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From: [e-mail] (Tom Lane)
Newsgroups: comp.graphics,alt.graphics.pixutils,alt.binaries.pictures.utilities,alt.binaries.pictures.d,alt.binaries.pictures.erotica.d,news.answers
Subject: JPEG image compression: Frequently Asked Questions
Summary: Useful info about JPEG (JPG) image files and programs
Keywords: JPEG, image compression, FAQ
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Date: 8 Sep 92 14:38:29 GMT
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Archive-name: jpeg-faq
Last-modified: 8 September 1992

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

New since version of 23 August 1992:
  * More on color quantization: 256 colors in does not mean 256 colors out.
  * More info on Mac and Amiga JPEG programs.
  * Some info on Acorn Archimedes programs too.


This article includes the following sections:

1)  What is JPEG?
2)  Why use JPEG?
3)  How well does it work?
4)  What are good "quality" settings for JPEG?
5)  When should I use JPEG, and when should I stick with GIF?
6)  Where can I get JPEG software?
    6A) "canned" software, viewers, etc.
    6B) source code
7)  What's all this hoopla about color quantization?
8)  How does JPEG work?
9)  What about lossless JPEG?
10)  Why all the argument about file formats?
11)  And what about arithmetic coding?
12)  Does loss accumulate with repeated compression/decompression?
13)  What are some rules of thumb for converting GIF images to JPEG?

Sections 1-6 are basic info that every JPEG user needs to know;
sections 7-13 are advanced info for the curious.

This article is posted every 2 weeks.  You can always find the latest version
in the news.answers archive at rtfm.mit.edu (18.172.1.27).  By FTP, fetch
/pub/usenet/news.answers/jpeg-faq; or if you don't have FTP, send e-mail to
[e-mail] with body "send usenet/news.answers/jpeg-faq".)

----------


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 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.

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.

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 indexing image
archives, making thumbnail views or icons, etc. etc.  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.


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 below.)

Unless your viewing software supports 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 (GIF, for instance).  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 much greater than the
extra time to decompress the file.  I'll let you do the arithmetic yourself.

The other reason why JPEG will gradually replace GIF as the standard Usenet
posting format is that JPEG can store full color information: 24 bits/pixel
(16 million colors) instead of 8 or less (256 or fewer colors).  If you have
only 8-bit display hardware then this may not seem like much of an advantage
to you.  Within a couple of years, though, 8-bit GIF will look 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 color display hardware.  Hence JPEG is considerably
more appropriate than GIF for use as a Usenet posting standard.


3)  How well does it work?

Pretty darn well.  Here are some sample file sizes for an image I have handy,
a 727x525 full-color image of a ship in a harbor.  The first three files are
for comparison purposes; the rest were created with the free JPEG software
described in section 6B.

File       Size in bytes        Comments

ship.ppm    1145040  Original file in PPM format (no compression; 24 bits
             or 3 bytes per pixel, plus a few bytes overhead)
ship.ppm.Z     963829  PPM file passed through Unix compress
             compress doesn't accomplish a lot, you'll note.
             Other text-oriented compressors give similar results.
ship.gif     240438  Converted to GIF with ppmquant -fs 256 | ppmtogif
             Most of the savings is the result of losing color
             info: GIF saves 8 bits/pixel, not 24.  (See sec. 7.)

ship.jpg95     155622  cjpeg -Q 95    (highest useful quality setting)
             This is indistinguishable from the 24-bit original,
             at least to my nonprofessional eyeballs.
ship.jpg75      58009  cjpeg -Q 75    (default setting)
             You have to look mighty darn close to distinguish this
             from the original, even with both on-screen at once.
ship.jpg50      38406  cjpeg -Q 50
             This has slight defects; if you know what to look
             for, you could tell it's been JPEGed without seeing
             the original.  Still as good image quality as many
             recent postings in Usenet pictures groups.
ship.jpg25      25192  cjpeg -Q 25
             JPEG's characteristic "blockiness" becomes apparent
             at this setting (djpeg -b helps some).  Still, I've
             seen plenty of Usenet postings that were of poorer
             image quality than this.
ship.jpg5o       6587  cjpeg -Q 5 -o    (-o reduces table overhead)
             Blocky, but perfectly satisfactory for preview or
             indexing purposes.  Note that this file is TINY:
             the compression ratio from the original is 173:1 !

In this case JPEG can make a file that's a factor of four or five smaller
than a GIF of comparable quality (the -Q 75 file is every bit as good as the
GIF, better if you have a full-color display).  This seems to be a typical
ratio for real-world scenes.


4)  What are good "quality" settings for JPEG?

(Note: the -Q settings discussed in this article apply to the free JPEG
software described in section 6B.  Other JPEG implementations, such as Image
Alchemy, may use a completely different quality scale.)

The name of the game in using JPEG is to pick the lowest quality setting
(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.

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.  Except for
experimental purposes, never go above -Q 95; saying -Q 100 will produce a
file two or three times as large as -Q 95, but of hardly any better quality.

If the image was less than perfect quality to begin with, you might be able to
go down to -Q 50 without objectionable degradation.  On the other hand, you
might need to go to a HIGHER quality setting to avoid further degradation.
The second case seems to apply most of the time when converting GIFs to JPEG.
The default -Q 75 is about right for compressing 24-bit images, but -Q 85 to
95 is usually better for converting GIFs (see section 13 for more info).

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".

Another recommendation: when you are making a final version of an image for
posting on Usenet or archiving, specify "-o" to cjpeg.  This will make the
file a little smaller without affecting image quality; it will take longer
to do the compression, but not any longer to decompress.


5)  When should I use JPEG, and when should I stick with GIF?

As a rule of thumb, JPEG is superior to GIF for storing full-color or
gray-scale images of "realistic" scenes; that means scanned photographs and
similar material.  JPEG is superior even if you don't have 24-bit display
hardware, and it is a LOT superior if you do.  (See section 7 for details.)

GIF does significantly better on images with only a few distinct colors,
such as cartoons and line drawings.  In particular, large areas of pixels
that are all *exactly* the same color are compressed very efficiently indeed
by GIF.  JPEG can't squeeze these files as much as GIF does without
introducing visible defects.  This sort of image is best kept in GIF form.
(In particular, single-color borders are quite cheap in GIF files, but they
should be avoided in JPEG files.)

JPEG also 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.  Again, this
sort of thing is not found in scanned photographs, but it shows up fairly
often 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.

Computer-drawn images (ray-traced scenes, for instance) usually fall between
scanned images and cartoons in terms of complexity.  The more complex and
subtly rendered the image, the more likely that JPEG will do well on it.

Plain black-and-white (two level) images should never be converted to JPEG.
You need at least about 16 gray levels before JPEG is useful for gray-scale
images.

If you have an existing library of GIF images, you may wonder whether you
should convert them to JPEG.  You will lose some image quality if you do.
(Section 7, which argues that JPEG image quality is superior to GIF, only
applies if both formats start from a full-color original.  If you start from
a GIF, you've already irretrievably lost a great deal of information; JPEG
can only make things worse.)  However, the disk space savings may justify
converting anyway.  This is a decision you'll have to make for yourself.
If you do convert a GIF library to JPEG, see section 13 for hints.


6)  Where can I get JPEG software?

6A) If you are looking for "canned" software, viewers, etc:

The first part of this list is system-specific programs that only run on one
kind of system.  If you don't see what you want for your machine, check out
the portable JPEG software described at the end of the list.

X Windows:

John Bradley's free XV (version 2.00 and up) is an excellent viewer for JPEG,
GIF, and other image formats.  It's available for FTP from export.lcs.mit.edu
or ftp.cis.upenn.edu.  The file is called 'xv-???.tar.Z' (where ??? is the
version number, currently 2.21); it is located in the 'contrib' directory on
export or the 'pub/xv' directory at upenn.  XV reduces all images to 8 bits
internally, which means it's not a real good choice if you have a 24-bit
display (you'll still get only 8-bit color).  Also, you shouldn't use XV to
convert full-color images to JPEG, because they'll get color-quantized first.
With the exception of those two limitations, XV is as good as they come.

"xli" is less featureful than XV, but it will do the right thing on 24-bit
displays.  xli is free and available from export.lcs.mit.edu, files
contrib/xli.*.  The current version is 1.10.  (At last report, the files at
export were a tar archive of 1.08 and a file of patches to bring 1.08 up to
1.10; be sure to retrieve both files.)  Alternately, you can use xloadimage
in combination with the free JPEG software described in 6B.

Another good choice for X Windows is John Cristy's free ImageMagick package,
also available from export, file contrib/ImageMagick.tar.Z.  The viewer
included in this package handles 24-bit displays correctly; for colormapped
displays, it does better (though slower) color quantization than XV or the
basic free JPEG software.

MS-DOS:

There are at least two freeware JPEG viewers for plain MS-DOS (non-Windows).

One is Eric Praetzel's DVPEG.  The current version, 2.0 beta, is available by
FTP from sunee.waterloo.edu (129.97.50.50), file pub/jpeg/viewers/dvpeg2.zip.
This is a good basic viewer that works on either 286 or 386/486 machines.
The user interface is not flashy, but it's functional (it does more than
Hiview, for instance).  Does not work with some display cards.

A more recent arrival is Mohammad Rezaei's Hiview.  The current version,
1.2, is available from Simtel20 and mirror sites (see NOTE below), file
msdos/graphics/hv12.zip.  Hiview is noticeably faster than DVPEG and works
on a fairly wide variety of display types.  However, Hiview requires a 386
or better CPU and a VCPI-compatible memory manager (QEMM386 and 386MAX work;
Windows and OS/2 do not).  Also installation is a bit tricky; read the
directions carefully!

If neither of these viewers work on your hardware, 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 Independent JPEG Group's free JPEG converters are FTPable from Simtel20
and mirror sites (see NOTE below), file msdos/graphics/jpeg3.zip (or
jpeg3386.zip if you have a 386 and extended memory).  The same files were
posted to comp.binaries.ibm.pc (volume 18, issues 123-130) and should be
available from any c.b.i.p archive site.  These files are DOS compilations
of the free source code described in section 6B.

Handmade Software offers two rather pricy shareware programs: Image Alchemy
and GIF2JPG/JPG2GIF (contact [e-mail] for details).  The PC versions
of these programs are FTPable from Simtel20 and mirror sites (see NOTE
below), files msdos/graphics/alch16.zip and gif2jpg5.zip.  GIF2JPG/JPG2GIF
only performs JPEG<=>GIF format conversion.  Image Alchemy converts files
between these and many other formats, and can also display images on some
types of hardware.  The display option is limited and not very high quality,
so you'll still want a separate viewer program.  (CAUTION: GIF2JPG produces
a proprietary file format unless you specify -j.  Be sure to use -j if you
want to exchange JPEG files with other Usenet users.  For that matter, it's
not real clear that you should be posting JPEG files made from GIFs; see
section 5.)

In my biased opinion, the free JPEG software is a better choice than
GIF2JPG/JPG2GIF; it's faster, as good or better image quality, and free :-).
On the other hand, Image Alchemy may be worth its price, if you need the
additional conversion and image manipulation capabilities it provides.

NOTE ABOUT SIMTEL20: The Internet's key archive site for PC-related programs
is Simtel20, full name wsmr-simtel20.army.mil (192.88.110.20).  Simtel20
runs a non-Unix operating system; where this document refers to directory
(eg) "msdos/graphics" at Simtel20, that really means "pd1:<msdos.graphics>".
If you are not physically on MILnet, you should expect rather slow FTP
transfer rates from Simtel20.  There are several Internet sites that
maintain copies (mirrors) of the Simtel20 archives; most FTP users should
go to one of the mirror sites instead.  A popular USA mirror site is
oak.oakland.edu (141.210.10.117); it keeps Simtel20 files in (eg)
"/pub/msdos/graphics".  If you have no FTP capability, you can retrieve
files from Simtel20 by e-mail; see informational postings in
comp.binaries.ibm.pc.archives to find out how.  If you are outside the USA,
consult the same newsgroup to learn where your nearest Simtel20 mirror is.

Microsoft Windows:

There are several Windows programs capable of displaying JPEG images.

JView is the newest kid on the block.  It's freeware, quite fast, has good
on-line help, and can write out the decompressed image in Windows BMP
format; but it can't create new JPEG files.  JView lacks some useful
features of the shareware viewers (such as brightness adjustment), but it's
an excellent basic viewer.  The current version, 0.9, is available from
ftp.cica.indiana.edu (129.79.20.84), file pub/pc/win3/desktop/jview090.zip.
(Mirrors of this archive can be found at some other Internet sites,
including wuarchive.wustl.edu.)

WinJPEG can display GIF, Targa, and BMP files as well as JPEG; it can write
all of these formats too, so it can be used as a converter.  It has some
other nifty features including color-balance adjustment and slideshow.
On the minus side, it lacks on-line help, the current version is a little
slower than the current version of JView, and it's shareware (only $15
though).  The current version is 1.2, available from Simtel20 and mirror
sites (see NOTE above), file msdos/windows3/winjp120.zip.

ColorView is another shareware entry ($30).  This was an early and promising
contender, but it has not been updated in some time, and at this point it
has no real advantages over WinJPEG.  If you want to try it anyway, the
current version is 0.97, available from ftp.cica.indiana.edu, file
pub/pc/win3/desktop/cview097.zip.

The DOS conversion programs described above will run inside a Windows DOS
window.  Note that Windows viewers are generally slower than non-Windows
viewers on the same hardware, due to Windows' system overhead.

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 not running System 7, you must also
install the 32-bit QuickDraw extension.)  You can get QuickTime from
ftp.apple.com, file dts/mac/quicktime/quicktime.hqx.

Apple has released a free program called PictPixie that can convert the
Usenet-standard JFIF JPEG format to and from QuickTime's internal JPEG
format.  PictPixie can also be used as a viewer for JFIF, QuickTime JPEG,
and GIF files.  You can get PictPixie from ftp.apple.com, file
dts/mac/quicktime/pictpixie.hqx.  Requires QuickTime.  PictPixie is fast but
requires lots of memory, and it has a relatively unfriendly user interface
(it was intended as a developer's tool).  PictPixie is an unsupported
program, meaning it has some minor bugs that Apple does not intend to fix.
(The QuickTime Starter Kit includes a less-buggy descendant of PictPixie
called PICTCompressor.  PICTCompressor is reputed to have a cleaner but less
flexible user interface.)

Another good choice is JPEGView, a free program for viewing both JFIF and
QuickTime JPEG files, as well as converting between the two formats.
The current version, 1.1, is much improved over the initial release (0.9).
Get it from sumex-aim.stanford.edu, file /info-mac/app/jpeg-view-11.hqx.
Requires System 7 and QuickTime.  JPEGView doesn't do quite as much as
PictPixie, but it can open multiple pictures and has a slide-show feature;
so it is much handier than PictPixie for skimming through lots of images.
JPEGView also needs less memory than PictPixie to view large images.  On
8-bit-color displays, JPEGView's image quality is inferior to PictPixie (see
below for details), but that will be fixed in the next release.

Storm Technology has released a free JPEG viewer/converter called Picture
Decompress.  This is much inferior to PictPixie or JPEGView in speed,
features, and memory demands, but it will run without System 7 or QuickTime,
so you may be forced to use it on older systems.  (You'll still need 32-bit
QuickDraw.)  This program can be FTPed from sumex-aim.stanford.edu, file
/info-mac/app/picture-decompress-201.hqx.  Make sure you get version 2.0.1
or later; earlier versions are not compatible with JFIF file format.  You'll
also need a tool for adjusting file type codes; you must set the type of a
downloaded image file to 'JPEG' to allow Picture Decompress to open it.

On 8-bit-color Macs, PictPixie produces visibly better results than the
other two programs, since it uses QuickTime's slower 2-pass color-reduction
algorithm, instead of the quick-and-dirty 1-pass method.  Many people think
that the IJG JPEG quantizer does better color reduction than either of
QuickTime's methods.  Real Soon Now there should be at least one Mac program
based on the IJG JPEG code, which will be slower but higher quality than
QuickTime.

The shareware image viewer/converter GIFConverter will support JPEG in its
next release.  There is already a beta version out (2.3b1), but it is pretty
flaky, so I recommend waiting for the real release.  Once stable, this
program should offer very nice viewing and format-conversion features.

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
file to 'JPEG' to allow Photoshop to recognize it.

Amiga:

The shareware program HamLab Plus is probably the best inexpensive JPEG
viewer/converter for Amigas.  It's cheap ($20) and can read several formats
besides JPEG.  The current version is 2.0.8.  A demo version is available by
FTP from amiga.physik.unizh.ch (130.60.80.80) and mirror sites (including
wuarchive.wustl.edu in the USA), file amiga/gfx/hamlab208d.lha.
The demo version will crop images larger than 512x512, but it is otherwise
fully functional.

If you're willing to spend real money, the commercial program Art Department
Professional is a very nifty piece of software that handles JPEG.

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.

Acorn Archimedes:

!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 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].

Portable software for almost any system:

If none of the above fits your situation, you can obtain and compile the free
JPEG conversion software described in 6B.  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.

If you are not reasonably handy at configuring and installing portable C
programs, you may have some difficulty installing the free source code.
Steve Davis ([e-mail]) has volunteered to maintain an archive of
pre-built executable versions of the free JPEG code for various machines.
His FTP archive is at ftp.cis.ksu.edu (129.130.10.80); look under /pub/JPEG
to see what he currently has.  (The administrators of this system ask that
FTP traffic be limited to non-prime hours.)  This archive is not maintained
by the Independent JPEG Group, and files in it may not represent the latest
free source code.  (Actually, Steve has gotten pretty lax about maintaining
his archive.  Any volunteers to set up a new one?)

There are numerous commercial JPEG offerings, with more popping up every
day.  I recommend that you not spend money on one of these unless you find
the available free or shareware software vastly too slow.  In that case,
purchase a hardware-assisted product.  Ask pointed questions about whether
the product complies with the final JPEG standard and about whether it can
handle the JFIF file format; many of the earliest commercial releases are
not and never will be compatible with anyone else's files.


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 several places.
The "official" archive site for this source code is ftp.uu.net (137.39.1.9
or 192.48.96.9).  Look under directory /graphics/jpeg; the current release
is jpegsrc.v3.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.
Folks in Europe may find it easier to FTP from nic.funet.fi (see directory
pub/graphics/programs/jpeg).  The source code is also available on
CompuServe, in the GRAPHSUPPORT forum (GO PICS), library 10, as jpsrc3.zip.
If you have no FTP access, you can retrieve the source from your nearest
comp.sources.misc archive; version 3 appeared as issues 1-18 of volume 29.
(If you don't know how to retrieve comp.sources.misc postings, see the FAQ
article "How to find sources".  This appears regularly in news.answers, or
you can get it by sending e-mail to [e-mail] with
"send usenet/news.answers/finding-sources" in the body.)

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.


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 map the image into an appropriate set of representative
colors.  This process is called "color quantization".  (This is something
of a misnomer, "color selection" 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 (except at
the very lowest JPEG quality settings).

Since JPEG is a full-color format, converting a color JPEG image for display
on 8-bit-or-less hardware requires color quantization.  This is *always* the
case: even if you feed a 256-or-less-color GIF into JPEG, what comes out of
the decompressor is *not* 256 colors, but thousands of colors.  JPEG's
lossiness affects each pixel a little differently, so two pixels that
started as identical colors will probably come out as 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.  (Incidentally, because
of this effect it's pretty much meaningless to talk about the number of
colors used by a JPEG image.  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.)  The only way to avoid
quantization is to ask for gray-scale output.

On the other hand, a GIF image by definition has already been quantized to
256 or fewer colors.  For purposes of Usenet picture distribution, GIF has
the advantage that the sender precomputes the color quantization, so
recipients don't have to.  This is also the *disadvantage* of GIF: you're
stuck with the sender's quantization.  If the sender quantized to a
different number of colors than what you can display, you have to
re-quantize, resulting in much poorer image quality than if you had
quantized once from a full-color image.  Furthermore, if the sender didn't
use a high-quality color quantization algorithm, you're out of luck.

For this reason, JPEG offers the promise of significantly better image quality
for all users whose machines don't match the sender's display hardware.
JPEG's full color image can be quantized to precisely match the user'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.

It's also worth mentioning that many GIF-viewing programs include rather
shoddy quantization routines.  If you view a 256-color GIF on a 16-color EGA
display, for example, you are probably getting a much worse image than you
need to.  This is partly an inevitable consequence of doing two color
quantizations (one to create the GIF, one to display it), but often it's
also due to sloppiness.  JPEG conversion programs will be forced to use
high quality quantizers in order to get acceptable results at all, and in
normal use they will quantize directly to the number of colors to be
displayed.  Thus, JPEG is likely to provide better results than the average
GIF program for low-color-resolution displays as well as high-resolution ones!

Finally, an ever-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.
Thus, JPEG is an all-around better choice than GIF for representing images
in a machine-independent fashion.


8)  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 long enough already, so I'm not going to say more
than that.  For a good technical introduction, see:
    Wallace, Gregory K.  "The JPEG Still Picture Compression Standard",
    Communications of the ACM, April 1991 (vol. 34 no. 4), pp. 30-44.
(Adjacent articles in that issue discuss MPEG motion picture compression,
applications of JPEG, and related topics.)  If you don't have the CACM issue
handy, a PostScript file containing a revised version of this article is
available at ftp.uu.net, graphics/jpeg/wallace.ps.Z.  The file (actually a
preprint for an article to appear in IEEE Trans. Consum. Elect.) omits the
sample images that appeared in CACM, but it includes corrections and some
added material.  Note: the Wallace article is copyright ACM and IEEE, and
it may not be used for commercial purposes.

An alternative, more leisurely explanation of JPEG can be found in "The Data
Compression Book" by Mark Nelson, published by M&T Books (Redwood City, CA),
1991, ISBN 1-55851-216-0.  This book provides excellent introductions to
many data compression methods including JPEG, plus sample source code in C.
The JPEG-related source code is far from industrial-strength, but it's a
pretty good learning tool.  (When you are ready to look at a real
implementation, see section 6B above.)


9)  What about 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.  At present, very few implementations of lossless JPEG exist,
and all of them are commercial.

Saying "-Q 100" to the free JPEG software DOES NOT get you a lossless image.
What it does get rid of is deliberate information loss in the coefficient
quantization step.  There is still a good deal of information loss in the
color subsampling step.  (There should be a command line switch to disable
subsampling, but as of today, there isn't one.)

Even with both quantization and subsampling turned off, the regular JPEG
algorithm is not lossless, because it is subject to roundoff errors in
various calculations.  The maximum error is a few counts in any one pixel
value; it's highly unlikely that this could be perceived by the human eye,
but it might be a concern if you are doing machine processing of an image.

At this minimum-loss setting, regular JPEG produces files that are perhaps
half the size of an uncompressed 24-bit-per-pixel image.  True lossless JPEG
provides roughly the same amount of compression, but it guarantees
bit-for-bit accuracy.

If you have an application requiring lossless storage of images with less
than 6 bits per pixel (per color component), you may want to look into the
JBIG bilevel image compression standard.  This performs better than JPEG
lossless on such images.  JPEG lossless is superior to JBIG on images with
8 or more bits per pixel; furthermore, it is public domain, while the JBIG
techniques are heavily covered by patents.


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 de-facto 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 may well prove less transportable,
    because different vendors have historically 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.

A particular case that people may be interested in is Apple's QuickTime
software for the Macintosh.  QuickTime uses a JFIF-compatible format wrapped
inside the Mac-specific PICT structure.  Conversion between JFIF and
QuickTime JPEG is pretty straightforward; in fact Apple has released a
utility program for the purpose (see PictPixie in section 6A).

I believe that Usenet should adopt JFIF as the replacement for GIF in
picture postings.  JFIF is simpler than TIFF and is available now; the
TIFF 6.0 spec has only recently been officially adopted, and it is still
unusably vague on some crucial details.  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.


11)  And 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 or 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.

There is some small chance that the legal situation may change in the
future.  Stay tuned for further details.


12)  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, though usually not as much as was lost the
first time around.

The next best thing would be that if you decompress an image and recompress
it *without changing it* then there is no further loss, i.e., you get an
identical JPEG file.  Even this is not true; at least, not with the current
free JPEG software.  It's essentially a problem of accumulation of roundoff
error.  If you repeatedly compress and decompress, the image will eventually
degrade to where you can see visible changes from the first-generation
output.  (It usually takes many such cycles to get visible change.)
One of the things on our to-do list is to see if accumulation of error can
be avoided or limited, but I am not optimistic about it.

In any case, the most that could possibly be guaranteed would be that
compressing the unmodified full-color output of djpeg, at the original
quality setting, would introduce no further loss.  Even such simple changes
as cropping off a border could cause further roundoff-error degradation.
(If you're wondering why, it's because the pixel-block boundaries move.
If you cropped off only multiples of 16 pixels, you might be safe, but
that's a mighty limited capability!)

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 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 :-).


13)  What are some rules of thumb for converting GIF images to JPEG?

As stated earlier, you *will* lose some amount of image information if you
convert an existing GIF image to JPEG.  If you can obtain the original
full-color data the GIF was made from, it's far better to make a JPEG from
that.  But if you need to save space and have only the GIF to work from,
here are some suggestions for getting maximum space savings with minimum
loss of quality.

The first rule when converting a GIF library is to look at each JPEG, to
make sure you are happy with it, before throwing away the corresponding GIF;
that will give you a chance to re-do the conversion with a higher quality
setting if necessary.  Some GIFs may be better left as GIFs, as explained in
section 5; there are images for which a JPEG file of reasonable quality will
be *larger* than a GIF.  (So check the sizes too.)

Experience to date suggests that large, high-visual-quality GIFs are the best
candidates for conversion to JPEG.  They chew up the most storage so offer
the most potential savings, and they convert to JPEG with least degradation.
Don't waste your time converting any GIF much under 100 Kbytes.  Also, don't
expect JPEG files converted from GIFs to be as small as those created
directly from full-color originals.  To maintain image quality you may have
to let the converted files be as much as twice as big as straight-through
JPEG files would be (i.e., shoot for 1/2 or 1/3rd the size of the GIF file,
not 1/4th as suggested in earlier comparisons).

cjpeg's default Q setting of 75 is appropriate for full-color input, but
for GIF inputs, Q settings of 85 to 95 often seem to be necessary to avoid
image degradation.

Many people have developed an odd habit of putting a large constant-color
border around a GIF image.  While useless, this was nearly free in terms of
storage cost in GIF files.  It is NOT free in JPEG files, and the sharp
border boundary can create visible artifacts ("ghost" edges).  Do yourself
a favor and crop off any border before JPEGing.  (If you are on an X Windows
system, XV's manual and automatic cropping functions are a very painless
way to do this.)

Sometimes, smoothing a GIF before compression will reduce the JPEG file size
*and* improve the output image quality.  The theory is that smoothing
reduces the dithering patterns found in most color GIFs; this helps because
dithering creates high-spatial-frequency noise which JPEG doesn't handle
well.  If you can see regular fine-scale patterns on the GIF image, then
smoothing is definitely indicated.  At some point good JPEG software will
probably include an input-smoothing option, but for now, you'll have to use
external tools.  If you have the PBMPLUS package, try
    giftoppm input.gif | pnmconvol weightfile | cjpeg >output.jpg
where weightfile contains
    P2 3 3 200
    101 101 101
    101 192 101
    101 101 101
(Thanks to Jef Poskanzer for these values.)  For GIFs with very heavy-handed
dithering, the stronger smoothing provided by pnmsmooth may work better.
You may be able to drop down to Q 75 or less without visible quality loss if
you smooth the input image this way.


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

For more information 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
Internet: [e-mail]    BITNET: tgl%cs.cmu.edu@carnegie