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JPEG FAQ - Web Design Group


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									 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?
[7] How do I view JPEG images posted on Usenet?

More advanced questions:

[8] What is color quantization?
[9] What are some rules of thumb for converting GIF images to JPEG?
[10] Does loss accumulate with repeated compression/decompression?
[11] What is progressive JPEG?
[12] Can I make a transparent JPEG?
[13] Isn't there a lossless JPEG?
[14] Why all the argument about file formats?
[15] How do I recognize which file format I have, and what do I do about
[16] How does JPEG work?
[17] What about arithmetic coding?
[18] Could an FPU speed up JPEG? How about a DSP chip?
[19] Isn't there an M-JPEG standard for motion pictures?
[20] What if I need more than 8-bit precision?


[21] Where are FAQ lists archived?

This article and its companion are posted every 2 weeks. If you can't
part 2, you can get it from the news.answers archive at rtfm.mit.edu
(see "[21] Where are FAQ lists archived?"). Part 2 changes very
get a new copy if the one you are reading is more than a couple months


Subject: [1] What is JPEG?

JPEG (pronounced "jay-peg") is a standardized image compression
JPEG stands for Joint Photographic Experts Group, the original name of
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,
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
the one you started with. (There are lossless image compression
but JPEG achieves much greater compression than is possible with lossless
methods.) JPEG is designed to exploit known limitations of the human
notably the fact that small color changes are perceived less accurately
small changes in brightness. Thus, JPEG is intended for compressing
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,
if they are invisible to the eye.

A useful property of JPEG is that the degree of lossiness can be varied
adjusting compression parameters. This means that the image maker can
off file size against output image quality. You can make *extremely*
files if you don't mind poor quality; this is useful for applications
as indexing image archives. Conversely, if you aren't happy with the
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
the required calculations. Some viewers obtain remarkable speedups in


Subject: [2] Why use JPEG?

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

Making image files smaller is a win for transmitting files across
and for archiving libraries of images. Being able to compress a 2 Mbyte
full-color file down to, say, 100 Kbytes makes a big difference in disk
space and transmission time! And JPEG can easily provide 20:1
of full-color data. If you are comparing GIF and JPEG, the size ratio is
usually more like 4:1 (see "[4] How well does JPEG compress images?").

If your viewing software doesn't support JPEG directly, you'll have to
convert JPEG to some other format to view the image. 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
or transmit an image more cheaply. But it's worth noting that when
or telephone transmission is involved, the time savings from transferring
shorter file can be greater than the 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
widely used on the net, can only store 8 bits/pixel (256 or fewer
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,
GIF will probably seem as obsolete as black-and-white MacPaint format
today. Furthermore, JPEG is far more useful than GIF for exchanging
among people with widely varying display hardware, because it avoids
prejudging how many colors to use (see "[8] What is color
Hence JPEG is considerably more appropriate than GIF for use as a Usenet
and World Wide Web standard format.

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 real disadvantage of lossy compression is that if you repeatedly
compress and decompress an image, you lose a little quality each time
(see "[10] Does loss accumulate with repeated
This is a serious objection for some applications but matters not at all
for many others.


Subject: [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
similar material. Any continuous variation in color, such as occurs in
highlighted or shaded areas, will be represented more faithfully and in
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
large areas of pixels that are all *exactly* the same color are
very efficiently indeed by GIF. JPEG can't squeeze such data as much as
does without introducing visible defects. (One implication of this is
large single-color borders are quite cheap in GIF files, while they are
avoided in JPEG files.)

Computer-drawn images, such as ray-traced scenes, 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).
But icons that use only a few colors are handled better by GIF.

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
consider 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. (If you want to
attach descriptive text to a JPEG image, put it in as a comment rather
trying to overlay it on the image. Most recent JPEG software can deal
textual comments in a JPEG file, although older viewers may just ignore

Plain black-and-white (two level) images should never be converted to
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
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-
photographic GIFs can often be converted with no visible quality loss,
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
or waste a lot of disk space ... quite possibly both. Read sections 8
and 9
if you want to convert GIFs to JPEG.


Subject: [4] How well does JPEG compress images?

Very well indeed, when working with its intended type of image
and suchlike). For full-color images, the uncompressed data is normally
bits/pixel. The best known lossless compression methods can compress
data about 2:1 on average. JPEG can typically achieve 10:1 to 20:1
compression without visible loss, bringing the effective storage
down to 1 to 2 bits/pixel. 30:1 to 50:1 compression is possible with
to moderate defects, while for very-low-quality purposes such as previews
archive indexes, 100:1 compression is quite feasible. An image
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
(8 bits/pixel). This provides 3:1 compression. GIF has additional "LZW"
compression built in, but LZW doesn't work very well on typical
data; at most you may get 5:1 compression overall, and it's not at all
uncommon for LZW to be a net loss (i.e., less than 3:1 overall
LZW *does* work well on simpler images such as line drawings, which is
GIF handles that sort of image so well. When a JPEG file is made from
full-color photographic 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
eye is much more sensitive to brightness variations than to hue
JPEG can compress hue data more heavily than brightness (gray-scale)
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
visible loss is often around 5:1 compression for gray-scale images.

The exact threshold at which errors become visible depends on your
conditions. The smaller an individual pixel, the harder it is to see an
error; so errors are more visible on a computer screen (at 70 or so
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 compression
ratios quoted above are typical for screen viewing. Also note that the
threshold of visible error varies considerably across images.


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

Most JPEG compressors let you pick a file size vs. image quality tradeoff
selecting a quality setting. There seems to be widespread confusion
the meaning of these settings. "Quality 95" does NOT mean "keep 95% of
information", as some have claimed. The quality scale is purely
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 IJG JPEG
software (described in part 2), and to many programs based on it. Some
other JPEG implementations use completely different quality scales; for
instance, Apple uses a scale running from 0-4, not 0-100. A few programs
even use a scale running in the opposite direction, with higher numbers
indicating lower image quality (more compression). Other programs don't
use a numeric scale at all, just "high"/"medium"/"low" style choices.
Fortunately, this confusion doesn't prevent different implementations
exchanging JPEG files. But you do need to keep in mind that quality
can vary from one JPEG-creating program to another.

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

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

If the image was less than perfect quality to begin with, you might be
to drop down to Q 50 without objectionable degradation. On the other
you might need to go to a *higher* quality setting to avoid further loss.
This is often necessary if the image contains dithering or moire patterns
(see "[9] What are some rules of thumb for converting GIF images to

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
better quality. Q 100 is a mathematical limit rather than a useful
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
prepared to tolerate large defects, a Q setting in the range of 5 to 10
about right. Q 2 or so may be amusing as "op art". (It's worth
that the current IJG software is not optimized for such low quality
Future versions may achieve better image quality for the same file size
low quality settings.)

If your image contains sharp colored edges, you may notice slight
or jagginess around such edges no matter how high you make the quality
setting. This can be suppressed, at a price in file size, by turning off
chroma downsampling in the compressor. The IJG encoder regards
as a separate option which you can turn on or off independently of the Q
setting. With the "cjpeg" program, the command line switch "-sample 1x1"
turns off downsampling; other programs based on the IJG library may have
checkboxes or other controls for downsampling. Other JPEG
may or may not provide user control of downsampling. Adobe Photoshop,
example, automatically switches off downsampling at its higher quality
settings. On most photographic images, we recommend leaving downsampling
on, because it saves a significant amount of space at little or no visual

For images being used on the World Wide Web, it's often a good idea to
give up a small amount of image quality in order to reduce download time.
Quality settings around 50 are often perfectly acceptable on the Web.
In fact, a user viewing such an image on a browser with a 256-color
is unlikely to be able to see any difference from a higher quality
because the browser's color quantization artifacts will swamp any
imperfections in the JPEG image itself. It's also worth knowing that
current progressive-JPEG-making programs use default progression
that are tuned for quality settings around 50-75: much below 50, the
scans will look really bad, while much above 75, the later scans won't
contribute anything noticeable to the picture.


Subject: [6] Where can I get JPEG software?

See part 2 of this FAQ for recommendations about programs for particular
systems. Part 2 also tells where to find free C source code for
implementing JPEG, in case you want to write your own programs using

The comp.graphics.* FAQs and the alt.binaries.pictures FAQ are more
sources of information about graphics programs available on the Internet
(see "[21] Where are FAQ lists archived?").


Subject: [7] How do I view JPEG images posted on Usenet?

Image files posted on the alt.binaries.pictures.* newsgroups are usually
"uuencoded". Uuencoding converts binary image data into text that can
safely be posted. Most posters also divide large posts into multiple
since some news software can't cope with big articles. Before your
will recognize the image, you must combine the parts into one file and
the text through a uudecode program. (This is all true for GIF as well
JPEG, by the way.) There are programs available to automate this

For more info see the alt.binaries.pictures FAQ, which is available from
(see also "[21] Where are FAQ lists archived?").


Subject: [8] What is 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
256 or fewer distinct colors at a time. To display a full-color image,
computer must choose an appropriate set of representative colors and map
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. But we're stuck with the standard usage.)

Clearly, color quantization is a lossy process. It turns out that for
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 method is a black art, and no single algorithm is best
for all images.

Since JPEG is a full-color format, displaying a color JPEG image 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
its quantization algorithm. Depending on whether a quick-and-dirty or
good-but-slow method is used, you'll see great variation in image quality
among viewers on 8-bit displays, much more than occurs on 24-bit

On the other hand, a GIF image has already been quantized to 256 or fewer
colors. (A GIF always has a specific number of colors in its palette,
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 image maker's quantization. If the maker quantized
a different number of colors than what you can display, you'll either
display capability or else have to requantize to reduce the number of
(which usually results in much poorer image quality than quantizing 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
is ruined.

For this reason, JPEG promises significantly better image quality than
for all users whose machines don't match the image maker's display
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
improvements in quantization algorithms, or purchase better display
hardware, to get a better view of JPEG images you already have. With a
you're stuck forevermore with what was sent.

A growing number of people have better-than-8-bit display hardware
15-bit "hi-color" PC displays, true 24-bit displays on workstations and
Macintoshes, etc. For these people, GIF is already obsolete, as it
represent an image to the full capabilities of their display. JPEG
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-
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
will usually come out with slightly different colors. Considering the
image, 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


Subject: [9] What are some rules of thumb for converting GIF images to

Converting GIF files to JPEG is a tricky business --- you are piling one
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
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
pixel-for-pixel identical to the GIF --- it won't. Especially not on an
8-bit display, because the color quantization process used to display the
JPEG probably won't quite match the quantization process used to make the
GIF from the original data (see "[8] What is color quantization?"). But
remember that the GIF itself is not all that faithful to the full-color
original, if you look at individual pixels. Looking at the overall
a converted JPEG can look as good as its GIF source. Some people claim
on 24-bit displays, a carefully converted JPEG can actually look better
the GIF source, because dither patterns have been eliminated. (More
dithering in a moment.)

On the other hand, JPEG conversion absolutely *will* degrade an
image or one that is converted carelessly. If you are not willing to
the amount of trouble suggested below, you're much better off leaving
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
(see "[3] When should I use JPEG, and when should I stick with GIF?").
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
converting any GIF that's much under 100 Kbytes; the potential savings
worth the hassle.)

The second rule is to know where the image came from. Repeated
conversions are guaranteed to turn an image into mush, because you pay a
quality-loss price on each round trip. Don't reconvert images that have
been converted before.

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
Worse, the sharp border boundary can create visible artifacts (ghost
Furthermore, when viewing a bordered JPEG on an 8-bit display, the
will think the border color is important because there's so much of it,
hence will waste color palette entries on the border, thus actually
the displayed quality of the main part of the image! So do yourself a
and crop off any border before JPEGing.

The final rule is to look at each JPEG, to make sure you are happy with
before throwing away the corresponding GIF. This will give you a chance
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
file of reasonable quality may come out *larger* than the GIF.

Gray-scale images usually convert without much problem. When using
be sure to use the -gray switch. (Otherwise, cjpeg treats a GIF as color
data; this works, but it wastes space and time if the image is really
gray-scale.) 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
adjacent pixels are often of significantly different colors; at normal
the eye averages these pixels together to produce the illusion of an
intermediate color value. The trouble with dithering is that, to JPEG,
looks like high-spatial-frequency color noise; and JPEG can't compress
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
image (if you had it). To get around this, you need to "smooth" the GIF
image before compression. Smoothing averages together nearby pixels,
approximating the color that you thought you saw anyway, and in the
getting rid of the rapid color changes that give JPEG trouble. Proper
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 IJG JPEG software (cjpeg or derived programs), a simple
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. GIFs with
heavy-handed dithering may require larger smoothing factors. (If you can
see regular fine-scale patterns on the GIF image even without
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
program, but appropriate use of such tools is beyond the scope of this

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 still higher quality settings if you can't hide the
dithering pattern with a reasonable smoothing factor. Really badly
GIFs are best left as GIFs.

Don't expect JPEG files converted from GIFs to be as small as those
directly from full-color originals. The dithering noise wastes space,
you won't be able to smooth away all the noise without blurring the
Typically, a good-quality converted JPEG will be one-half to one-third
size of the GIF file, not one-fourth as suggested in section 4. If the
comes out much more than half the size of the GIF, this is a good sign
the image shouldn't be converted at all.

The upshot of all this is that "cjpeg -quality 85 -smooth 10" is probably
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
Blindly converting a large GIF library at this or any other setting is a
recipe for disaster.


Subject: [10] Does loss accumulate with repeated

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
of generations of JPEG compression between initial and final versions of
It turns out that if you decompress and recompress an image at the same
quality setting first used, little or no further degradation occurs.
means that you can make local modifications to a JPEG image without
degradation of other areas of the image. (The areas you change will
degrade, however.) Counterintuitively, this works better the lower the
quality setting. But you must use *exactly* the same setting, or all
are off. Also, the decompressed image must be saved in a full-color
if you do JPEG=>GIF=>JPEG, the color quantization step loses lots of

Unfortunately, cropping doesn't count as a local change! JPEG processes
the image in small blocks, and cropping usually moves the block
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, PNG, TIFF, etc) while working on the image, then JPEG it
you are ready to file it away or send it out on the net. Aside from
avoiding degradation, you will save a lot of compression/decompression
this way :-).


Subject: [11] What is progressive JPEG?

A simple or "baseline" JPEG file is stored as one top-to-bottom scan of
image. Progressive JPEG divides the file into a series of scans. The
scan shows the image at the equivalent of a very low quality setting, and
therefore it takes very little space. Following scans gradually improve
quality. Each scan adds to the data already provided, so that the total
storage requirement is about the same as for a baseline JPEG image of the
same quality as the final scan. (Basically, progressive JPEG is just a
rearrangement of the same data into a more complicated order.)

The advantage of progressive JPEG is that if an image is being viewed
on-the-fly as it is transmitted, one can see an approximation to the
image very quickly, with gradual improvement of quality as one waits
this is much nicer than a slow top-to-bottom display of the image. The
disadvantage is that each scan takes about the same amount of computation
display as a whole baseline JPEG file would. So progressive JPEG only
sense if one has a decoder that's fast compared to the communication
(If the data arrives quickly, a progressive-JPEG decoder can adapt by
skipping some display passes. Hence, those of you fortunate enough to
T1 or faster net links may not see any difference between progressive and
regular JPEG; but on a modem-speed link, progressive JPEG is great.)

Up until recently, there weren't many applications in which progressive
looked attractive, so it hasn't been widely implemented. But with the
popularity of World Wide Web browsers running over slow modem links, and
with the ever-increasing horsepower of personal computers, progressive
has become a win for WWW use. IJG's free JPEG software (see part 2) now
supports progressive JPEG, and the capability is spreading fast in WWW
browsers and other programs.

Except for the ability to provide progressive display, progressive JPEG
baseline JPEG are basically identical, and they work well on the same
of images. It is possible to convert between baseline and progressive
representations of an image without any quality loss. (But specialized
software is needed to do this; conversion by decompressing and
is *not* lossless, due to roundoff errors.)

A progressive JPEG file is not readable at all by a baseline-only JPEG
decoder, so existing software will have to be upgraded before progressive
JPEG can be used widely. See question 15 in part 2 for the latest news
about which programs support it.


Subject: [12] Can I make a transparent JPEG?

No. JPEG does not support transparency and is not likely to do so any
soon. It turns out that adding transparency to JPEG would not be a
task; read on if you want the gory details.

The traditional approach to transparency, as found in GIF and some other
file formats, is to choose one otherwise-unused color value to denote a
transparent pixel. That can't work in JPEG because JPEG is lossy: a
won't necessarily come out *exactly* the same color that it started as.
Normally, a small error in a pixel value is OK because it affects the
only slightly. But if it changes the pixel from transparent to normal or
vice versa, the error would be highly visible and annoying, especially if
the actual background were quite different from the transparent color.

A more reasonable approach is to store an alpha channel (transparency
percentage) as a separate color component in a JPEG image. That could
since a small error in alpha makes only a small difference in the result.
The problem is that a typical alpha channel is exactly the sort of image
that JPEG does very badly on: lots of large flat areas and sudden jumps.
You'd have to use a very high quality setting for the alpha channel. It
could be done, but the penalty in file size is large. A transparent JPEG
done this way could easily be double the size of a non-transparent JPEG.
That's too high a price to pay for most uses of transparency.

The only real solution is to combine lossy JPEG storage of the image
with lossless storage of a transparency mask using some other algorithm.
Developing, standardizing, and popularizing a file format capable of
doing that is not a small task. As far as I know, no serious work is
being done on it; transparency doesn't seem worth that much effort.


Subject: [13] Isn't there a lossless JPEG?

There's a great deal of confusion on this subject. The JPEG standard
include a truly lossless compression algorithm, i.e., one that guarantees
its decompressed 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.

Lossless JPEG typically compresses full-color data by around 2:1.
JPEG works well only on continuous-tone images; it does not provide
compression of palette-color images or low-bit-depth images. (Early
with the new PNG standard suggest that PNG outcompresses lossless JPEG on
most images. Once PNG becomes common, lossless JPEG will probably be

There are very few implementations of true lossless JPEG. The PVRG code
mentioned in part 2 handles lossless JPEG. Another free implementation
is available from ftp.cs.cornell.edu:/pub/multimed/ljpg.tar.Z. This is a
smaller program that handles *only* lossless JPEG.

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

Many implementations won't even let you get to the maximum possible
because it's such an inefficient way to use regular JPEG. With the IJG
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
files generated at more reasonable settings. And they're still slightly
lossy! If you really need lossless storage, don't try to approximate it
with regular JPEG.


Subject: [14] Why all the argument about file formats?

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

Since we can't actually exchange images with anyone else unless we agree
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
"do their own thing", and as a result their programs aren't compatible
anyone else's.

The closest thing we have to a standard JPEG format is some work that's
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
     pixels and not much else.
   * TIFF/JPEG, aka TIFF 6.0, an extension of the Aldus TIFF format. TIFF
     a "high-end" format that will let you record just about everything
     ever wanted to know about an image, and a lot more besides :-).
JFIF has emerged as the de-facto standard on Internet, and is what is
commonly meant by "a JPEG file". Most JFIF readers are also capable of
handling some not-quite-JFIF-legal variant formats.

The TIFF 6.0 spec for incorporating JPEG is not widely implemented,
because it has some serious design flaws. A revised TIFF/JPEG design is
described by TIFF Technical Note #2; this design will be the one used in
TIFF 7.0. New implementations of TIFF should use the Tech Note's design
embedding JPEG, not the TIFF 6.0 design. (As far as I know, NeXTStep
systems are the only ones making any significant use of TIFF 6.0 style
TIFF/JPEG.) Even when TIFF/JPEG is stable, it will never be as widely
as JFIF. TIFF is far more complex than JFIF, and is generally less
transportable because different vendors often implement slightly
nonoverlapping subsets of TIFF. Adding JPEG to the mix hasn't helped

Apple's Macintosh QuickTime software uses a JFIF-compatible datastream
wrapped inside the Mac-specific PICT format. Conversion between JFIF and
PICT/JPEG is pretty straightforward, and several Mac programs are
to do it (see Mac portion of part 2). If you have an editor that handles
binary files, you can even strip a PICT/JPEG file down to JFIF by hand;
the next section for details.

News flash: the ISO JPEG committee seems to have won their turf wars.
have defined a complete file format spec called SPIFF in the new "Part 3"
extensions to the JPEG standard. It's pretty late in the game though, so
whether this will have much impact on real-world files remains to be
SPIFF is upward compatible with JFIF, so if it does get widely adopted,
users probably won't even notice.


Subject: [15] 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
to be HSI format or some other proprietary JPEG-based format. You can
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
       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
       usually 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
    Look for the 3-byte sequence (hex) FF D8 FF. The text 'Photo - JPEG'
    will usually appear shortly before this header, and 'AppleMark' or
    'JFIF' will usually appear shortly after it. Strip off everything
    before the FF D8 FF and you will usually be able to decode the file.
    (This will fail if the PICT image is divided into multiple "bands";
    fortunately banded PICTs aren't very common. A banded PICT contains
    multiple JPEG datastreams whose heights add up to the total image
    height. These need to be stitched back together into one image.
    Bailey Brown has some simple tools for this purpose on a Web page at

5.   If the file came from a Macintosh, it could also be a standard JFIF
     file with a MacBinary header attached. In this case, the JFIF header
     will appear 128 bytes into the file. Get rid of the first 128 bytes
     and you're set.

6.   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
     If you can identify the start of the JPEG data stream (look for FF
     try stripping off everything before that.

HSI files used to be rather common in alt.binaries.pictures.* postings,
although thankfully they have gotten less so. You can spot an HSI
by looking at the first few characters of the uuencoded data. The
characteristic HSI pattern is
      "begin" line
      M:'-I ...
whereas standard JFIF files begin with
      "begin" line
      M_]C_X ...
If you learn to spot the HSI pattern, you can save yourself the trouble
of downloading unusable files.

At least one release of HiJaak Pro writes JFIF files that claim to be
revision 2.01. There is no such spec; the latest JFIF revision is 1.02.
It looks like HiJaak got the high and low bytes backwards.
most JFIF readers will give up on encountering these files, because the
spec defines a major version number change to mean an incompatible format
change. If there ever *were* a version 2.01, it would be so numbered
because current software could not read it and should not try. (One
if HiJaak has ever heard of cross-testing with other people's software.)
If you run into one of these misnumbered files, you can fix it with a
binary-file editor, by changing the twelfth byte of the file from 2 to 1.

If the file header seems valid, but your decoder chokes with a complaint
like "Unsupported marker type 0xC2", then you have a progressive JPEG
and a non-progressive-capable decoder. See part 2 of this FAQ for
information about more up-to-date programs.


Subject: [16] How does JPEG work?

You can find an introduction and references for further reading in 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]
(see also "[21] Where are FAQ lists archived?").

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 perhaps 10
years behind JPEG in the standardization pipeline. Fractals have been
terribly over-hyped by their chief commercial proponent, and it's
to say what their true capabilities are.


Subject: [17] What about arithmetic coding?

The JPEG spec defines two different "back end" modules for the final
of compressed data: either Huffman coding or arithmetic coding is
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
JPEG standard is subject to patents owned by IBM, AT&T, and Mitsubishi.
Thus *you cannot legally use JPEG arithmetic coding* unless you obtain
licenses from these companies. (Patent law's "experimental use"
allows people to test a patented method in the context of scientific
research, but any commercial or routine personal use is infringement.)

I recommend that people not use JPEG arithmetic coding; the space savings
isn't great enough to justify the potential legal hassles. In
arithmetic coding *should not* be used for any images to be exchanged on
the Internet. Even if you don't care about US patent law, other folks


Subject: [18] 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.
Most production-quality JPEG programs use only integer arithmetic and so
they are unaffected by the presence or absence of floating-point

It is possible to save a few math operations by doing the DCT step in
floating point. On most PC-class machines, FP operations are enough
than integer operations that the overall speed is still much worse with
Some high-priced workstations and supercomputers have fast enough FP
hardware to make an FP DCT method be a win.

DSP (digital signal processing) chips are ideally suited for fast
integer arithmetic, so programming a DSP to do JPEG can yield significant
speedups. DSPs are available as add-ons for some PCs and workstations;
if you have such hardware, look for a JPEG program that can exploit it.


Subject: [19] Isn't there an M-JPEG standard for motion pictures?

As was stated in section 1, JPEG is only for still images. Nonetheless,
you will frequently see references to "motion JPEG" or "M-JPEG" for
*There is no such standard*. Various vendors have applied JPEG to
individual frames of a video sequence, and have called the result "M-
Unfortunately, in the absence of any recognized standard, they've each
it differently. The resulting files are usually not compatible across
different vendors.

MPEG is the recognized standard for motion picture compression. It uses
many of the same techniques as JPEG, but adds inter-frame compression to
exploit the similarities that usually exist between successive frames.
Because of this, MPEG typically compresses a video sequence by about a
factor of three more than "M-JPEG" methods can for similar quality.
The disadvantages of MPEG are (1) it requires far more computation to
generate the compressed sequence (since detecting visual similarities is
hard for a computer), and (2) it's difficult to edit an MPEG sequence on
frame-by-frame basis (since each frame is intimately tied to the ones
it). This latter problem has made "M-JPEG" methods rather popular for
editing products.

It's a shame that there isn't a recognized M-JPEG standard. But there
isn't, so if you buy a product identified as "M-JPEG", be aware that you
are probably locking yourself into that one vendor.

Recently, both Microsoft and Apple have started pushing (different :-()
"standard" M-JPEG formats. It remains to be seen whether either of these
efforts will have much impact on the current chaos. Both companies were
spectacularly unsuccessful in getting anyone else to adopt their ideas
still-image JPEG file formats, so I wouldn't assume that anything will
happen this time either...

See the MPEG FAQ for more information about MPEG.


Subject: [20] What if I need more than 8-bit precision?

Baseline JPEG stores images with 8 bits per color sample, in other words
24 bits per pixel for RGB images, 8 bits/pixel for grayscale, 32
for CMYK, etc. There is an extension that stores 12 bits/sample for
applications that need higher accuracy. Medical images, for example, are
often 12-bit grayscale. The 12-bit extension is not very widely
however. One package that does support it is the free IJG source code
part 2 of this FAQ).

For lossless JPEG, the standard permits any data precision between 2 and
bits per sample, but high-precision lossless JPEG is even less widely
supported than high-precision lossy JPEG. I don't know of any working
noncommercial implementations. (The Cornell lossless JPEG coder tries to
support it, but reportedly has bugs with data of more than 8 bits.)


Subject: [21] Where are FAQ lists archived?

Many FAQs are crossposted to news.answers.    Well-run netnews sites will
the latest versions available in that newsgroup. However, there are a
of postings in news.answers, and they can be hard to sort through.

The latest versions of news.answers postings are archived at
You can retrieve this FAQ by FTP as /pub/usenet/news.answers/jpeg-
and /pub/usenet/news.answers/jpeg-faq/part2. With a Web browser, use the
URL ftp://rtfm.mit.edu/pub/usenet/news.answers/jpeg-faq/. If you have no
FTP access, send e-mail to mail-server@rtfm.mit.edu containing the lines
      send usenet/news.answers/jpeg-faq/part1
      send usenet/news.answers/jpeg-faq/part2
(If you don't get a reply, the server may be misreading your return
add a line such as "path myname@mysite" to specify your correct e-mail
address to reply to.) For more info about the FAQ archive, retrieve the
file rtfm.mit.edu:/pub/usenet/news.answers/news-answers/introduction.

The same FAQs are also available in the World Wide Web, although you
be wary of out-of-date copies at some sites. Well-run WWW FAQ archive
include http://www.lib.ox.ac.uk/internet/news/ and

                 tom lane
                 organizer, Independent JPEG Group
                 tgl@netcom.com or tgl@sss.pgh.pa.us

From tgl@netcom.com Sun Jul 21 17:53:14 1996
Path: szdc!zdc!zdc-
From: tgl@netcom.com (Tom Lane)
Subject: JPEG image compression FAQ, part 2/2
Message-ID: <jpeg-faq-p2_837989585@netcom.com>
Followup-To: comp.graphics.misc
Summary: System-specific hints and program recommendations for JPEG
Keywords: JPEG, image compression, FAQ, JPG, JFIF
Supersedes: <jpeg-faq-p2_836768094@netcom.com>
Reply-To: jpeg-info@uunet.uu.net
Organization: Independent JPEG Group
References: <jpeg-faq-p1_837989585@netcom.com>
Date: Sun, 21 Jul 1996 22:53:14 GMT
Approved: news-answers-request@MIT.Edu
Expires: Sun, 18 Aug 1996 22:53:05 GMT
Lines: 761
Sender: tgl@netcom21.netcom.com
Xref: szdc comp.graphics.misc:7855
comp.infosystems.www.authoring.images:9644 alt.graphics.pixutils:2000
alt.binaries.pictures.utilities:5041 alt.binaries.pictures.d:2417
alt.binaries.pictures.erotica.d:7726 comp.answers:4418 alt.answers:5233

Archive-name: jpeg-faq/part2
Posting-Frequency: every 14 days
Last-modified: 21 July 1996

This article answers Frequently Asked Questions about JPEG image
This is part 2, covering system-specific hints and program
for a variety of computer systems. Part 1 covers general questions and
answers about JPEG. As always, suggestions for improvement of this FAQ

New since version of 7 July 1996:
  * New entry for ThumbsPlus (image browser/cataloger for Windows).
  * New version of Image Alchemy (1.8).

Tho decide whether to list it. If you
have an opinion pro or con on any program, I'd appreciate hearing it.

This FAQ also includes a few hints that are specific to a machine or
program, and thus don't belong in the general discussion of part 1.


Subject: [2] How do I retrieve these programs?

All the files mentioned in this FAQ are available by standard Internet
If you don't know how to use FTP, please read the article "Anonymous FTP
FAQ List", which you can get by sending e-mail to mail-
with the single line "send usenet/news.answers/ftp-list/faq" in the body.
(See also "[16] Where are FAQ lists archived?") This section gives some
quick reminders which are not meant as a substitute for reading the FTP

If you do not have direct access to FTP, you can use an "ftpmail" server
obtain files by e-mail. See the FTP FAQ for details.

If you use a WWW browser such as Mosaic or Lynx, it will do FTP for you.
To retrieve a file described here as "site.name:/path/to/file", tell the
browser to open the URL "ftp://site.name/path/to/file". (If you are
this FAQ in the WWW FAQ archive, the file names should appear as links
you can just click on.) Don't forget to set save-to-disk mode first.

Many of the pointers given here refer to popular central archive sites,
such as ftp.simtel.net for DOS software or sumex-aim.stanford.edu for
These sites are often overloaded, and are likely to refuse your
request when they are busy. You can try again at a less popular time of
day, or you can look for a "mirror site". Most central archive sites
groups of mirror sites that keep copies of their files. Find out the
of the mirror site closest to you, and visit that site instead; it's good
net citizenship and you'll get faster response. Check the FAQs for the
newsgroups specific to your system type to find lists of mirror sites.
(The archive site may list some mirror sites in its connection-refused
message. Unfortunately, some FTP programs won't show you the whole
WWW browsers are often bad about this.)

If you are able to reach the archive site, but the file you want doesn't
exist, most likely it's been replaced by a newer version. Get a
listing of the directory that's supposed to contain the file, and look
a file with a similar name but a higher version number. (If you find an
out-of-date reference in a *current* version of the JPEG FAQ, I'd
appreciate hearing about it by e-mail.)

Practically all of the files listed here are compressed archive files.
This means you need to retrieve them in binary mode. (WWW browsers do
automatically, but many older FTP programs must be told to use binary
Once you've got the archive file, you'll need a decompressor/dearchiver
to extract the program and documentation files inside it. Check the FAQs
for your system type to find out where to get dearchiver programs.


Subject: [3] X Windows

XV is an excellent viewer for JPEG, GIF, and many other image formats.
It can also do format conversion and some simple image manipulations.
Get it from ftp.cis.upenn.edu:/pub/xv/xv-3.10a.tar.gz. Shareware, $25.
Version 3.10 has some nifty new features, and it loads JPEGs noticeably
faster than any prior version. If you're still using version 2.anything,
it's definitely time to upgrade. HINT: 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
from the 8-bit version, thus costing you image quality.) You can set
mode to be default by adding "xv.force8: true" to your .Xdefaults file.

Another excellent choice is John Cristy's free ImageMagick package,
package handles many image processing and conversion tasks. The
viewer handles 24-bit displays correctly; for colormapped displays, it
better (though slower) color quantization than XV or the basic IJG JPEG

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
prior versions and is easier to install. xloadimage is free and
from ftp.x.org:/R5contrib/xloadimage.4.1.tar.gz. xli is a variant
of xloadimage; xli is slightly better as an interactive viewer, but it
be used as a converter, and it supports fewer file formats. xli is also
free and available from ftp.x.org:/contrib/applications/xli.1.16.tar.gz.


Subject: [4] Unix (without X)

If you want a command-line JPEG conversion program, see the IJG source
described in section 14. (This code is included as a subdirectory in
of the X programs described above, although they may not have the latest

Non-X viewers are hard to come by, since they are very hardware
Linux users with VGA/SVGA displays may like zgv. Version 2.7 is
from sunsite.unc.edu:/pub/Linux/apps/graphics/viewers/zgv2.7-bin.tar.gz.
(Several other alternatives are available in the same directory.)
If you use a less popular platform, you're probably out of luck.


Subject: [5] MS-DOS

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

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 ftp.simtel.net,
which is the site referenced by the FTP pointers given below. However,
there are numerous mirror sites that 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
nearest mirror site. If you have no FTP capability, the same postings
tell you how to retrieve Simtel files by e-mail.

QPV (formerly called QPEG) is an extremely fast JPEG viewer. In exchange
speed, QPV gives up some image quality, particularly on 256-or-less-color
displays. Its best feature is a really-fast small preview window, which
great for searching through lots of image files. Also views
Requires 386-or-better CPU and VGA-or-better display card. Current
is 1.7c, available from ftp.tu-
In the USA, a closer site is ftp.best.com:/pub/bryanw/qpv/. Shareware,

SEA is a new JPEG/PNG/GIF/etc viewer and file-format converter. It is
very very fast --- faster than QPV in most cases, according to the
Also, it can read progressive JPEGs; QPV can't. Current version is 1.2c,
available from ftp.simtel.net:/pub/simtelnet/msdos/graphics/sea12c.zip.
Shareware, $24. Requires 386-or-better CPU and VESA-compatible display.

DVPEG is a free viewer for JPEG, GIF, Targa, and PPM files. Current
is 3.0l, available from
(That's lower case l, not digit 1.) This is a good basic viewer that
in both 286 and 386-and-up versions. The user interface is clunky but
functional. DVPEG is substantially faster than it used to be; on hi-
displays it is nearly as fast as QPV. On 8-bit displays, its two-pass
quantization mode is slow but gives much better image quality than QPV

Lesser-used DOS viewers include:
* DISPLAY, alias DISP. The Swiss army knife of DOS viewers. Does almost
  everything, but a bit intimidating for newcomers. User interface is
  improved over early versions, but still awkward in places. Requires
  or better. Freeware. Current version is 1.89, available from
  nctuccca.edu.tw:/PC/graphics/disp/disp189a.zip and disp189b.zip.
* GDS. A well-done viewer and image converter for many image formats.
  Installation is simple, and the on-line documentation is very good.
  JPEG loading is a bit slower than the above viewers, though.
  $40. Current version is 3.1f. A slightly restricted demo version is
  available from ftp.photodex.com:/gds31f.exe.
* NVIEW. Views JPEG and half a dozen other image formats. Easy to use,
  very easy to install. Only moderately fast, but it has lots of
  Supports hi-color and true-color modes on some cards, but not mine :-(.
  Requires 386 or better. Current version is 1.50, available from
  ftp.simtel.net:/pub/simtelnet/msdos/graphics/nview150.zip. Shareware,
* CSHOW or CompuShow (recently renamed 2SHOW). This is a widely used
  for GIF and other formats. Versions prior to CSHOW 9.00 or 2SHOW 2.00
  absolutely abysmal JPEG support; if you have one of those, toss it and
  a newer version. The current release is still the slowest DOS JPEG
  listed here, but it's faster than it used to be, and image quality and
  robustness have improved substantially. The main reason to use CSHOW
  as a JPEG viewer is that it supports a wide range of pre-VGA display
  hardware (most of the above viewers require VGA or better). Also,
  doesn't require a 386. Current version is 2.04, available from
  ftp.simtel.net:/pub/simtelnet/msdos/graphics/2show204.zip. Shareware,

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
of them to work, you'll need to use one of the following conversion
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 hi-color BMP- or TARGA-capable viewer instead.)

The free IJG JPEG converters are available from
ftp.simtel.net:/pub/simtelnet/msdos/graphics/jpeg6a.zip (or jpeg6a32.zip
if you have a 386-or-better CPU and extended memory). These programs
convert JPEG to and from GIF, BMP, Targa, and PPM formats; they are DOS
compilations of the free source code described in section 14.

Handmade Software offers free JPEG<=>GIF conversion tools,
These are quite slow and are limited to conversion to and from GIF
thus they can't produce 24-bit color output from a JPEG. The sole
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
are rather widespread on BBSes, this is a useful capability. Version 2.0
of these tools is free (prior versions were shareware), and is available
from ftp.simtel.net:/pub/simtelnet/msdos/graphics/gif2jpg2.zip.
NOTE: do not use HSI format for files to be posted on Usenet, since it is
not readable by any non-HSI software.

Handmade Software also has a shareware image conversion and manipulation
package, Image Alchemy. This will translate JPEG files (both JFIF and
formats) to and from many other image formats. It can also display
A demo version of Image Alchemy version 1.8 is available from

JPGINDEX is a useful tool for making indexes of JPEG image collections.
Available from ftp.simtel.net:/pub/simtelnet/msdos/graphics/jpgidx13.zip.


Subject: [6] Microsoft Windows

ACDSee is a very fast, easy to use JPEG/GIF/PNG/etc viewer. Good viewing
and browsing capabilities, including a fast preview display; but no image
editing or conversion functions. Current version is 1.31, available from
ftp.simtel.net:/pub/simtelnet/win3/graphics/acdc131.zip (and other Simtel
mirror sites; see NOTE in previous section). Shareware, $15.

LView Pro is a viewer/editor/converter for JPEG, GIF, BMP, and other
formats. It offers a wide array of image editing functions and can load
JPEGs in either fast/low-quality or slow/high-quality modes. Requires
or better CPU. The current version, 1.D, runs under Windows 95, Windows
or Windows 3.1 with Win32s 32-bit extension. It's available from
ftp.std.com:/ftp/vendors/mmedia/lview/lviewpro.zip. Shareware, $30.
An older version that can run under vanilla Windows 3.1 is

WinECJ is a fast, no-frills viewer with image quality noticeably worse
most other JPEG viewers. (You can purchase a version with better image
quality for AUD$30.) Version 1.2 is free and available from
ftp.coast.net:/SimTel/win3/graphics/winecj12.zip. Requires Windows 3.1
256-or-more-colors mode.

WinJPEG displays and converts JPEG, GIF, TIFF, BMP, and other file
It has some other nifty features including screen capture, color-balance
adjustment, and slideshow. Shareware, $25. The current version is 2.76,
available from ftp.coast.net:/SimTel/win3/graphics/winjp276.zip. (This
is a
286-compatible version; if you register, you'll get the 386-and-up
which is roughly twice as fast.)

VuePrint is a widely used viewer and printer for JPEG, GIF, BMP, and
formats. It's slower at loading JPEGs than most other programs listed
but it has an impressive array of features. Shareware, $40. Version 4.6
available from ftp.simtel.net:/pub/simtelnet/win3/graphics/vuepri46.zip.

ThumbsPlus is an image browser and cataloger that handles many file
It can also do some editing and format conversion, but indexing a large
image collection is what it's really aimed at. Current version is 3.0c,
ftp.cerious.com:/pub/cerious/thmpls32.exe. Shareware, $70. Requires
Windows 95 or NT, or Windows 3.1 with Win32s.

Some people prefer Paint Shop Pro. It's not very impressive as just a
viewer (especially since image quality is not very good on 8-bit
but it has *lots* of image manipulation and conversion features. Current
version is 3.11, available from
ftp.coast.net:/SimTel/win3/graphics/psp311.zip. Shareware, $69.

QPV and DVPEG (see previous section) 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.

Other Windows 95 native releases include:
* PolyView. Reads JPEG, PNG, GIF, BMP, Photo-CD. Current version is
  available from ftp.winsite.com:/pub/pc/win95/desktop/polyv221.zip.
  Shareware, $20.

Other popular Windows NT-only viewers include:
* NTj. Reads JPEG, GIF, and BMP. Available from
  ftp.digital.com:/pub/DEC/NTj/NTj-x86.exe (or NTj.exe for the DEC Alpha
  version). Freeware.
* PolyView. Reads JPEG, GIF, BMP, Photo-CD. Current version is 1.70,
  available from ftp.winsite.com:/pub/pc/winnt/misc/polyv170.zip.
  Shareware, $20.


Subject: [7] OS/2

The most widely used OS/2 JPEG viewers are:

PMJPEG 1.73: OS/2 2.x port of WinJPEG, a popular viewer/converter for
Windows (see description in previous section). Shareware, $20.
from hobbes.nmsu.edu:/os2/graphics/pmjpg173.zip.

PMView 0.92: JPEG/GIF/BMP/Targa/etc viewer. GIF viewing very fast, JPEG
viewing roughly the same speed as the above two programs. Has image
manipulation & slideshow functions. Shareware, $35. Available from

Galleria 2.2: JPEG/BMP/PCX/Targa/TIFF viewer/editor/converter.
Shareware, $65. Available from

All of these viewers require Palette Manager for best display quality.
Opinion seems to be about equally split as to which is the best, so try
them all to see which one you like.

JPEGPROC enables all OS/2 multimedia applications to read and write JPEG
files. Available as part of the "Practice Viewer Upgrade" which also
includes a multimedia browser alleged to be better than IB.EXE.
Shareware. Available from hobbes.nmsu.edu:/os2/graphics/pvu110.zip.

Very out-of-date OS/2 executables of the free IJG conversion programs
are available from hobbes.nmsu.edu:/os2/graphics/jpegv4.zip (32-bit) or
hobbes.nmsu.edu:/os2/graphics/jpeg4_16.zip (16-bit, for OS/2 1.x).
I hope someone will submit newer versions soon.

Note: the hobbes OS/2 collection is mirrored at ftp-os2.cdrom.com.


Subject: [8] Macintosh

Most Mac JPEG programs rely on Apple's JPEG implementation, which is part
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
System 6.0.7 or later. (If you're running System 6, you must also
the 32-bit QuickDraw extension; in System 7, that is built in.) The
version of QuickTime is 2.1, available from

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 part 1 for
details.) If you post images on Usenet, make sure they are in JFIF
Most of the programs mentioned here can handle either format.

The two major Internet sites for Mac software are sumex-aim.stanford.edu
mac.archive.umich.edu. Unfortunately they are both very busy, so you may
have better luck getting files from a mirror site. (In fact, these days
you *must* use a mirror site for sumex-aim; the primary archive only
mirror sites to log in! The sumex-aim FTP pointers given below are
therefore useless as-is; you *must* substitute a mirror site name.) See
"Introductory Macintosh Frequently Asked Questions" in the comp.sys.mac.*
newsgroups for the current locations of mirrors.

JPEGView is an excellent free program for viewing
and other image files. It can convert between the two JPEG formats and
create preview images for files. The current version is 3.3.1, available
from sumex-aim.stanford.edu:/info-mac/gst/grf/jpeg-view-331.hqx.
System 7; QuickTime is optional. JPEGView usually produces the best
image quality of all the currently available Mac JPEG viewers, and it
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
rather than presenting scroll bars like most other viewers. (You can
in on any desired portion, though.) Some people like this behavior, some
don't. Overall, JPEGView's user interface is very well thought out.

Jade is a new, very promising freeware viewer for JPEG and GIF images.
It's fast, simple to use, and has preview and slideshow capabilities.
And it supports progressive JPEGs. Since JPEGView is no longer being
updated, Jade will probably supersede it as the most popular Mac JPEG
viewer before long. Current version is 1.03, available from
sumex-aim.stanford.edu:/info-mac/gst/grf/jade-103.hqx. Requires 68020
(or higher) or PowerPC, as well as System 7.5 (or later) or Thread

GIFConverter, a shareware ($40) image viewer/editor/converter, supports
JFIF,PICT/JPEG,GIF, and many other image formats. Current version is
Requires System 6.0.5 or later. GIFConverter is not better than JPEGView
a plain JPEG/GIF viewer, but it has much more extensive image
and format conversion capabilities. Also, GIFConverter is your best bet
if your machine is too old to run System 7 and/or QuickTime. Hint: if
GIFConverter runs out of memory while loading a large JPEG, try
the file to GIF with JPEG Convert, then viewing the GIF version.

GraphicConverter is another popular viewer/editor/converter.   It has even
more functionality than GIFConverter, but is correspondingly larger.
if you like lots of options. Shareware, $35. Current version is 2.4.3,

JPEG Convert, a Mac version of the free IJG JPEG conversion utilities, is
available at sumex-aim.stanford.edu:/info-mac/gst/grf/jpeg-convert-
This will run on any Mac, but it only does file conversion, not viewing.
You can use it in conjunction with any GIF viewer. (Note: JPEG Convert
not yet been updated to support progressive JPEG :-(.)

JPEG Transmogrifier is a simple drag-and-drop application to convert
JPEGs to progressive and back. It can also convert GIF and PICT images
either flavor of JPEG. Shareware, $12. Available from

Progressify is an even simpler drag-and-drop application that converts
regular JPEGs to progressive and back, and does nothing else. Although
less capable than Transmogrifier, it's free. Available from

More and more commercial Mac applications are supporting JPEG, although
all can deal with the Usenet-standard JFIF format. Adobe Photoshop,
2.0.1 or later, can read and write JFIF-format JPEG files. (In 2.0.1,
the JPEG plug-in on the Acquire menu; 2.5 and later handle JPEG the same
other file types.) You must set the file type code of a downloaded JPEG
file to 'JPEG' to allow Photoshop to recognize it.

Photoshop does not yet support progressive JPEG by itself, but you can
two different plugins that enable progressive JPEG support in Photoshop
(and other applications that support Photoshop plugins). One is ProJPEG,
available from aris.com:/boxtop/ProJPEGv1.0.1.sit.hqx (shareware, $25).
The other is the plugin version of JPEG Transmogrifier, available from
ftp.best.com:/pub/gyld/plugin.sea.hqx (shareware, $22).

HINT: if you use Fetch to retrieve files by FTP, add ".jpg" to its list
binary file types under Customize/Suffix Mapping. Otherwise Fetch's
"automatic" retrieval mode will retrieve JPEGs in text mode, thus
the data.


Subject: [9] Amiga
Most programs listed in this section are available from "AmiNet" archive
sites. The master AmiNet site is wuarchive.wustl.edu, 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
Opinions here are mostly stolen from his article.

FastView is a fast, high-quality JPEG/GIF/ILBM viewer. Works well on
ECS and AGA displays. Shareware, $15; requires OS 2.0. Version 2.0 is
available from Aminet sites, file /pub/aminet/gfx/show/FView20.lha.

FastJPEG is a free JPEG viewer; it's fast and has good image quality, but
doesn't view any formats except JPEG. Somewhat faster than FastView on
machines, slower on AGA. Version 1.10 is available from Aminet sites,

PPShow is a 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
as good (particularly   on ECS 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
several formats besides JPEG. The current version is 2.0.8. A demo
is available from AmiNet sites, file /pub/aminet/gfx/edit/hamlab208d.lha.
The demo version will crop images larger than 512x512, but it is
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

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.
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 preferences. Viewtek has poor display
on OCS/ECS (HAM6) screens; but it looks very good on AGA (HAM8).
There is finally a good JPEG datatype for use with datatype-based viewers
(such as Multiview or ShowDT). Available from AmiNet sites, file
/pub/aminet/util/dtype/jfif_dtc.lha. (The version dated 12/12/94
has a bug; you should also get /pub/aminet/util/dtype/jfif_FIX.lha.)

The free IJG JPEG software is available compiled for Amigas from AmiNet
sites, file /pub/aminet/gfx/conv/jpegV6bin.lha. (Despite the name, this
is now version 6a.) These programs convert JPEG to/from PPM, GIF, BMP,
Targa formats.

If you have a DCTV box or a compatible display, try JPEGonDCTV.
from AmiNet sites, file /pub/aminet/gfx/show/JPEGonDCTV100.lha.   Viewtek
also reported to work well with DCTV.


Subject: [10] Atari ST

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

MGIF is a good free viewer/editor for JPEG and many other image formats.
It's particularly good on monochrome monitors, where it manages to
four-level gray-scale effect by flickering; but it works on all Ataris.
Version 5.00 is at atari.archive.umich.edu:/atari/Graphics/mgif500.lzh.

1stGuide is a small, fast viewer for all ST/TT/Falcon systems; it
JPEG, PNG, and other file formats. Shareware, $35. Available from

The free IJG JPEG software is available compiled for Atari ST/TT/etc
from micros.hensa.ac.uk:/micros/atari/tos/p/p108/jpeg5abn.zoo.
These programs convert JPEG to/from PPM, GIF, BMP, Targa formats.


Subject: [11] Acorn Archimedes

The Acorn archive at micros.hensa.ac.uk contains several JPEG-capable
programs. Read the file micros.hensa.ac.uk:/micros/arch/riscos/index
for retrieval instructions. Recommended archive entries include:

a022 Translator 7.18: image file format converter (shareware)
b008 FYEO 2.02: For Your Eyes Only, fast JPEG/GIF image viewer
a110 JPEG 6.a: IJG v6a software (JPEG<=>PPM,GIF,Targa) w/ desktop front
a121 ChangeFSI 1.15: image format conversion and viewing
e018 SwiftJPEG 0.09: fast JPEG viewer, requires SpriteExtend 0.99

SpriteExtend 0.99 comes with ROS 3.6, and is available for ROS 3.5 from
ftp.acorn.co.uk:/pub/riscos/releases/spriteextend.arc. It provides very
fast JPEG decoding, but sacrifices image quality on 256-color displays.


Subject: [12] NeXT

OmniImageFilter is a filter package that converts NeXTStep TIFF to and
about 30 image formats. It reads JPEG but does not write it. It works
most NeXTStep programs that handle drag-and-drop. OmniImage is a simple
image viewer that uses the filter package. Both are free. Available
ftp.omnigroup.com:/pub/software/OmniImageFilter-3.0.pkg.tar and

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
cs.orst.edu:/pub/next/3.0/bin/ImageViewer0.9i.tar.Z. Note that there
is an older version floating around that does not support JPEG.

The "imagetools" archive at ftp.thoughtport.com:/pub/next/graphics/
NeXTStep compiled binaries for a wide array of free image manipulation
including the IJG JPEG tools.

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


Subject: [13] Other systems

If you don't see what you want for your machine, check out the free IJG
source code described in the next section. Assuming you have a C
and at least a little knowledge of compiling C programs, you should be
to prepare JPEG conversion programs from the source code. You'll also
a viewer program. If your display is 8 bits or less, any GIF viewer will
fine; if you have a display with more color capability, try to find a
that can read Targa, BMP, or PPM 24-bit image files.


Subject: [14] Freely available source code for JPEG

Free, portable C code for JPEG compression is available from the
JPEG Group. Source code, documentation, and test files are included.
Version 6a is available from
If you are on a PC you may prefer ZIP archive format, which you can find
ftp.simtel.net:/pub/simtelnet/msdos/graphics/jpegsr6a.zip (or at any
Simtel mirror site). On CompuServe, see the Graphics Support forum
(GO CIS:GRAPHSUP), library 12 "JPEG Tools", file jpegsr6a.zip.

The IJG code includes a reusable JPEG compression/decompression library,
plus sample applications "cjpeg" and "djpeg", which perform conversion
between JPEG JFIF format and image files in PPM/PGM (PBMPLUS), GIF, BMP,
Utah RLE, and Targa formats. A third application "jpegtran" provides
lossless transcoding between different JPEG formats --- for example, it
convert a baseline JPEG file to an equivalent progressive JPEG file. Two
small applications "wrjpgcom" and "rdjpgcom" insert and extract textual
comments in JFIF files. The package is highly portable; it has been used
successfully on many machines ranging from Apple IIs to Crays.

The IJG code is free for both noncommercial and commercial use; only an
acknowledgement in your documentation is required to use it in a product.
(See the README file in the distribution for details.)

A different free JPEG implementation, written by the PVRG group at
is available from havefun.stanford.edu:/pub/jpeg/JPEGv1.2.1.tar.Z. The
code is designed for research and experimentation rather than production
use; it is slower, harder to use, and less portable than the IJG code,
the PVRG code is easier to understand. Also, the PVRG code supports
lossless JPEG, while the IJG code does not. (But PVRG does not support
progressive JPEG.)

There's also a lossless-JPEG-only implementation available from Cornell,

Neither the PVRG nor Cornell codecs are being actively maintained, but
IJG code is.


Subject: [15] Which programs support progressive JPEG?
With luck, this will only be a Frequently Asked Question for a short
after which most JPEG-supporting programs will have been upgraded to
p-JPEG capability. But right now it's a hot topic. Here's the latest
I've heard (if you have newer info, please send mail):

WWW Browsers:

Netscape 2.0b1 (Unix/X, Windows, Mac): full implementation

Spyglass Enhanced Mosaic 2.1 (Unix/X, Windows, Mac): full implementation
(Note: lots of other people license Spyglass' code, but I don't know
which licensees are shipping the latest version.)

Netshark 1.1 (Windows, Mac): full implementation

Microsoft Internet Explorer 2.0 (Windows, Mac): full implementation

Java 1.0 (Windows 95/NT, Solaris, more coming): full implementation

OmniWeb 2.0 (NeXTStep): full implementation

Wollongong's Emissary 1.1 (Windows): full implementation (? not sure)

I-Comm 1.09beta (Windows): full implementation (?)

UdiWWW 1.0.010 (Windows): full implementation

NCSA Mac Mosaic 3.0a2 (Mac): full implementation

NCSA Windows Mosaic 2.1.1 (Windows): reads p-JPEG, no incremental display

NCSA X Mosaic 2.7b2 (Unix/X): reads p-JPEG, no incremental display

Arena beta-1e (Unix/X): reads p-JPEG, no incremental display

Fresco 0.72 (Acorn): reads p-JPEG, no incremental display

(A browser that doesn't do incremental display of images won't be able to
give you the progressive effect, but it's still useful to have p-JPEG
compatibility so that you can at least see the image.)

See "BrowserWatch" at http://www.browserwatch.com/ for contact
for these browsers. Versions mentioned are the first to support p-JPEG,
not necessarily the current release.

Image Viewers & Converters:

See the appropriate prior sections for exact pointers to these programs.
Note that image viewers generally won't bother with doing incremental
display of p-JPEG files; they'll just read them in one pass for speed.

IJG command-line programs (almost any platform): see section 14 for
code. Precompiled executables are also available for some platforms;
see subject heading for your system. You need v6 or later.

XV (Unix/X): recompile v3.10 with IJG v6 to read p-JPEG

ImageMagick (Unix/X): 3.6.6 or later

JPEGPROC (OS/2): 1.1.0 or later

LView Pro (Windows 95/NT, or Win 3.1 + Win32s): 1.C or later

PolyView (Windows 95): 2.18 or later

ThumbsPlus (Windows 95/NT, or Win 3.1 + Win32s): 3.0c or later

PMView (OS/2): 0.92 or later

SEA (DOS): 1.2b or later

Jade (Mac): all versions

GraphicConverter (Mac): 2.3.1 or later

ProJPEG Photoshop plugin (Mac): all versions

JPEG Transmogrifier (Mac): all versions

DeBabelizer (Mac): 1.6.5 or later

1stGuide (Atari): 10.Jan.96 or later


Subject: [16] Where are FAQ lists archived?

Many FAQs are crossposted to news.answers. Well-run netnews sites will
the latest versions available in that newsgroup. However, there are a
of postings in news.answers, and they can be hard to sort through.

The latest versions of news.answers postings are archived at
You can retrieve this FAQ by FTP as /pub/usenet/news.answers/jpeg-
and /pub/usenet/news.answers/jpeg-faq/part2. With a Web browser, use the
URL ftp://rtfm.mit.edu/pub/usenet/news.answers/jpeg-faq/. If you have no
FTP access, send e-mail to mail-server@rtfm.mit.edu containing the lines
      send usenet/news.answers/jpeg-faq/part1
      send usenet/news.answers/jpeg-faq/part2
(If you don't get a reply, the server may be misreading your return
add a line such as "path myname@mysite" to specify your correct e-mail
address to reply to.) For more info about the FAQ archive, retrieve the
file rtfm.mit.edu:/pub/usenet/news.answers/news-answers/introduction.

The same FAQs are also available in the World Wide Web, although you
be wary of out-of-date copies at some sites. Well-run WWW FAQ archive
include http://www.lib.ox.ac.uk/internet/news/ and

                tom lane
                organizer, Independent JPEG Group
                tgl@netcom.com or tgl@sss.pgh.pa.us

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