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From: [e-mail] (Tom Lane)
Subject: JPEG image compression: Frequently Asked Questions
Message-ID: <[e-mail]>
Followup-To: alt.binaries.pictures.d
Summary: Useful info about JPEG (JPG) image files and programs
Keywords: JPEG, image compression, FAQ
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Organization: School of Computer Science, Carnegie Mellon
Date: Mon, 3 May 1993 03:15:23 GMT
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Archive-name: jpeg-faq
Last-modified: 2 May 1993

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

New since version of 18 April 1993:
  * New version of XV supports 24-bit viewing for X Windows.
  * New versions of DVPEG & Image Alchemy for DOS.
  * New versions of Image Archiver & PMView for OS/2.
  * New listing: MGIF for monochrome-display Ataris.


This article includes the following sections:

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

Sections 1-6 are basic info that every JPEG user needs to know;
sections 7-14 are advanced info for the curious.

This article is posted every 2 weeks.  You can always find the latest version
in the news.answers archive at rtfm.mit.edu (18.70.0.226).  By FTP, fetch
/pub/usenet/news.answers/jpeg-faq; or if you don't have FTP, send e-mail to
[e-mail] with body "send usenet/news.answers/jpeg-faq".
Many other FAQ articles are also stored in this archive.  For more
instructions on use of the archive, send e-mail to the same address with the
words "help" and "index" (no quotes) on separate lines.  If you don't get a
reply, the server may be misreading your return address; add a line such as
"path myname@mysite" to specify your correct e-mail address to reply to.


----------


[1]  What is JPEG?

JPEG (pronounced "jay-peg") is a standardized image compression mechanism.
JPEG stands for Joint Photographic Experts Group, the original name of the
committee that wrote the standard.  JPEG is designed for compressing either
full-color or gray-scale digital images of "natural", real-world scenes.
It does not work so well on non-realistic images, such as cartoons or line
drawings.

JPEG does not handle black-and-white (1-bit-per-pixel) images, nor does it
handle motion picture compression.  Standards for compressing those types
of images are being worked on by other committees, named JBIG and MPEG
respectively.

JPEG is "lossy", meaning that the image you get out of decompression isn't
quite identical to what you originally put in.  The algorithm achieves much
of its compression by exploiting known limitations of the human eye, notably
the fact that small color details aren't perceived as well as small details
of light-and-dark.  Thus, JPEG is intended for compressing images that will
be looked at by humans.  If you plan to machine-analyze your images, the
small errors introduced by JPEG may be a problem for you, even if they are
invisible to the eye.

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


[2]  Why use JPEG?

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

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

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

It's worth noting that when network or phone transmission is involved, the
time savings from transferring a shorter file can be much greater than the
extra time to decompress the file.  I'll let you do the arithmetic yourself.

The other reason why JPEG will gradually replace GIF as a standard Usenet
posting format is that JPEG can store full color information: 24 bits/pixel
(16 million colors) instead of 8 or less (256 or fewer colors).  If you have
only 8-bit display hardware then this may not seem like much of an advantage
to you.  Within a couple of years, though, 8-bit GIF will look as obsolete as
black-and-white MacPaint format does today.  Furthermore, for reasons detailed
in section 7, JPEG is far more useful than GIF for exchanging images among
people with widely varying color display hardware.  Hence JPEG is considerably
more appropriate than GIF for use as a Usenet posting standard.


[3]  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.

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

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

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

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

Plain black-and-white (two level) images should never be converted to JPEG.
You need at least about 16 gray levels before JPEG is useful for gray-scale
images.  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 an existing library of GIF images, you may wonder whether you
should convert them to JPEG.  You will lose a little image quality if you do.
(Section 7, which argues that JPEG image quality is superior to GIF, only
applies if both formats start from a full-color original.  If you start from
a GIF, you've already irretrievably lost a great deal of information; JPEG
can only make things worse.)  However, the disk space savings may justify
converting anyway.  This is a decision you'll have to make for yourself.
If you do convert a GIF library to JPEG, see section 14 for hints.  Be
prepared to leave some images in GIF format, since some GIFs will not
convert well.


[4]  How well does JPEG compress images?

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

File	   Size in bytes		Comments

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

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

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


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

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

The name of the game in using JPEG is to pick the lowest quality setting
(smallest file size) that decompresses into an image indistinguishable from
the original.  This setting will vary from one image to another and from one
observer to another, but here are some rules of thumb.

The default quality setting (-Q 75) is very often the best choice.  This
setting is about the lowest you can go without expecting to see defects in a
typical image.  Try -Q 75 first; if you see defects, then go up.  Except for
experimental purposes, never go above -Q 95; saying -Q 100 will produce a
file two or three times as large as -Q 95, but of hardly any better quality.

If the image was less than perfect quality to begin with, you might be able to
go down to -Q 50 without objectionable degradation.  On the other hand, you
might need to go to a HIGHER quality setting to avoid further degradation.
The second case seems to apply much of the time when converting GIFs to JPEG.
The default -Q 75 is about right for compressing 24-bit images, but -Q 85 to
95 is usually better for converting GIFs (see section 14 for more info).

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

(Note: the quality settings discussed in this article apply to the free JPEG
software described in section 6B, and to many programs based on it.  Other
JPEG implementations, such as Image Alchemy, may use a completely different
quality scale.  Some programs don't even provide a numeric scale, just
"high"/"medium"/"low"-style choices.)