Web Design in a Nutshell, 3rd Edition

Reduce the number of colors (bit depth)

Although GIF format can support 8-bit color information with a maximum of 256 colors, you don’t necessarily have to use all of them. In fact, you can reduce the size of a file considerably by saving it at a lower bit depth, which corresponds to fewer colors. Photoshop and Fireworks allow you to select the number of colors you’d like in the image. Other tools may ask you to choose from a list of bit depths. The effect is the same; it’s just useful to know how bit depth translates into numbers of colors for the latter (see Table 29-1 for translations).

Table 29-1. Color depth equivalents for bit depths

Bit depth	Number of colors
1-bit	2 (black and white)
2-bit	4
3-bit	8
4-bit	16
5-bit	32
6-bit	64
7-bit	128
8-bit	256

The goal is to find the minimum number of colors (smallest bit depth) that still maintains the integrity and overall character of the image. You may be surprised to find how many images survive a reduction to just 32 colors. Of course, the bit depth at which the image quality becomes unacceptable depends on the specific image and your personal preferences. I personally look at most images at 32 colors first, and add colors from there if I can’t live with the results.

Reducing the number of colors decreases the file size in two ways. First, lower bit depths include less data in the file. In addition, clusters of similarly colored pixels become the same color, creating more pockets of repeating pixels for LZW compression ...