OpenGL Programming Guide

Chapter 5Color

RGBA versus Color-Index Mode

Color-Index Display Mode

With color-index mode, OpenGL uses a color map (or lookup table), which is similar to using a palette to mix paints to prepare for a paint-by-number scene. A painter's palette provides spaces to mix paints together; similarly, a computer's color map provides indices where the primary red, green, and blue values can be mixed, as shown in Figure 5-4 .

Figure 5-4 : A Color Map

A painter filling in a paint-by-number scene chooses a color from the color palette and fills the corresponding numbered regions with that color. A computer stores the color index in the bitplanes for each pixel. Then those bitplane values reference the color map, and the screen is painted with the corresponding red, green, and blue values from the color map, as shown in Figure 5-5 .

Figure 5-5 : Using a Color Map to Paint a Picture

In color-index mode, the number of simultaneously available colors is limited by the size of the color map and the number of bitplanes available. The size of the color map is determined by the amount of hardware dedicated to it. Typical sizes range from 256 (28) to 4096 (212). The size of the color map is a power of 2, indexed by the number of bitplanes available in color-index mode. If there are 2n indices in the color map and m available bitplanes, the number of usable entries is the smaller of 2n and 2m.

With RGBA mode, each pixel's color is independent of other pixels. However, in color-index mode, each pixel with the same index stored in its bitplanes shares the same color-map location. If the contents of an entry in the color map change, then all pixels of that color index change their color.

OpenGL Programming Guide