Web Design in a Nutshell, 3rd Edition

Optimizing Methods

In addition to the standard image-compressing methods, GIF animation tools optimize 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 equally efficient. These methods, in order from least to most compression, include:

Bounding box (also called “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.

Redundant pixel removal (or frame differencing)

In frame differencing, only the individual pixels that change are stored for each frame. This is a more efficient method than minimum bounding rectangle, which includes a lot of unnecessary pixel information to make up the rectangle.

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, SuperGIF 1.0, and WebPainter 3 all take advantage of LZW compression for animations.