Web Design in a Nutshell, 3rd Edition

Disposal Methods

The disposal method gives instructions on what to do with the previous frame once a new frame is displayed.

Most GIF animation utilities offer “optimization,” a file size-reducing process that takes advantage of the fact that previous frames will “show through” transparent areas of a later frame. In order for this process to work, the disposal method must be set to Do Not Dispose (or Leave Alone, Leave As Is, and the like). With this method, areas of previous frames continue to display unless covered up by an area in a succeeding frame.

The most common disposal method choices are listed here, but not all of these are available in all animation tools.

Unspecified (Nothing)

Use this option to replace one full-size, nontransparent frame with another.

Do Not Dispose (Leave As Is)

In this option, any pixels not covered by the next frame continue to display. Use this when you want a frame to continue to show throughout the animation.

Restore to Background

The background color or background tile shows through the transparent pixels of the new frame (replacing the image areas of the previous frame).

Restore to Previous

This option restores to the state of the previous, undisposed frame. For example, if you have a static background that is set to Do Not Dispose, that image will reappear in the areas left by a replaced frame.

This disposal method is not correctly supported in Netscape Navigator (it is treated like Do Not Dispose), leading to all the frames being visible and ...