formats.  xli is also
free and available from ftp.x.org, file contrib/xli.1.15.tar.Z.

If you want a command-line JPEG conversion program, see the IJG source code
described in section 6B.  (This code is included as a subdirectory in most
of the above-mentioned packages.)

MS-DOS:

This covers plain DOS; for Windows or OS/2 programs, see the next headings.

QPEG is the fastest noncommercial JPEG viewer I know of.  In exchange for
speed, QPEG gives up some image quality, particularly on 256-or-less-color
displays.  Its best feature is a really-fast small preview window, which is
great for searching through lots of image files.  Also views GIF and Targa.
Requires 386-or-better CPU and VGA-or-better display card.  Shareware, $20.
Current version is 1.3n, available from ftp.tu-clausthal.de (139.174.2.10),
file pub/msdos/graphics/qpeg13n.zip.  From the USA, access to that site is
very slow; instead try ftp.rahul.net in directory pub/bryanw/pc/jpeg/.

DVPEG is a free viewer for JPEG, GIF, Targa, and PPM files.  The current
version, 3.0l, is available by FTP from sunee.uwaterloo.ca (129.97.50.50),
file pub/jpeg/viewers/dvpeg30l.zip.  (That's lower case l, not digit 1.)
This is a good basic viewer that works
on either 286 or 386/486 machines.  The user interface is clunky but
functional.  DVPEG is substantially faster than it used to be; on
hi-color displays it is nearly as fast as QPEG.  On 8-bit displays, its
two-pass quantization mode is slow but gives much better image quality than
QPEG can manage.

Lesser-used DOS viewers include:
* DISPLAY, alias DISP.  Has many nice image manipulation features including
  contact-sheet generation.  User interface is much improved over versions
  prior to 1.70, but installation is still a little tricky.  Requires 386
  or better.  Current version is 1.80a, available from nctuccca.edu.tw:
  /PC/graphics/disp/disp180a.zip (also at Simtel sites).  Freeware.
* NVIEW.  Views JPEG and half a dozen other image formats.  Easy to use,
  very easy to install.  Fairly fast with moderate image quality on 8-bit
  displays.  Does not support hi-color or true-color modes (yet), so this
  is not the viewer for you if you have such a card.  Requires 386 or
  better.  Current version is 1.2, available from Simtel archive sites (see
  NOTE below), file msdos/graphics/nview120.zip.  Shareware, $25.
* ColorView for DOS.  This viewer's main advantage is easy installation.
  Menu interface is spiffy-looking but I find it a bit clunky to use.
  Does not require 386, should work with any display that has a VESA
  driver.  Current version is 2.1, available from Simtel archive sites (see
  NOTE below), file msdos/graphics/dcview21.zip.  Requires a VESA graphics
  driver; if you don't have one, look in vesadrv2.zip or vesa-tsr.zip from
  the same directory.  Shareware, $30.

DISRECOMMENDATION: The well-known GIF viewer CompuShow (CSHOW, recently
renamed 2SHOW) supports JPEG, but CSHOW's JPEG implementation is crummy:
it's much slower than any of the above viewers, and its image quality is
poor except on hi-color displays.  Too bad ... it'd have been nice to see a
good JPEG capability in CSHOW.  If you want it anyway, see Simtel archive
sites (see NOTE below), file msdos/gif/cshw101a.zip.  Shareware, $25.

Due to the remarkable variety of PC graphics hardware, any one of these
viewers might not work on your particular machine.  If you can't get *any*
of them to work, you'll need to use one of the following conversion programs
to convert JPEG to GIF, then view with your favorite GIF viewer.  (If you
have hi-color hardware, don't use GIF as the intermediate format; try to
find a TARGA-capable viewer instead.  VPIC5.0 is reputed to do the right
thing with hi-color displays.)

The free IJG JPEG converters are available from Simtel archive sites (see
NOTE below), file msdos/graphics/jpeg4.zip (or jpeg4386.zip if you have a
386 and extended memory).  These programs will convert JPEG to and from GIF,
Targa, and PPM formats; they are DOS compilations of the free source code
described in section 6B.

Handmade Software offers free JPEG<=>GIF conversion tools, GIF2JPG/JPG2GIF.
These are slow and are limited to conversion to and from GIF format; in
particular, they can't produce 24-bit color output from a JPEG.  The sole
advantage of these tools is that they will read and write HSI's proprietary
JPEG format as well as the Usenet-standard JFIF format.  Since HSI-format
files are rather widespread on BBSes, this is a useful capability.  Version
2.0 of these tools is free (prior versions were shareware).  Get it from
Simtel archive sites (see NOTE below), file msdos/graphics/gif2jpg2.zip.
NOTE: do not use HSI format for files to be posted on Usenet, since it is
not readable on non-PC platforms.

Handmade Software also has a shareware image conversion and manipulation
package, Image Alchemy.  This will translate JPEG files (both JFIF and HSI
formats) to and from many other image formats.  It can also display images.
A demo version of Image Alchemy version 1.7 is available from Simtel archive
sites (see NOTE below), file msdos/graphics/alch17.zip.

JPGINDEX is a useful tool for making indexes of JPEG image collections.
Available from Simtel archive sites, file msdos/graphics/jpgidx13.zip.

NOTE ABOUT SIMTEL FILES:  The largest Internet collection of PC-related
programs is the Simtel archives (named for the original archive site, now
defunct).  The principal archive site for these files is oak.oakland.edu
(141.210.10.117), which keeps Simtel files under /pub; so look in, eg,
/pub/msdos/graphics.  There are many mirror sites which keep copies of the
Simtel files; for quickest response you should use the mirror site closest
to you.  Consult the periodic postings in comp.archives.msdos.announce to
find your nearest mirror site.  If you have no FTP capability, the same
postings will tell you how to retrieve Simtel files by e-mail.

Microsoft Windows:

LView is a freeware viewer/converter for JPEG, GIF, Targa, and BMP files.
The latest version is 3.1, available from ftp.std.com (192.74.137.7),
file src/pc/graphics/lview/lview31.zip.  Requires 386 or better CPU.
This is a very good, feature-laden program.  LView can load JPEGs with
either fast/low-quality (ECJ) code or slow/high-quality (IJG) code.

WinECJ is a fast, no-frills viewer with image quality noticeably worse than
most other JPEG viewers.  (You can purchase a version with better image
quality for AUD$30.)  Version 1.2 is free and available from Simtel archive
sites (see NOTE above), file msdos/windows3/winecj12.zip.  Requires Windows
3.1 and 256-or-more-colors mode.  If you want more than bare-bones features,
try LView instead; LView with the ECJ option is only fractionally slower
than WinECJ, and it does much more.

WinJPEG (shareware, $20) displays JPEG,GIF,Targa,TIFF,PCX, and BMP files;
it can write all of these formats too, so it can be used as a converter.
It has some other nifty features including screen capture, color-balance
adjustment, and slideshow.  The current version is 2.43, available from
ftp.cica.indiana.edu, file pub/pc/win3/desktop/winjp243.zip.  (This is a
286-compatible version; if you register, you'll get the 386-and-up version,
which is roughly twice as fast.)

Some people prefer Paint Shop Pro.  It's not very impressive as just a JPEG
viewer (especially since image quality is not very good on 8-bit displays),
but if you need more functionality than LView or WinJPEG offer, PSP may be
what you need.  Shareware, $69.  Current version is 2.00, available from
ftp.cica.indiana.edu, file pub/pc/win3/desktop/pspro200.zip.

QPEG and DVPEG (see DOS heading) work under Windows, but only in full-screen
mode, not in a window.  Also note that you can run the DOS conversion
programs described earlier inside a Windows DOS window.

OS/2:

The following files are available from ftp-os2.cdrom.com (192.153.46.2):

os2/2_x/graphics/jovw122b.zip
    JoeView 1.22b: free JPEG/GIF/BMP/PCX/TIFF viewer.
os2/2_x/graphics/pmjpg163.zip
    PMJPEG 1.63: OS/2 2.x port of WinJPEG, a popular viewer for Windows
    (see description in Windows section).  Shareware, $20.
os2/2_x/graphics/pmvu86b.zip
    PMView 0.86b: JPEG/GIF/BMP/Targa/PCX viewer.  GIF viewing very fast,
    JPEG viewing roughly the same speed as the above two programs.  Has
    image manipulation & slideshow functions.  Shareware, $35.
os2/2_x/graphics/imgarc13.zip
    Image Archiver 1.03: image conversion/viewing with PM graphical interface.
    Strong on conversion functions, viewing is a bit weaker.  Shareware, $15.
os2/2_x/graphics/jpegv4.zip
    32-bit version of free IJG conversion programs, version 4.
os2/all/graphics/jpeg4_16.zip
    16-bit version of same, for OS/2 1.x.

All of the OS/2 viewers require Palette Manager for best display quality.

Macintosh:

Most Mac JPEG programs rely on Apple's JPEG implementation, which is part of
the QuickTime system extension; so you need to have QuickTime installed.
To use QuickTime, you need a 68020 or better CPU and you need to be running
System 6.0.7 or later.  (If you're running System 6, you must also install
the 32-bit QuickDraw extension; it is built-in on System 7.)  The latest
version of QuickTime is 1.6.1, available by FTP from ftp.apple.com, file
dts/mac/sys.soft/quicktime/quicktime-1-6-1.hqx.

Mac users should keep in mind that QuickTime's JPEG format, PICT/JPEG, is
not the same as the Usenet-standard JFIF JPEG format.  (See section 10 for
details.)  If you post images on Usenet, make sure they are in JFIF format.
Most of the programs mentioned here can handle either format.

JPEGView is an excellent free program for viewing JFIF, PICT/JPEG, GIF, TIFF,
and other image files.  It can convert between the two JPEG formats and can
create preview images for files.  The current version is 3.2.1, available from
sumex-aim.stanford.edu (36.44.0.6), file info-mac/grf/util/jpeg-view-321.hqx.
Requires System 7 and QuickTime.  JPEGView usually produces the best color
image quality of all the currently available Mac JPEG viewers, and it needs
much less memory to view large images than most other Mac viewers.  Given a
large image, JPEGView automatically scales it down to fit on the screen,
rather than presenting scroll bars like most other viewers.  (You can zoom
in on any desired portion, though.)  Some people like this behavior, some
don't.  Overall, JPEGView's user interface is very well thought out.

GIFConverter, a shareware ($40) image viewer/converter, supports JFIF,
PICT/JPEG, GIF, and many other image formats.  Current version is 2.3.7,
available at mac.archive.umich.edu (141.211.120.11), file
mac/graphics/graphicsutil/gifconverter2.37.cpt.hqx.  Requires System 6.0.5
or later.  GIFConverter is not better than JPEGView as a plain JPEG/GIF
viewer, but it has much more extensive image manipulation and format
conversion capabilities.  Also, GIFConverter can load and save JFIF images
*without* QuickTime, so it is your best bet if your machine is too old to
run QuickTime.  (But it's faster with QuickTime.)  Hint: if GIFConverter
runs out of memory while loading a large JPEG, try converting the file to
GIF with JPEG Convert, then viewing the GIF version.

A competitor to GIFConverter is GraphicConverter, also shareware ($35).
Current version is 1.7.8, available from sumex-aim.stanford.edu, file
info-mac/grf/util/graphic-converter-178.hqx.  Requires System 7 and QuickTime
to handle JPEG.  I haven't used these two programs enough to say which one is
better ... try 'em both.

JPEG Convert, a Mac version of the free IJG JPEG conversion utilities, is
available from sumex-aim.stanford.edu, file info-mac/app/jpeg-convert-10.hqx.
This will run on any Mac, but it only does file conversion, not viewing.
You can use it in conjunction with any GIF viewer.

Storm Technology's Picture Decompress is a free JPEG viewer/converter.
This rather old program is inferior to the above programs in many ways, but
it will run without System 7 or QuickTime, so you may be forced to use it on
older systems.  (It does need 32-bit QuickDraw, so really old machines can't
use it.)  You can get it from sumex-aim.stanford.edu, file
info-mac/app/picture-decompress-201.hqx.

If your machine is too old to run 32-bit QuickDraw (a Mac Plus for instance),
GIFConverter is your only choice for single-program JPEG viewing.  If you
don't want to pay for GIFConverter, use JPEG Convert and a free GIF viewer.

More and more commercial Mac applications are supporting JPEG, although not
all can deal with the Usenet-standard JFIF format.  Adobe Photoshop, version
2.0.1 or later, can read and write JFIF-format JPEG files (use the JPEG
plug-in from the Acquire menu).  You must set the file type of a downloaded
JPEG file to 'JPEG' to allow Photoshop to recognize it.

Amiga:

Most programs listed in this section are available from "AmiNet" archive
sites.  The master AmiNet site is wuarchive.wustl.edu (128.252.135.4), but
there are many mirror sites and you should try to use the closest one.

Osma Ahvenlampi posted a good review of Amiga picture viewers in
comp.sys.amiga.reviews in March 1994.  You can retrieve it from math.uh.edu,
pub/Amiga/comp.sys.amiga.reviews/software/graphics/PictureViewerSurvey_2.
Opinions here are mostly stolen from his article.

FastJPEG is a free JPEG viewer; it's fast and has good image quality, but it
doesn't view any formats except JPEG.  Version 1.10 is even faster than the
original, and it works with grayscale JPEGs now.  Available from Aminet
sites, file pub/aminet/gfx/show/FastJPEG_1.10.lha.

PPShow is a very good free JPEG/GIF/ILBM/ANIM/Datatype viewer.  Version 4.0
is available from Aminet sites, file pub/aminet/gfx/show/PPShow40.lha.  For
viewing JPEGs it is a little slower than FastJPEG, and image quality is not
as good (particularly on non-AGA machines); but if you want to use just one
viewer, PPShow is the one.

HamLab Plus is an excellent JPEG viewer/converter, as well as being a
general image manipulation tool.  It's cheap (shareware, $20) and can read
several formats besides JPEG.  The current version is 2.0.8.  A demo version
is available from AmiNet sites, file pub/aminet/gfx/edit/hamlab208d.lha.
The demo version will crop images larger than 512x512, but it is otherwise
fully functional.

Rend24 (shareware, $30) is an image renderer that can display JPEG, ILBM,
and GIF images.  The program can be used to create animations, even
capturing frames on-the-fly from rendering packages like Lightwave.
The current version is 1.05, available from AmiNet sites, file
pub/aminet/gfx/aga/rend105.lha.

Viewtek is a free JPEG/ILBM/GIF/ANIM viewer.  The current version is 2.1,
available from AmiNet sites, file pub/aminet/gfx/show/ViewTEK21.lha.  This
is reported to be a considerable improvement over 2.0 (which was actually a
very buggy beta version).  Viewtek used to be the best free JPEG viewer for
Amiga, but it now faces stiff competition from FastJPEG and PPShow.  The
choice depends on your display hardware and personal preference.

The free IJG JPEG software is available compiled for Amigas from AmiNet
sites, file pub/aminet/gfx/conv/AmigaJPEGV4.lha.  These programs convert
JPEG to/from PPM,GIF,Targa formats.

If you're willing to spend real money, there are several commercial packages
that support JPEG.  Well regarded products include CineMorph (from Great
Valley Products), ImageFX (ditto), Art Department Professional or ADPro
(ASDG Inc), and ImageMaster (Black Belt Systems).

The Amiga world is heavily infested with quick-and-dirty JPEG programs, many
based on an ancient beta-test version of the free IJG JPEG software (thanks
to a certain magazine that published same on its disk-of-the-month, without
so much as notifying the authors).  Among these are "AugJPEG", "NewAmyJPEG",
"VJPEG", and probably others I have not even heard of.  In my opinion,
anything older than IJG version 3 (March 1992) is not worth the disk space
it's stored on; if you have such a program, trash it and get something newer.

Atari ST:

GEM-View (shareware, $26) displays JPEG, GIF, and other image formats.
Current version is 2.48, available from atari.archive.umich.edu, file
/atari/Graphics/Gemview/gview248.lzh.  This is a well regarded viewer.
The English documentation tends to be a few versions behind, though.

The free IJG JPEG software is available compiled for Atari ST/TT/etc
from atari.archive.umich.edu, file /atari/Graphics/jpeg4bin.zoo.
These programs convert JPEG to/from PPM, GIF, Targa formats.

For monochrome ST monitors, try MGIF, which manages to achieve four-level
gray-scale effect by flickering.  Version 4.2 loads JPEG files much faster
than 4.1 did.  Available from atari.archive.umich.edu, file
/atari/Graphics/mgif42b.zoo.

Acorn Archimedes:

The Acorn archive at micros.hensa.ac.uk (148.88.8.84) contains several
JPEG-capable programs.  Read the file /micros/arch/riscos/index for
retrieval instructions.  Recommended archive entries include:
a022 Translator 7.18: image file format converter (shareware)
b008 FYEO 2.00: For Your Eyes Only, fast JPEG/GIF image viewer (shareware)
b024 ARCJPEG 1.14R: IJG v4 software (JPEG<=>PPM,GIF,Targa) w/ desktop front end

!ChangeFSI, supplied with RISC OS 3 version 3.10, can convert from and view
JPEG JFIF format.  Provision is also made to convert images to JPEG,
although this must be done from the CLI rather than by double-clicking.

There's also a commercial product called !JPEG which provides JPEG read/write
functionality and direct JPEG viewing, as well as a host of other image
format conversion and processing options.  Contact: DT Software, FREEPOST,
Cambridge, UK.  Tel: [phone].

NeXT:

ImageViewer is a PD utility that displays images and can do some format
conversions.  The current version reads JPEG but does not write it.
ImageViewer is available from the NeXT archives at sonata.cc.purdue.edu and
cs.orst.edu, file pub/next/3.0/bin/ImageViewer0.9i.tar.Z.  Note that there
is an older version floating around that does not support JPEG.

NeXTStep includes built-in support for TIFF/JPEG, but not for the
Usenet-standard JFIF format.  Be warned that the TIFF/JPEG standard is
likely to change away from the flavor currently produced by NeXTStep,
so compatibility with other platforms is doubtful.


[6B]  If you are looking for source code to work with:

Free, portable C code for JPEG compression is available from the Independent
JPEG Group, which I lead.  A package containing our source code,
documentation, and some small test files is available from ftp.uu.net
(192.48.96.9) in directory /graphics/jpeg.  The current release is v4, file
jpegsrc.v4.tar.Z.  (This is a compressed TAR file; don't forget to retrieve
in binary mode.)  You can retrieve this file by FTP or UUCP.  Copies can
also be found at many other Internet sites.  If you are on a PC and don't
know how to cope with .tar.Z format, you may prefer ZIP format, which you
can find at Simtel archive sites (see NOTE above), file
msdos/graphics/jpegsrc4.zip.  On CompuServe, see the GRAPHSUPPORT forum (GO
GRAPHSUP), library 15, file jpgs4a.zip.  If you have no FTP access, you can
retrieve the code via an ftpmail server; see the Anonymous FTP FAQ article
referred to at the top of section 6.

The free JPEG code provides conversion between JPEG "JFIF" format and image
files in GIF, PBMPLUS PPM/PGM, Utah RLE, and Truevision Targa file formats.
The core compression and decompression modules can easily be reused in other
programs, such as image viewers.  The package is highly portable; we have
tested it on many machines ranging from PCs to Crays.

We have released this software for both noncommercial and commercial use.
Companies are welcome to use it as the basis for JPEG-related products.
We do not ask a royalty, although we do ask for an acknowledgement in
product literature (see the README file in the distribution for details).
We hope to make this software industrial-quality --- although, as with
anything that's free, we offer no warranty and accept no liability.

The Independent JPEG Group is a volunteer organization; if you'd like to
contribute to improving our software, you are welcome to join.


A different free JPEG implementation, written by the PVRG group at Stanford,
is available from havefun.stanford.edu in directory pub/jpeg.  This program
is designed for research and experimentation rather than production use;
it is slower, harder to use, and less portable than the IJG code, but it
implements a larger subset of the JPEG standard.


[7]  What's all this hoopla about color quantization?

Most people don't have full-color (24 bit per pixel) display hardware.
Typical display hardware stores 8 or fewer bits per pixel, so it can display
256 or fewer distinct colors at a time.  To display a full-color image, the
computer must choose an appropriate set of representative colors and map the
image into these colors.  This process is called "color quantization".
(This is something of a misnomer; "color selection" or "color reduction"
would be a better term.  We're stuck with the standard usage though.)

Clearly, color quantization is a lossy process.  It turns out that for most
images, the details of the color quantization algorithm have *much* more
impact on the final image quality than do any errors introduced by JPEG
itself (except at the very lowest JPEG quality settings).  Making a good
color quantization algorithm is a black art, and no single algorithm is best
for all images.

Since JPEG is a full-color format, converting a color JPEG image for display
on 8-bit-or-less hardware requires color quantization.  The speed and image
quality of a JPEG viewer running on such hardware are largely determined by
its quantization algorithm.  You'll see great variation in image quality
among viewers on 8-bit displays, much more than occurs on 24-bit displays.

On the other hand, a GIF image has already been quantized to 256 or fewer
colors.  (A GIF *does* have a specific number of colors in its palette, and
the format doesn't allow more than 256 palette entries.)  GIF has the
advantage that the image maker precomputes the color quantization, so
viewers don't have to; this is one of the things that make GIF viewers
faster than JPEG viewers.  But this is also the *disadvantage* of GIF:
you're stuck with the maker's quantization.  If the maker quantized to a
different number of colors than what you can display, you'll either waste
display capability or have to quantize again to further reduce the number of
colors (which results in much poorer image quality than if you had quantized
once from a full-color image).  Furthermore, if the maker didn't use a
high-quality color quantization algorithm, you're out of luck --- the image
is ruined.

For this reason, JPEG promises significantly better image quality than GIF
for all users whose machines don't match the image maker's display hardware.
JPEG's full color image can be quantized to precisely match the viewer's
display hardware.  Furthermore, you will be able to take advantage of future
improvements in quantization algorithms (there is a lot of active research
in this area), or purchase better display hardware, to get a better view of
JPEG images you already have.  With a GIF, you're stuck forevermore with
what was sent.

A growing number of people have better-than-8-bit display hardware already:
15-bit "hi-color" PC displays, true 24-bit displays on workstations and
Macintoshes, etc.  For these people, GIF is already obsolete, as it cannot
represent an image to the full capabilities of their display.  JPEG images
can drive these displays much more effectively.

In short, JPEG is an all-around better choice than GIF for representing
photographic images in a machine-independent fashion.


It's sometimes thought that a JPEG converted from a GIF shouldn't require
color quantization.  This is false: even when you feed a 256-or-less-color
GIF into JPEG, what comes out of the decompressor is not 256 colors, but
thousands of colors.  This happens because JPEG's lossiness affects each
pixel a little differently, so two pixels that started with identical colors
will usually come out with slightly different colors.  Each original color
gets "smeared" into a group of nearby colors.  Therefore quantization is
always required to display a color JPEG on a colormapped display, regardless
of the image source.

The same effect makes it nearly meaningless to talk about the number of
colors used by a JPEG image.  Even if you tried to count the number of
distinct pixel values, different JPEG decoders would give you different
results because of roundoff error differences.  I occasionally see posted
images described as "256-color JPEG".  This tells me that the poster
(a) hasn't read this FAQ and (b) probably converted the JPEG from a GIF.
JPEGs can be classified as color or gray-scale, but number of colors just
isn't a useful concept for JPEG, any more than it is for a real photograph.


[8]  What are some rules of thumb for converting GIF images to JPEG?

Converting GIF files to JPEG is a tricky business --- you are piling one set
of limitations atop a quite different set, and the results can be awful.
Certainly a JPEG made from a GIF will never be as good as a JPEG made from
true 24-bit color data.  But if what you've got is GIFs, and you need to
save space, here are some hints for getting the best results.

With care and a clean source image, it's often possible to make a JPEG of
quality equivalent to the GIF.  This does *not* mean that the JPEG looks
identical to the GIF --- it probably won't on an 8-bit display, because the
color quantization process used to display the JPEG won't exactly match the
GIF's quantization.  (See section 7 for more about that.)  But given a good
viewer, the JPEG will look as good as the GIF.  Some people claim that on
24-bit displays, a carefully converted JPEG can look better than the GIF
source, because dither patterns have been eliminated.  (More about dithering
in a moment.)

On the other hand, JPEG conversion *will* degrade an unsuitable image or one
that is converted carelessly.  If you are not willing to take the amount of
trouble suggested below, you're much better off leaving your GIF images
alone.  Simply cranking the JPEG quality setting up to a very high value
wastes space (which defeats the whole point of the exercise...) and some
images will be degraded anyway.

The first rule is never to convert an image that's not appropriate for JPEG
(see section 3 about that).  Large, high-visual-quality photographic images
are usually the best material.  And they take up lots of space in GIF form,
so they offer significant potential space savings.  (A good rule of thumb is
not to bother converting any GIF that's much under 100 Kbytes; the potential
space savings isn't worth the hassle.)

The second rule is to look at each JPEG, to make sure you are happy with it,
before throwing away the corresponding GIF; this will give you a chance to
re-do the conversion with a higher quality setting if necessary.  Also
compare the file sizes --- if the image isn't suitable JPEG material, a JPEG
file of reasonable quality may come out *larger* than the GIF.

The third rule is to get rid of the border.  Many people have developed
an odd habit of putting a large single-color border around a GIF image.
While useless, this is nearly free in terms of storage cost in GIF files.
It is NOT free in JPEG files, either in storage space or in decoding time;
and the sharp border boundary can create visible artifacts ("ghost" edges).
Furthermore, when viewing a bordered JPEG on an 8-bit display, the quantizer
will think the border color is important because there's so much of it, and
hence will waste color palette entries on the border, thus actually reducing
the displayed quality of the main part of the image!  So do yourself a favor
and crop off any border before JPEGing.

Gray-scale images usually convert without much problem.  When using cjpeg,
be sure to specify -gray.  (Otherwise, cjpeg treats a GIF as color data;
this works but wastes space and time for gray-scale data.)  Quality settings
around the default (75) are usually fine.

Color images are much trickier.  Color GIFs of photographic images are
usually "dithered" to fool your eye into seeing more than the 256 colors
that GIF can actually store.  If you enlarge the image, you will find that
adjacent pixels are often of significantly different colors; at normal size
the eye averages these pixels together to produce the illusion of an
intermediate color value.  The trouble with dithering is that, to JPEG, it
looks like high-spatial-frequency color noise; and JPEG can't compress noise
very well.  The resulting JPEG file is both larger and of lower image
quality than what you would have gotten from JPEGing the original full color
image (if you had it).  To get around this, you need to "smooth" the GIF
image before compression.  Smoothing averages together nearby pixels, thus
approximating the color that you thought you saw anyway, and in the process
getting rid of the rapid color changes that give JPEG trouble.  Proper use
of smoothing will both reduce the size of the compressed file and give you a
better-looking output image than you'd get without smoothing.

With the V4 free JPEG software (or programs based on it), a simple smoothing
capability is built in.  Try "-smooth 10" or so when converting GIFs.
Values of 10 to 25 seem to work well for high-quality GIFs.  Heavy-handed
dithering may require larger smoothing factors.  (If you can see regular
fine-scale patterns on the GIF image even without enlargement, then strong
smoothing is definitely called for.)  Too large a smoothing factor will blur
the output image, which you don't want.  If you are an image processing
wizard, you can also do smoothing with a separate filtering program, but
appropriate use of such tools is beyond the scope of this FAQ.

Quality settings around 85 (a bit higher than default) usually work well
when converting color GIFs, assuming that you've picked a good smoothing
factor.  You may need to go higher if you can't hide the dithering pattern
with a reasonable smoothing factor.  Really badly dithered GIFs are best
left as GIFs.

Don't expect JPEG files converted from GIFs to be as small as those created
directly from full-color originals.  You won't be able to smooth away all of
the dithering noise (without blurring the image) and this noise wastes space.
Typically, a good-quality converted JPEG will be 1/2 to 1/3rd the size of
the GIF file, not 1/4th as suggested in section 4.  If the JPEG comes out
much more than half the size of the GIF, this is a good sign that the image
shouldn't be converted at all.

The upshot of all this is that "cjpeg -quality 85 -smooth 10" is probably a
good starting point for converting color GIFs.  But if you care about the
image, you'll want to check the results and maybe try a few other settings.
Blindly converting a large GIF library at this or any other setting is a
recipe for disaster.


[9]  Does loss accumulate with repeated compression/decompression?

It would be nice if, having compressed an image with JPEG, you could
decompress it, manipulate it (crop off a border, say), and recompress it
without any further image degradation beyond what you lost initially.
Unfortunately THIS IS NOT THE CASE.  In general, recompressing an altered
image loses more information.  Hence it's important to minimize the number
of generations of JPEG compression between initial and final versions of an
image.

It turns out that if you decompress and recompress an image at the same
quality setting first used, little or no further degradation occurs.
(Counterintuitively, this works better the lower the quality setting.
But you must use *exactly* the same setting, or all bets are off.)
This means that you can make local modifications to a JPEG image without
material degradation of other areas of the image.  The areas you change
will degrade, though.

Unfortunately, cropping doesn't count as a local change!  JPEG processes
the image in small blocks, and cropping usually moves the block boundaries,
so that the image looks completely different to JPEG.  You can take
advantage of the low-degradation behavior if you are careful to crop the
top and left margins only by a multiple of the block size (typically 16
pixels), so that the remaining blocks start in the same places.

The bottom line is that JPEG is a useful format for archival storage and
transmission of images, but you don't want to use it as an intermediate
format for sequences of image manipulation steps.  Use a lossless 24-bit
format (PPM, RLE, TIFF, etc) while working on the image, then JPEG it when
you are ready to file it away.  Aside from avoiding degradation, you will
save a lot of compression/decompression time this way :-).


[10]  Why all the argument about file formats?

Strictly speaking, JPEG refers only to a family of compression algorithms;
it does *not* refer to a specific image file format.  The JPEG committee was
prevented from defining a file format by turf wars within the international
standards organizations.

Since we can't actually exchange images with anyone else unless we agree on
a common file format, this leaves us with a problem.  In the absence of
official standards, a number of JPEG program writers have just gone off to
"do their own thing", and as a result their programs aren't compatible with
anybody else's.

The closest thing we have to a standard JPEG format is some work that's been
coordinated by people at C-Cube Microsystems.  They have defined two
JPEG-based file formats:
  * JFIF (JPEG File Interchange Format), a "low-end" format that transports
    pixels and not much else.
  * TIFF/JPEG, aka TIFF 6.0, an extension of the Aldus TIFF format.  TIFF is
    a "high-end" format that will let you record just about everything you
    ever wanted to know about an image, and a lot more besides :-).  TIFF is
    a lot more complex than JFIF, and is generally less transportable,
    because different vendors have often implemented slightly different
    and incompatible subsets of TIFF.  It's not likely that adding JPEG to
    the mix will do anything to improve this situation.
Both of these formats were developed with input from all the major vendors
of JPEG-related products; it's reasonably likely that future commercial
products will adhere to one or both standards.

JFIF has emerged as the de-facto standard on Usenet.  JFIF is simpler than
TIFF and is available now; the TIFF 6.0 spec for incorporating JPEG is not
widely implemented, partly because it has some serious design flaws.  It is
likely that the TIFF 6.0 JPEG section will be changed significantly before
widespread adoption occurs.  Even when TIFF/JPEG is well defined, the JFIF
format is likely to be a widely supported "lowest common denominator";
TIFF/JPEG files may never be as transportable.

A particular case of wide interest is Apple's Macintosh QuickTime software.
QuickTime uses a JFIF-compatible format wrapped inside the Mac-specific PICT
structure.  Conversion between JFIF and PICT/JPEG is pretty straightforward,
and several Mac programs are available to do it (see Mac portion of section
6A).  If you have an editor that handles binary files, you can even strip a
PICT/JPEG file down to JFIF by hand; see section 11 for details.

Another particular case is Handmade Software's DOS programs (GIF2JPG/JPG2GIF
and Image Alchemy).  These programs are capable of reading and writing JFIF
format.  By default, though, they write a proprietary format developed by
HSI.  This format is NOT readable by any non-HSI programs and should not be
used for Usenet postings.  Use the -j switch to get JFIF output.  (This
applies to old versions of these programs; the current releases emit JFIF
format by default.  You still should be careful not to post HSI-format
files, unless you want to get flamed by people on non-PC platforms.)


[11]  How do I recognize which file format I have, and what do I do about it?

If you have an alleged JPEG file that your software won't read, it's likely
to be HSI format or some other proprietary JPEG-based format.  You can tell
what you have by inspecting the first few bytes of the file:

1.  A JFIF-standard file will start with the four bytes (hex) FF D8 FF E0,
    followed by two variable bytes (often hex 00 10), followed by 'JFIF'.

2.  If you see FF D8 at the start, but not the 'JFIF' marker, you may have a
    "raw JPEG" file.  This is probably decodable as-is by JFIF software ---
    it's worth a try, anyway.

3.  HSI files start with 'hsi1'.  You're out of luck unless you have HSI
    software.  Portions of the file may look like plain JPEG data, but they
    won't decompress properly with non-HSI programs.

4.  A Macintosh PICT file, if JPEG-compressed, will have several hundred
    bytes of header (often 726 bytes, but not always) followed by JPEG data.
    Look for the 3-byte sequence (hex) FF D8 FF --- the text 'Photo - JPEG'
    will usually appear shortly before this header, and 'JFIF' or 'AppleMark'
    will usually appear shortly after it.  Strip off everything before the
    FF D8 FF and you should be able to decode the file.

5.  Anything else: it's a proprietary format, or not JPEG at all.  If you are
    lucky, the file may consist of a header and a raw JPEG data stream.
    If you can identify the start of the JPEG data stream (look for FF D8),
    try stripping off everything before that.

In uuencoded Usenet postings, the characteristic JFIF pattern is

    "begin" line
    M_]C_X ...

whereas uuencoded HSI files will start with

    "begin" line
    M:'-I ...

If you learn to check for the former, you can save yourself the trouble of
downloading non-JFIF files.


[12]  How does JPEG work?

The buzz-words to know are chrominance subsampling, discrete cosine
transforms, coefficient quantization, and Huffman or arithmetic entropy
coding.  This article's too long already, so I'm not going to say more
than that here.  For technical information see the comp.compression FAQ,
which is available from the news.answers archive at rtfm.mit.edu, in files
/pub/usenet/news.answers/compression-faq/part[1-3].  If you need help in
using the news.answers archive, see the top of this article.

The comp.compression FAQ is also a good starting point for information on
other state-of-the-art image compression methods, such as wavelets and
fractals.  A quick comparison: wavelets are likely to be the basis of the
next generation of image-compression standards, but they are 5 to 10 years
behind JPEG in the standardization pipeline; as for fractals, it's very
difficult to separate real performance from hype.


[13]  Isn't there a lossless JPEG?

There's a great deal of confusion on this subject.  The JPEG committee did
define a truly lossless compression algorithm (i.e., one that guarantees the
final output is bit-for-bit identical to the original input).  However, this
lossless mode has almost nothing in common with the regular lossy JPEG
algorithm, and it offers much less compression.  At present, very few
implementations of lossless JPEG exist.  The PVRG code mentioned in section
6B is the only noncommercial code I know of that handles lossless JPEG.

Lossless JPEG typically compresses full-color data by around 2:1.  Lossless
JPEG works well only on continuous-tone images; it does not provide useful
compression of palette-color images or low-bit-depth images.  (The JBIG
standard is considered superior to lossless JPEG for images of less than 6
bits/sample.)

Cranking a regular JPEG implementation up to its maximum quality setting
*does not* get you lossless storage.  Even at the maximum possible quality
setting, regular JPEG is not lossless, because it is subject to roundoff
errors in various calculations.  Roundoff errors are nearly always too small
to be seen, but they will accumulate if you put the image through multiple
cycles of compression.

Many implementations won't even let you get to the maximum possible setting,
because it's such an inefficient way to use regular JPEG.  With the IJG JPEG
software, for example, you have to say not only "-quality 100" but also
"-sample 1x1" to eliminate all deliberate loss of information.  The
resulting files are far larger and of only fractionally better quality than
files generated at more reasonable settings.  If you really need lossless
storage, don't try to approximate it with regular JPEG.


[14]  What about arithmetic coding?

The JPEG spec defines two different "back end" modules for the final output
of compressed data: either Huffman coding or arithmetic coding is allowed.
The choice has no impact on image quality, but arithmetic coding usually
produces a smaller compressed file.  On typical images, arithmetic coding
produces a file 5 to 10 percent smaller than Huffman coding.  (All the
file-size numbers previously cited are for Huffman coding.)

Unfortunately, the particular variant of arithmetic coding specified by the
JPEG standard is subject to patents owned by IBM, AT&T, and Mitsubishi.
Thus *you cannot legally use arithmetic coding* unless you obtain licenses
from these companies.  (The "fair use" doctrine allows people to implement
and test the algorithm, but actually storing any images with it is dubious
at best.)

At least in the short run, I recommend that people not worry about
arithmetic coding; the space savings isn't great enough to justify the
potential legal hassles.  In particular, arithmetic coding *should not*
be used for any images to be exchanged on Usenet.


[15]  Could an FPU speed up JPEG?  How about a DSP chip?

Since JPEG is so compute-intensive, many people suggest that using an
FPU chip (a math coprocessor) should speed it up.  This is not so.
Production-quality JPEG programs use only integer arithmetic and so are
unaffected by the presence or absence of floating-point hardware.
Converting the key operations to floating point would only slow things down.
(On some very expensive machines, where floating point arithmetic is
actually faster than integer, such a rewrite would indeed be a win.  Most
such hardware has "Cray" on the nameplate :-).)

On the other hand, DSP chips are ideally suited for fast repetitive integer
arithmetic, so programming a DSP to do JPEG can yield significant speedups.
DSPs are starting to be found as add-ons for workstations and PCs, so you
can expect to see DSP-based JPEG programs popping up.


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

If you have more questions about JPEG in general or the free JPEG software
in particular, contact the Independent JPEG Group at [e-mail].

--
            tom lane
            organizer, Independent JPEG Group
            [e-mail] or [e-mail]