Graphics for the web: GIF Format
The GIF (Graphics Interchange Format) format was invented in 1987 by
Compuserve to allow images to be displayed. This format allows for 256
colors (which was a lot at the time), compression, interlacing and
animation. It is a very powerful format, suitable for many different types
of images.
Due to the limited number of colors, GIF is primarily useful in images with
a distinct separation of colors. A cartoon, for example, is ideal for the
GIF format.
When you save an image in GIF format, you have the option to specify how
many colors will be saved. By doing this you can decrease the size of an
image even further. All of the tools which are available to optimize GIF
images work by reducing the number of colors to the bare minimum. This can
produce astounding results in the size of the finished file.
Unlike JPEG, GIF uses a non-lossy compression algorithm. This means that
images do not loose bits when they are decompressed. In order to accomplish
this, GIF uses a proprietary encoding/decoding scheme called LZW (Lempel Zev
Welch). LZW is an excellent compression algorithm which typically results in
very small files (in comparison to fully expanded BMP files).
This compression method is actually the cause of a bit of controversy. As it
turns out, LZW is owned by UniSys, and over the past few years they have
made some attempts to collect licensing fees for products which save in the
GIF format. These attempts have had mixed results, and has caused the
development of a new non-proprietary standard called PNG.
You may indicate that a color in a GIF image is transparent, meaning
whatever is below the image will show through.
The GIF89a standard included animation, which basically allows multiple
images to be included in a single GIF file. In that image you can specify
the timing that the frames are shown and whether or not the animation stops
or loops forever.
Interlacing is a cool feature which allows graphics to be "faded in".
Initially a very low quality image is displayed, and bits are slowly added
as the entire image is received. GIF supports this feature, which is useful
for displaying very large images quickly.
An interesting albeit little known fact about GIF images is that they can
include comments, which take up space and make the graphic larger. There are
a number of products which will remove these comments. I found that the
freeware program called GIFclean is simple to use and works well.
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