OpenGL Programming Guide

Chapter 6Lighting

Lighting in Color-Index Mode

The Mathematics of Color-Index Mode Lighting

Advanced

As you might expect, since the allowable parameters are different for color-index mode than for RGBA mode, the calculations are different as well. Since there's no material emission and no ambient light, the only terms of interest from the RGBA equations are the diffuse and specular contributions from the light sources and the shininess. Even these need to be modified, however, as explained below.

Begin with the diffuse and specular terms from the RGBA equations. In the diffuse term, instead of diffuselight * diffusematerial , substitute dci as defined in the previous section for color-index mode. Similarly, in the specular term, instead of specularlight * specularmaterial , use sci as defined in the previous section. (Calculate the attenuation, spotlight effect, and all other components of these terms as before.) Call these modified diffuse and specular terms d and s, respectively. Now let s' = min{ s, 1 }, and then compute

c = am + d(1-s')(dm-am) + s'(sm-am)

where am, dm, and sm are the ambient, diffuse, and specular material indexes specified using GL_COLOR_INDEXES. The final color index is

c' = min { c, sm }

After lighting calculations are performed, the color-index values are converted to fixed-point (with an unspecified number of bits to the right of the binary point). Then the integer portion is masked (bitwise ANDed) with 2n-1, where n is the number of bits in a color in the color-index buffer.

OpenGL Programming Guide