Optimizing Animated GIFs 329Animated GIFsOptimizing Animated GIFsWeb Design in a Nutshell, eMatter Edition- Reducing the number of colors
- Reducing the bit depth
Optimizing Methods
In addition to the standard image-compressing methods, GIF animation tools opti-
mize animations by eliminating the repetition of pixels in unchanging image areas.
Only the pixels that change are recorded for each frame. Different tools use
different optimizing methods, which are not created equal in terms of efficiency.
These methods, in order from least to most compression, include:
Minimum Bounding Rectangle
In this method, the changed portion of the image is saved, but it is always
saved in the smallest rectangular area necessary to contain the changed pixels.
Frame Differencing
In frame differencing,onlythe individual pixels that change are stored for
each frame. This is a more efficient method than Minimum Bounding Rect-
angle, which includes a lot of unnecessary pixel information to make up the
rectangle. Of the tools listed above, only GIFmation, GMG, and Ulead GIF
Animator use the Frame Differencing method for optimization.
LZW Interframe Optimization
This optimization method uses the LZW compression scheme to minimize the
frequency of changes in pixel patterns between frames. This compression
method, when used in conjunction with frame difference, is capable of
producing the smallest possible file sizes. Macromedia Fireworks is currently
the only commercial product that takes advantage of LZW compression for
animations (the GIF Wizard online service also uses it).
GIF Wizard
GIF Wizard is an online service that evaluates and optimizes graphics on users’
web sites, using compression techniques created by Tom Kirchman. Notably, GIF
Wizard is the only current compression product that takes advantage of all three of
the above-mentioned optimization methods. The LZW compression support
enables GIF Wizard to compress animations smaller than any other utility currently
available.
GIF Wizard is located athttp://www.gifwizard.com/. You can upload your anima-
tion for a free demo of its powers. GIF Wizard instantly evaluates your graphic
and provides several alternative versions, each with a report of the percentage of
bytes saved.
GIF Wizard can also do an evaluation of your entire site. There are annual fees for
access to its full range of optimization and link-checking services.