Web Design in a Nutshell, 3rd Edition

8-Bit Indexed Color

GIF files are indexed color images that can contain a maximum of 8-bit color information (they can also be saved at lower bit rates). This means they can contain up to 256 colors—the maximum number that 8 bits of information can define (2⁸ = 256). GIFs may be saved at lower bit depths as well. Lower bit depths result in fewer colors and also reduce file size. This is discussed in the section "Minimizing GIF File Sizes" later in this chapter.

Indexed color means that the set of colors in the image, its palette, is stored in a color table (also called a color map). Each pixel in the image contains a reference (or “index “) to a position in the color table. Figure 29-2 illustrates how a 2-bit indexed color image references its color table for display. In Adobe Photoshop, you can view the table for an indexed color image by selecting Image → Mode → Color Table. The color table is also displayed when you choose GIF in the Save for Web window in Photoshop and the Optimize panel in Adobe ImageReady and Macromedia Fireworks.

Figure 29-2. A 2-bit indexed color image and its color table

When you convert a 24-bit (millions of colors) image to GIF, the colors in the image must be reduced to a palette of 256 colors or fewer. In Photoshop and Fireworks, the conversion to indexed color happens as part of the Save to Web or Export function. Other image editing programs may require ...