This article answers Frequently Asked Questions about JPEG
image compression.
Covering general questions and answers about JPEG and gives
system-specific hints and program recommendations. As always, suggestions
for improvement of this FAQ are welcome.
This article includes the following sections:
Part 1
Basic questions:
What is JPEG?
Why use JPEG?
When
should I use JPEG, and when should I stick with GIF?
How well does JPEG compress
images?
What are
good "quality" settings for JPEG?
Where can I get JPEG software?
How do I view JPEG
images posted on Usenet?
More advanced questions:
What is color quantization?
What
are some rules of thumb for converting GIF images to JPEG?
Does
loss accumulate with repeated compression/decompression?
What is progressive JPEG?
Can I make a transparent JPEG?
Isn't there a lossless JPEG?
Why all the argument
about file formats?
How
do I recognize which file format I have, and what do I do about it?
What other
common compatibility problems are there?
How does JPEG work?
What about arithmetic coding?
Could an FPU
speed up JPEG? How about a DSP chip?
Isn't there
an M-JPEG standard for motion pictures?
What if I need more
than 8-bit precision?
How
can my program extract image dimensions from a JPEG file?
Miscellaneous:
Where
can I learn about using images on the World Wide Web?
Where are FAQ lists archived?
Part 2
What is covered in this FAQ?
How do I retrieve these
programs?
Programs and hints for specific systems:
X Windows
Unix (without X)
MS-DOS
Microsoft Windows
OS/2
Macintosh
Amiga
Atari ST
Acorn Archimedes
NeXT
Tcl/Tk
Other systems
Source code for JPEG:
Freely available source
code for JPEG
Miscellaneous:
Which programs
support progressive JPEG?
Where are FAQ lists archived?
Part 1
What is JPEG?
JPEG (pronounced "jay-peg") is a standardized image compression
mechanism. JPEG stands for Joint Photographic Experts Group, the
original name of the committee that wrote the standard.
JPEG is designed for compressing either full-color or gray-scale images
of natural, real-world scenes. It works well on photographs, naturalistic
artwork, and similar material; not so well on lettering, simple cartoons, or
line drawings. JPEG handles only still images, but there is a related
standard called MPEG for motion pictures.
JPEG is "lossy," meaning that the decompressed image isn't
quite the same as the one you started with. (There are lossless image
compression algorithms, but JPEG achieves much greater compression than is
possible with lossless methods.) JPEG is designed to exploit known
limitations of the human eye, notably the fact that small color changes are
perceived less accurately than small changes in brightness. Thus, JPEG is
intended for compressing images that will be looked at by humans. If you
plan to machine-analyze your images, the small errors introduced by JPEG may
be a problem for you, even if they are invisible to the eye.
A useful property of JPEG is that the degree of lossiness can be varied
by adjusting compression parameters. This means that the image maker can
trade off file size against output image quality. You can make *extremely*
small files if you don't mind poor quality; this is useful for applications
such as indexing image archives. Conversely, if you aren't happy with the
output quality at the default compression setting, you can jack up the
quality until you are satisfied, and accept lesser compression.
Another important aspect of JPEG is that decoders can trade off decoding
speed against image quality, by using fast but inaccurate approximations to
the required calculations. Some viewers obtain remarkable speedups in this
way. (Encoders can also trade accuracy for speed, but there's usually less
reason to make such a sacrifice when writing a file.)
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 win for transmitting files across
networks 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
compression of full-color data. If you are comparing GIF and JPEG, the size
ratio is usually more like 4:1 (see "How
well does JPEG compress images?").
Now, it takes longer to decode and view a JPEG image than to view an
image of a simpler format such as GIF. Thus using JPEG is essentially a
time/space tradeoff: you give up some time in order to store or transmit an
image more cheaply. But it's worth noting that when network transmission is
involved, the time savings from transferring a 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 format
widely used on the net, can only store 8 bits/pixel (256 or fewer colors).
GIF is reasonably well matched to inexpensive computer displays --- most
run-of-the-mill PCs can't display more than 256 distinct colors at once. But
full-color hardware is getting cheaper all the time, and JPEG photos 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 does today.
Furthermore, JPEG is far more useful than GIF for exchanging images among
people with widely varying display hardware, because it avoids prejudging
how many colors to use (see "What
is color quantization?"). Hence JPEG is considerably more
appropriate than GIF for use as a Usenet and World Wide Web standard photo
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 more quality each time
(see "Does
loss accumulate with repeated compression/decompression?"). This is
a serious objection for some applications but matters not at all for many
others.
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, continuous-tone artwork, and similar material. Any smooth
variation in color, such as occurs in highlighted or shaded areas, will be
represented more faithfully and in less space by JPEG than by GIF.
GIF does significantly better on images with only a few distinct colors,
such as line drawings and simple cartoons. Not only is GIF lossless for such
images, but it often compresses them more than JPEG can. For example, large
areas of pixels that are all *exactly* the same color are compressed very
efficiently indeed by GIF. JPEG can't squeeze such data as much as GIF does
without introducing visible defects. (One implication of this is that large
single-color borders are quite cheap in GIF files, while they are best
avoided in JPEG files.)
Computer-drawn images, 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 files:
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 than
trying to overlay it on the image. Most recent JPEG software can deal with
textual comments in a JPEG file, although older viewers may just ignore the
comments.)
Plain black-and-white (two level) images should never be converted to
JPEG; they violate all of the conditions given above. You need at least
about 16 gray levels before JPEG is useful for gray-scale images. It should
also be noted that GIF is lossless for gray-scale images of up to 256
levels, while JPEG is not.
If you have a large library of GIF images, you may want to save space by
converting the GIFs to JPEG. This is trickier than it may seem --- even when
the GIFs contain photographic images, they are actually very poor source
material for JPEG, because the images have been color-reduced.
Non-photographic images should generally be left in GIF form. Good-quality
photographic GIFs can often be converted with no visible quality loss, but
only if you know what you are doing and you take the time to work on each
image individually. Otherwise you're likely to lose a lot of image quality
or waste a lot of disk space ... quite possibly both. Read "What
is color quantization?" and "What
are some rules of thumb for converting GIF images to JPEG?"
if you want to convert GIFs to JPEG.
How
well does JPEG compress images?
Very well indeed, when working with its intended type of image
(photographs and suchlike). For full-color images, the uncompressed data is
normally 24 bits/pixel. The best known lossless compression methods can
compress such data about 2:1 on average. JPEG can typically achieve 10:1 to
20:1 compression without visible loss, bringing the effective storage
requirement down to 1 to 2 bits/pixel. 30:1 to 50:1 compression is possible
with small to moderate defects, while for very-low-quality purposes such as
previews or archive indexes, 100:1 compression is quite feasible. An image
compressed
100:1 with JPEG takes up the same space as a full-color one-tenth-scale
thumbnail image, yet it retains much more detail than such a thumbnail.
For comparison, a GIF version of the same image would start out by
sacrificing most of the color information to reduce the image to 256 colors
(8 bits/pixel). This provides 3:1 compression. GIF has additional "LZW"
compression built in, but LZW doesn't work very well on typical photographic
data; at most you may get 5:1 compression overall, and it's not at all
uncommon for LZW to be a net loss (i.e., less than 3:1 overall compression).
LZW *does* work well on simpler images such as line drawings, which is why
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
human eye is much more sensitive to brightness variations than to hue
variations, JPEG can compress hue data more heavily than brightness
(gray-scale) data. A gray-scale JPEG file is generally only about 10%-25%
smaller than a full-color JPEG file of similar visual quality. But the
uncompressed gray-scale data is only 8 bits/pixel, or one-third the size of
the color data, so the calculated compression ratio is much lower. The
threshold of visible loss is often around 5:1 compression for gray-scale
images.
The exact threshold at which errors become visible depends on your
viewing conditions. The smaller an individual pixel, the harder it is to see
an error; so errors are more visible on a computer screen (at 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.
What
are good "quality" settings for JPEG?
Most JPEG compressors let you pick a file size vs. image quality tradeoff
by selecting a quality setting. There seems to be widespread confusion about
the meaning of these settings. "Quality 95" does NOT mean
"keep 95% of the information", as some have claimed. The quality
scale is purely arbitrary; it's not a percentage of anything.
In fact, quality scales aren't even standardized across JPEG programs.
The quality settings discussed in this article apply to the free IJG JPEG
software (see "Freely
available source code for JPEG"), and to many programs based on it.
Some other JPEG implementations use completely different quality scales.
For example:
* Apple used to use a scale running from 0 to 4, not 0 to 100.
* Recent Apple software uses an 0-100 scale that has nothing to do with the
IJG scale (their Q 50 is about the same as Q 80 on the IJG scale).
* Paint Shop Pro's scale is the exact opposite of the IJG scale, PSP setting
N = IJG 100-N; thus lower numbers are higher quality in PSP.
* Adobe Photoshop doesn't use a numeric scale at all, it just gives you
"high"/"medium"/"low" choices. (But I hear
this is changing in 4.0.)
Fortunately, this confusion doesn't prevent different implementations from
exchanging JPEG files. But you do need to keep in mind that quality scales
vary considerably from one JPEG-creating program to another, and that just
saying "I saved this at Q 75" doesn't mean a thing if you don't
say which program you used.
In most cases the user's goal is to pick the lowest quality setting, or
smallest file size, that decompresses into an image indistinguishable from
the original. This setting will vary from one image to another and from one
observer to another, but here are some rules of thumb.
For good-quality, full-color source images, the default IJG quality
setting (Q 75) is very often the best choice. This setting is about the
lowest you can go without expecting to see defects in a typical image. Try Q
75 first; if you see defects, then go up.
If the image was less than perfect quality to begin with, you might be
able to drop down to Q 50 without objectionable degradation. On the other
hand, you might need to go to a *higher* quality setting to avoid further
loss. This is often necessary if the image contains dithering or moire
patterns (see "What
are some rules of thumb for converting GIF images to JPEG?").
Except for experimental purposes, never go above about Q 95; using Q 100
will produce a file two or three times as large as Q 95, but of hardly any
better quality. Q 100 is a mathematical limit rather than a useful setting.
If you see a file made with Q 100, it's a pretty sure sign that the maker
didn't know what he/she was doing.
If you want a very small file (say for preview or indexing purposes) and
are prepared to tolerate large defects, a Q setting in the range of 5 to 10
is about right. Q 2 or so may be amusing as "op art". (It's worth
mentioning that the current IJG software is not optimized for such low
quality factors. Future versions may achieve better image quality for the
same file size at low quality settings.)
If your image contains sharp colored edges, you may notice slight
fuzziness 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
downsampling 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 implementations may or may not provide user control of
downsampling. Adobe Photoshop, for 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 penalty.
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 display is
unlikely to be able to see any difference from a higher quality setting,
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 sequences that are
tuned for quality settings around 50-75: much below 50, the early scans will
look really bad, while much above 75, the later scans won't contribute
anything noticeable to the picture.
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 source code for implementing
JPEG, in case you want to write your own programs using JPEG.
The comp.graphics.* FAQs and the alt.binaries.pictures FAQ are more
general sources of information about graphics programs available on the
Internet (see "Where are FAQ
lists archived?").
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
parts, since some news software can't cope with big articles. Before your
viewer will recognize the image, you must combine the parts into one file
and run the text through a uudecode program. (This is all true for GIF as
well as JPEG, by the way.) There are programs available to automate this
process.
For more info see the alt.binaries.pictures FAQ, which is available from http://www.faqs.org/faqs/pictures-faq/
(see also "Where are FAQ lists
archived?").
What is color
quantization?
Many people don't have full-color (24 bit per pixel) display hardware.
Inexpensive display hardware stores 8 bits per pixel, so it can display at
most 256 distinct colors at a time. To display a full-color image, the
computer must choose an appropriate set of representative colors and map the
image into these colors. This process is called "color
quantization".
(This is something of a misnomer; "color selection" or "color
reduction" would be a better term. But we're stuck with the standard
usage.)
Clearly, color quantization is a lossy process. It turns out that for
most images, the details of the color quantization algorithm have *much*
more impact on the final image quality than do any errors introduced by JPEG
itself (except at the very lowest JPEG quality settings). Making a good
color quantization 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 by
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 displays.
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, and
the format doesn't allow more than 256 palette entries.) GIF has the
advantage that the image maker pre-computes 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 to a
different number of colors than what you can display, you'll either waste
display capability or else have to re-quantize to reduce the number of
colors (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 image
is ruined.
For this reason, JPEG promises significantly better image quality than
GIF for all users whose machines don't match the image maker's display
hardware. JPEG's full color image can be quantized to precisely match the
viewer's display hardware. Furthermore, you will be able to take advantage
of future improvements in quantization algorithms, or purchase better
display hardware, to get a better view of JPEG images you already have. With
a GIF, you're stuck forevermore with what was sent.
A closely related problem is seen in many current World Wide Web
browsers: when running on an 8-bit display, they force all images into a
pre-chosen palette. (They do this to avoid having to worry about how to
allocate the limited number of available color slots among the various items
on a Web page.) A GIF version of a photo usually degrades very badly in this
situation, because it's effectively being forced through a second
quantization step. A JPEG photo won't look wonderful either, but it will
look less bad than the GIF equivalent because it's been quantized only once.
A growing number of people have better-than-8-bit display hardware
already: 15- or 16-bit/pixel "high color" displays are now quite
common, and true 24-bit/pixel displays are no longer rare. For these people,
GIF is already obsolete, as it cannot represent an image to the full
capabilities of their display. JPEG images can drive these displays much
more effectively.
In short, JPEG is an all-around better choice than GIF for representing
photographic images in a machine-independent fashion. It's sometimes thought
that a JPEG converted from a GIF shouldn't require color quantization. That
is false; even when you feed a 256-or-less-color GIF into JPEG, what comes
out of the decompressor is not 256 colors, but thousands of colors. This
happens because JPEG's lossiness affects each pixel a little differently, so
two pixels that started with identical colors will usually come out with
slightly different colors. Considering the whole image, each original color
gets "smeared" into a cluster of nearby colors. Therefore
quantization is always required to display a color JPEG on a colormapped
display, regardless of the image source.
The same effect makes it nearly meaningless to talk about the number of
colors used by a JPEG image. Even if you tried to count the number of
distinct pixel values, different JPEG decoders would give you different
results because of roundoff error differences. I occasionally see posted
images described as "256-color JPEG". This tells me that the
poster (a) hasn't read this FAQ and (b) probably converted the JPEG from a
GIF. JPEGs can be classified as color or gray-scale, but number of colors
just isn't a useful concept for JPEG, any more than it is for a real
photograph.
What
are some rules of thumb for converting GIF images to JPEG?
Converting GIF files to JPEG is a tricky business --- you are piling one
set of limitations atop a quite different set, and the results can be awful.
Certainly a JPEG made from a GIF will never be as good as a JPEG made from
true 24-bit color data. But if what you've got is GIFs, and you need to save
space, here are some hints for getting the best results.
With care and a clean source image, it's often possible to make a JPEG of
quality equivalent to the GIF. This does not mean that the JPEG looks
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 "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 image, a converted JPEG can look as good as
its GIF source. Some people claim that on 24-bit displays, a carefully
converted JPEG can actually look better than the GIF source, because dither
patterns have been eliminated. (More about dithering in a moment.)
On the other hand, JPEG conversion absolutely *will* degrade an
unsuitable image or one that is converted carelessly. If you are not willing
to take the amount of trouble suggested below, you're much better off
leaving your GIF images alone. Simply cranking the JPEG quality setting up
to a very high value wastes space (which defeats the whole point of the
exercise, no?) and some images will be degraded anyway.
The first rule is never to convert an image that's not appropriate for
JPEG (see "When
should I use JPEG, and when should I stick with GIF?"). Large,
high-visual-quality photographic images are usually the best source
material. And they take up lots of space in GIF form, so they offer
significant potential space savings. (A good rule of thumb is not to bother
converting any GIF that's much under 100 Kbytes; the potential savings isn't
worth the hassle.)
The second rule is to know where the image came from. Repeated
GIF<=>JPEG conversions are guaranteed to turn an image into mush,
because you pay a steep quality 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 time.
Worse, the sharp border boundary can create visible artifacts (ghost edges).
Furthermore, when viewing a bordered JPEG on an 8-bit display, the quantizer
will think the border color is important because there's so much of it, and
hence will waste color palette entries on the border, thus actually reducing
the displayed quality of the main part of the image! So do yourself a favor
and crop off any border before JPEGing.
The final rule is to look at each JPEG, to make sure you are happy with
it, before throwing away the corresponding GIF. This will give you a chance
to re-do the conversion with a higher quality setting if necessary. Also
compare the file sizes --- if the image isn't suitable JPEG material, a JPEG
file of reasonable quality may come out *larger* than the GIF.
Gray-scale photos usually convert without much problem. When using cjpeg,
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 only
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
that adjacent pixels are often of significantly different colors; at normal
size the eye averages these pixels together to produce the illusion of an
intermediate color value. The trouble with dithering is that, to JPEG, it
looks like high-spatial-frequency color noise; and JPEG can't compress noise
very well. The resulting JPEG file is both larger and of lower image quality
than what you would have gotten from JPEGing the original full color image
(if you had it). To get around this, you need to "smooth" the GIF
image before compression. Smoothing averages together nearby pixels, thus
approximating the color that you thought you saw anyway, and in the process
getting rid of the rapid color changes that give JPEG trouble. Proper use of
smoothing will both reduce the size of the compressed file and give you a
better-looking output image than you'd get without smoothing.
With the IJG JPEG software (cjpeg or derived programs), a simple
smoothing capability is built in. Try "-smooth 10" or so when
converting GIFs. Values of 10 to 25 seem to work well for high-quality GIFs.
GIFs with heavy-handed dithering may require larger smoothing factors. (If
you can see regular fine-scale patterns on the GIF image even without
enlargement, then strong smoothing is definitely called for.) Too large a
smoothing factor will blur the output image, which you don't want. If you
are an image processing wizard, you can also do smoothing with a separate
filtering program, but appropriate use of such tools is beyond the scope of
this FAQ.
Quality settings around 85 (a bit higher than default) usually work well
when converting color GIFs, assuming that you've picked a good smoothing
factor. You may need still higher quality settings if you can't hide the
dithering pattern with a reasonable smoothing factor. Really badly dithered
GIFs are best left as GIFs.
Don't expect JPEG files converted from GIFs to be as small as those
created directly from full-color originals. The dithering noise wastes
space, but you won't be able to smooth away all the noise without blurring
the image. Typically, a good-quality converted JPEG will be one-half to
one-third the size of the GIF file, not one-fourth as suggested in section
4. If the JPEG comes out much more than half the size of the GIF, this is a
good sign that the image shouldn't be converted at all.
The upshot of all this is that "cjpeg -quality 85 -smooth 10"
is probably a good starting point for converting color GIFs. But if you care
about the image, you'll want to check the results and maybe try a few other
settings. Blindly converting a large GIF library at this or any other
setting is a recipe for disaster.
Does
loss accumulate with repeated compression/decompression?
It would be nice if, having compressed an image with JPEG, you could
decompress it, manipulate it (crop off a border, say), and recompress it
without any further image degradation beyond what you lost initially.
Unfortunately THIS IS NOT THE CASE. In general, recompressing an altered
image loses more information. Hence it's important to minimize the number of
generations of JPEG compression between initial and final versions of an
image.
It turns out that if you decompress and recompress an image at the same
quality setting first used, little or no further degradation occurs. This
means that you can make local modifications to a JPEG image without material
degradation of other areas of the image. (The areas you change will still
degrade, however.) Counter intuitively, this works better the lower the
quality setting. But you must use *exactly* the same setting, or all bets
are off. Also, the decompressed image must be saved in a full-color format;
if you do JPEG=>GIF=>JPEG, the color quantization step loses lots of
information.
Unfortunately, cropping doesn't count as a local change! JPEG processes
the image in small blocks, and cropping usually moves the block boundaries,
so that the image looks completely different to JPEG. You can take advantage
of the low-degradation behavior if you are careful to crop the top and left
margins only by a multiple of the block size (typically 16 pixels), so that
the remaining blocks start in the same places.
The bottom line is that JPEG is a useful format for compact 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 when
you are ready to file it away or send it out on the net. If you expect to
edit your image again in the future, keep a lossless master copy to work
from. The JPEG you put up on your Web site should be a derived copy, not
your editing master.
What is
progressive JPEG?
A simple or "baseline" JPEG file is stored as one top-to-bottom
scan of the image. Progressive JPEG divides the file into a series of scans.
The first scan shows the image at the equivalent of a very low quality
setting, and therefore it takes very little space. Following scans gradually
improve the quality. Each scan adds to the data already provided, so that
the total storage requirement is roughly 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 whole
image very quickly, with gradual improvement of quality as one waits longer;
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 to
display as a whole baseline JPEG file would. So progressive JPEG only makes
sense if one has a decoder that's fast compared to the communication link.
(If the data arrives quickly, a progressive-JPEG decoder can adapt by
skipping some display passes. Hence, those of you fortunate enough to have
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
JPEG 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 JPEG
has become a win for WWW use. IJG's free JPEG software (see "Freely
available source code for JPEG") 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
and baseline JPEG are basically identical, and they work well on the same
kinds 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 recompressing
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 "Which
programs support progressive JPEG?") for the latest news about
which programs support it.
Can I make
a transparent JPEG?
No. JPEG does not support transparency and is not likely to do so any
time soon. It turns out that adding transparency to JPEG would not be a
simple 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 pixel
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 image
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 work
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.
Isn't there
a lossless JPEG?
There's a great deal of confusion on this subject, which is not
surprising because there are several different compression methods all known
as "JPEG". The commonly used method is "baseline JPEG"
(or its variant "progressive JPEG"). The same ISO standard also
defines a very different method called "lossless JPEG". And if
that's not confusing enough, a new lossless standard called
"JPEG-LS" is about to hit the streets.
When I say "lossless", I mean mathematically lossless: a
lossless compression algorithm is one that guarantees its decompressed
output is bit-for-bit identical to the original input. This is a much
stronger claim than "visually indistinguishable from the
original". Baseline JPEG can reach visual indistinguishability for most
photo-like images, but it can never be truly lossless.
Lossless JPEG is a completely different method that really is lossless.
However, it doesn't compress nearly as well as baseline JPEG; it typically
can compress full-color data by around 2:1. And lossless JPEG works well
only on continuous-tone images. It does not provide useful compression of
palette-color images or low-bit-depth images.
Lossless JPEG has never been popular --- in fact, no common applications
support it --- and it is now largely obsolete. (For example, the new PNG
standard outcompresses lossless JPEG on most images.) Recognizing this, the
ISO JPEG committee recently finished an all-new lossless compression
standard called JPEG-LS (you may have also heard of it under the name LOCO).
JPEG-LS gives better compression than original lossless JPEG, but still
nowhere near what you can get with a lossy method. It's anybody's guess
whether this new standard will achieve any popularity.
It's worth repeating that cranking a regular JPEG implementation up to
its maximum quality setting *does not* get you lossless storage; even at the
highest possible quality setting, baseline JPEG is lossy because it is
subject to roundoff errors in various calculations. Roundoff errors alone
are nearly always too small to be seen, but they will accumulate if you put
the image through multiple cycles of compression (see "Does
loss accumulate with repeated compression/decompression?").
Many implementations won't even let you get to the maximum possible
setting, because it's such an inefficient way to use regular JPEG. With the
IJG JPEG software, for example, you have to not only select "quality
100" but also turn off chroma downsampling to minimize loss of
information. The resulting files are far larger and of only fractionally
better quality than 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.
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 anyone else's.
The closest thing we have to a standard JPEG format is some work that's
been coordinated by people at C-Cube Microsystems.
They have defined two JPEG-based file formats:
* JFIF (JPEG File Interchange Format), a "low-end" format that
transports pixels and not much else.
* TIFF/JPEG, aka TIFF 6.0, an extension of the Aldus TIFF format. TIFF is a
"high-end" format that will let you record just about everything
you ever wanted to know about an image, and a lot more besides :-).
JFIF has emerged as the de-facto standard on Internet, and is what is
most 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,
partly because it has some serious design flaws. A revised TIFF/JPEG design
is now 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
for 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 used
as JFIF. TIFF is far more complex than JFIF, and is generally less
transportable because different vendors often implement slightly different,
nonoverlapping subsets of TIFF. Adding JPEG to the mix hasn't helped any.
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 available
to do it (see part 2, item 8). If you have an editor that handles binary
files, you can even strip a PICT/JPEG file down to JFIF by hand; see the
next section for details.
News flash: the ISO JPEG committee seems to have won their turf wars.
They 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 seen. SPIFF is upward compatible with JFIF, so if it does get
widely adopted, most users probably won't even notice.
How
do I recognize which file format I have, and what do I do about it?
If you have an alleged JPEG file that your software won't read, it's
likely to be some proprietary JPEG-based format. You can tell what you have
by inspecting the first few bytes of the file:
1. A JFIF-standard file will start with the four bytes (hex) FF D8 FF E0,
followed by two variable bytes (often hex 00 10), followed by 'JFIF'.
2. If you see FF D8 FF at the start, but not the 'JFIF' marker, you
probably have a not-quite-JFIF JPEG file. Most JFIF software should read it
without complaint. If you are using something that is picky enough to
complain about the lack of a JFIF marker, try another decoder. (Both very
old JPEG files and very new ones may lack JFIF markers --- the new SPIFF
standard mentioned above doesn't use a JFIF marker. So gripe to your
software vendor if you find this to be the problem.)
3. A Macintosh PICT file, if JPEG-compressed, will have several hundred
bytes of header (often 726 bytes, but not always) followed by JPEG data.
Look for the 3-byte sequence (hex) FF D8 FF. The text 'Photo - JPEG' will
usually appear shortly before this header, and '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 http://www.accessone.com/~bbrown/photo-jpeg/photo-jpeg.html.)
4. 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.
5. Anything else: it's a proprietary format, or not JPEG at all. If you
are lucky, the file may consist of a header and a raw JPEG data stream. If
you can identify the start of the JPEG data stream (look for FF D8), try
stripping off everything before that.
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. Unfortunately, most
JFIF readers will give up on encountering these files, because the JFIF 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 wonders 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.
What
other common compatibility problems are there?
Aside from the file format difficulties mentioned in the previous
section, there are a few other common causes of trouble with transferring
JPEGs.
Old decoders that don't handle progressive JPEG will often give rather
cryptic error messages when fed a progressive JPEG. If you get a complaint
like "Unsupported marker type 0xC2", then you definitely have a
progressive JPEG file and a non-progressive-capable decoder. (See part 2 of
this FAQ for information about more up-to-date programs.) Or you may get a
generic error message that claims the file is corrupted or isn't JPEG at
all.
Adobe Photoshop and some other prepress-oriented applications will
produce four-channel CMYK JPEG files when asked to save a JPEG from CMYK
image mode. Hardly anything that's not prepress-savvy will cope with CMYK
JPEGs (or any other CMYK format for that matter). When making JPEGs for Web
use, be sure to save from RGB or grayscale mode.
Photoshop also has a habit of stuffing a rather large thumbnail/preview
image into an application-private segment of JPEG files. Some other
applications (notably early releases of Sun's Java library) are known to
choke on this data. This is definitely a bug in those other applications,
but the best available workaround is still to tell Photoshop not to save a
thumbnail. If you're putting up an image on the Web, having a thumbnail
embedded in it is just a waste of download time anyway.
When transferring images between machines running different operating
systems, be very careful to get a straight "binary" transfer ---
any sort of text format conversion will corrupt a JPEG file. Actually that's
true for all image formats not just JPEG.
How does JPEG work?
Technical details are outside the scope of this FAQ, but you can find an
introduction and references for further reading in the comp.compression FAQ,
which is available from http://www.faqs.org/faqs/compression-faq/
(see also "[24] 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 lossy 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 seem to be
losing favor as people learn more about their true capabilities and
limitations.
What about
arithmetic coding?
The JPEG spec defines two different "back end" modules for the
final output of compressed data: either Huffman coding or arithmetic coding
is allowed. The choice has no impact on image quality, but arithmetic coding
usually produces a smaller compressed file. On typical images, arithmetic
coding produces a file 5 to 10 percent smaller than Huffman coding. (All the
file-size numbers previously cited are for Huffman coding.)
Unfortunately, the particular variant of arithmetic coding specified by
the JPEG standard is subject to patents owned by IBM, AT&T, and
Mitsubishi. Thus *you cannot legally use JPEG arithmetic coding* unless you
obtain licenses from these companies. (Patent law's "experimental
use" exception 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 particular,
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 do.
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 hardware.
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 slower
than integer operations that the overall speed is still much worse with FP.
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
repetitive 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.
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 video. *There is no such standard*. Various vendors
have applied JPEG to individual frames of a video sequence, and have called
the result "M-JPEG". Unfortunately, in the absence of any
recognized standard, they've each done 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 a frame-by-frame basis (since each frame is intimately tied to
the ones around it). This latter problem has made "M-JPEG" methods
rather popular for video 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
about still-image JPEG file formats, so I wouldn't assume that anything good
will happen this time either...
See the MPEG FAQ for more information about MPEG.
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 bits/pixel
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 supported,
however. One package that does support it is the free IJG source code (see
part 2, item 15).
For lossless JPEG, the standard permits any data precision between 2 and
16 bits per sample, but high-precision lossless JPEG is even less widely
supported than high-precision lossy JPEG. The Stanford PVRG codec (see "Freely
available source code for JPEG") reportedly supports up to 16
bits/sample for lossless JPEG.
How
can my program extract image dimensions from a JPEG file?
The header of a JPEG file consists of a series of blocks, called
"markers". The image height and width are stored in a marker of
type SOFn (Start Of Frame, type N). To find the SOFn you must skip over the
preceding markers; you don't have to know what's in the other types of
markers, just use their length words to skip over them. The minimum logic
needed is perhaps a page of C code. (Some people have recommended just
searching for the byte pair representing SOFn, without paying attention to
the marker block structure. This is unsafe because a prior marker might
contain the SOFn pattern, either by chance or because it contains a
JPEG-compressed thumbnail image. If you don't follow the marker structure
you will retrieve the thumbnail's size instead of the main image size.) A
profusely commented example in C can be found in rdjpgcom.c in the IJG
distribution (see "Freely
available source code for JPEG"). Perl code can be found in wwwis,
from http://www.tardis.ed.ac.uk/~ark/wwwis/.
Where
can I learn about using images on the World Wide Web?
If you want to display still images on the World Wide Web, you have a
choice of using JPEG or GIF; those two formats are by far the most widely
supported by WWW browsers. (We can hope that PNG will soon become popular
enough to replace GIF on the Web; see http://www.cdrom.com/pub/png/
for PNG info.) For most images it's pretty obvious which format to choose
(see "When
should I use JPEG, and when should I stick with GIF?"). JPEG's
ability to trade off file size against image quality is especially helpful
for trimming download times of Web photos.
But there's a good many things to know that are specific to Web design,
and even specific to the currently-most-popular browsers. This FAQ doesn't
try to cover Web graphics design. Good basic information can be found at:
http://www.boutell.com/faq/
http://www.servtech.com/public/dougg/graphics/index.html
http://www.webreference.com/dev/graphics/
http://www.adobe.com/studio/tipstechniques/GIFJPGchart/main.html
http://ppewww.ph.gla.ac.uk/~flavell/www/palette.html
http://the-light.com/netcol.html
and here are some sites with more advanced info:
http://www.inforamp.net/~poynton/Poynton-colour.html
http://www.photo.net/philg/how-to-scan-photos.html
http://www.scantips.com/
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