Web Design in a Nutshell, 3rd Edition

Limit dithering

Dithering is the random dot pattern that results when colors are approximated by mixing similar colors from a limited palette. Dithering is relevant to GIF file size because it interrupts the clean areas of flat color that are conducive to efficient LZW compression, and can make the file size larger than it needs to be.

Nearly all image editing tools allow you to turn dithering on and off. Current web graphics tools (Fireworks and Photoshop/ImageReady) go one step further by allowing the amount of dithering to be selected on a sliding scale from 0 to 100. You can preview the results of various settings, making it easy to select the best balance of file size and image quality.

Bear in mind, however, that dithering is usually beneficial to image quality in photographic or continuous tone areas, as it prevents “banding” of colors in the image. Dithering may even permit the image to be saved at a lower bit depth, which generally results in smaller file sizes, so it is not appropriate to simply set dithering to zero. The optimal setting will depend on the image.