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.
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 transparentand 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.