JPEG (Joint
Photographic Experts Group)
JPEG is a standardised image compression mechanism.
JPEG is designed for compressing either full-colour (24 bit) or grey-scale
digital images of "natural" (real-world) scenes.
It works well on photographs, naturalistic artwork,
and similar material; not so well on lettering, simple cartoons, or
black-and-white line drawings (files come out very large). JPEG handles only
still images, but there is a related standard called MPEG for motion pictures.
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
limitation of the human eye, notably the fact that small colour 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.
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.
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.
GIF
(Graphics Interchange Format)
The Graphics Interchange Format was developed in
1987 at the request of CompuServe, who needed a platform independent image
format that was suitable for transfer across slow connections. It is a
compressed (lossless) format (it uses the LZW compression) and compresses at a
ratio of between 3:1 and 5:1
It is an 8 bit format which means the maximum
number of colours supported by the format is 256.
There are two GIF standards, 87a and 89a (developed
in 1987 and 1989 respectively). The 89a standard has additional features such
as improved interlacing, the ability to define one colour to be
transparent and the ability to store multiple images in one file to create
a basic form of animation.
Both Mosaic and Netscape will display 87a and 89a
GIFs, but while both support transparency and interlacing, only Netscape
supports animated GIFs.
PNG
(Portable Network Graphics format)
In January 1995 Unisys, the company CompuServe
contracted to create the GIF format, announced that they would be enforcing the
patent on the LZW compression technique the GIF format uses. This means that
commercial developers that include the GIF encoding or decoding algorithms have
to pay a license fee to CompuServe. This does not concern users of GIFs or
non-commercial developers.
However, a number of people banded together and
created a completely patent-free graphics format called PNG (pronounced
"ping"), the Portable Network Graphics format. PNG is superior to GIF
in that it has better compression and supports millions of colours. PNG files
end in a .png suffix.
PNG is supported in Netscape 4.03 .
When should we
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
Generally speaking, JPEG is superior to GIF for
storing full-colour or grey-scale images of "realistic" scenes; that
means scanned photographs and similar material. Any continuous variation in
colour, 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 colours, 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 colour
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-colour borders are quite cheap in GIF files, while they
are best avoided in JPEG files.)
Computer-drawn images (ray-traced scenes, for
instance) usually fall between photographs and cartoons in terms of complexity.
The more complex and subtly rendered the image, the more likely that JPEG will
do well on it. The same goes for semi-realistic artwork (fantasy drawings and
such).
JPEG has a hard time with very sharp edges: a row
of pure-black pixels adjacent to a row of pure-white pixels, for example. Sharp
edges tend to come out blurred unless you use a very high quality setting.
Edges this sharp are rare in scanned photographs, but are fairly common in GIF
files: borders, overlaid text, etc. The blurriness is particularly
objectionable with text that's only a few pixels high. If you have a GIF with a
lot of small-size overlaid text, don't JPEG it.
Plain black-and-white (two level) images should
never be converted to JPEG; they violate all of the conditions given above. You
need at least about 16 grey levels before JPEG is useful for grey-scale images.
It should also be noted that GIF is lossless for grey-scale images of up to 256
levels, while JPEG is not.
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