GIF (Graphics
Interchange Format)
Introduction
The GIF format is one of the most popular image formats
used on the Internet. GIF images are normally used for logos,
shapes and icons. The format is based on the lossless
LZW compression algorithm and is very suitable for
compressing large image areas of the same color. Another
important feature of GIF is that it supports animation.
However, since GIF supports a maximum of 256 colors, it
is not suitable for graphics which have long stretches of continuous-tone in
them (for example, photographs). The number of colors
can be reduced in order to reduce the file size.
Gradients and anti-aliasing should not be used within
GIF images as the GIF format does not implement these
features efficiently.
Transparency and Interlacing
GIF supports transparency and interlacing. Transparency
is supported by specifying which pixels of the image is
unused and as such, to reuse the background color.
Interlacing creates the illusion of faster loading
graphics as the image is presented to the viewer in a
series of interlaced frames. This is especially useful
in the Internet where the image becomes more and more defined until the entire image has been downloaded.
As interlaced images need to store additional
information, it's file size is usually a little larger.
Licensing Controversy
In 1987 GIF was release by Compuserve as a free specification. GIF soon became a world standard,
especially among the Internet community. GIF was relatively simple and very well documented.
The technique used to compress GIF images is called LZW and was first described in the June 1984 issue of IEEE's Computer magazine by Terry A. Welch. The compression algorithm was patented by Unisys but the article had no mention of this. It appears that neither the GIF
designers, nor the computer world in general were aware of the patent. Besides GIF, many other specifications (including TIFF) also includes the LZW
compression algorithm.
CompuServe Inc and Unisys Corporation announced at the end of December 1994 that developers would have to pay a license fee in order to continue to use
the LZW (Lempel-Ziv-Welch) compression technology patented by Unisys in certain categories of software supporting the GIF format.
GIF files are not covered by the patents. Only the LZW
compression algorithm is patented.
Although Unisys had the legal right to seek damages, Unisys has so far been very accommodating and fair. It is widely believed that Unisys was unprepared by the phenomenal success of LZW, especially among small developers.
The patents expire at midnight US Eastern Standard Time on June 19, 2003 for the US patent, and midnight local time on June 18 and 19, 2004 for the European and Japanese
counterparts respectively.
See
http://www.unisys.com/about__unisys/lzw
for more information.
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