OpenGL Programming Guide

Chapter 6Lighting

Lighting in Color-Index Mode

In color-index mode, the parameters comprising RGBA values either have no effect or have a special interpretation. Since it's much harder to achieve certain effects in color-index mode, you should use RGBA whenever possible. In fact, the only light-source, lighting-model, or material parameters in an RGBA form that are used in color index mode are the light-source parameters GL_DIFFUSE and GL_SPECULAR and the material parameter GL_SHININESS. These parameters (dl and sl, respectively) are used to compute color-index diffuse and specular light intensities (dci and sci) as follows:

dci = 0.30 R(dl) + 0.59 G(dl) + 0.11 B(dl)

sci = 0.3 R(sl) + 0.59 G(sl) + 0.11 B(sl)

where R(x), G(x), and B(x) refer to the red, green, and blue components, respectively, of color x. The weighting values 0.30, 0.59, and 0.11 reflect the "perceptual" weights that red, green, and blue have for your eye - your eye is most sensitive to green and least sensitive to blue.

To specify material colors in color-index mode, use glMaterial*() with the special parameter GL_COLOR_INDEXES, as follows:

GLfloat mat_colormap[] = { 16.0, 47.0, 79.0 };
glMaterialfv(GL_FRONT, GL_COLOR_INDEXES, mat_colormap);

The three numbers supplied for GL_COLOR_INDEXES specify the color indices for the ambient, diffuse, and specular material colors, respectively. In other words, OpenGL regards the color associated with the first index (16.0 in this example) as the pure ambient color, with the second index (47.0) as the pure diffuse color, and with the third index (79.0) as the pure specular color. (By default, the ambient color index is 0.0, and the diffuse and specular color indices are both 1.0. Note that glColorMaterial() has no effect on color-index lighting.)

As it draws a scene, OpenGL uses colors associated with indices in between these numbers to shade objects in the scene. Therefore, you must build a color ramp between the indicated indices (in this example, between indices 16 and 47, and then between 47 and and 79). Often, the color ramp is built smoothly, but you might want to use other formulations to achieve different effects. Here's an example of a smooth color ramp that starts with a black ambient color and goes through a magenta diffuse color to a white specular color:

for (i = 0; i < 32; i++) {
   auxSetOneColor (16 + i, 1.0 * (i/32.0), 0.0, 1.0 * (i/32.0));
   auxSetOneColor (48 + i, 1.0, 1.0 * (i/32.0), 1.0);
}

The auxiliary library command auxSetOneColor() takes four arguments. It associates the color index indicated by the first argument to the RGB triplet specified by the last three arguments. When i = 0, the color index 16 is assigned the RGB value (0.0, 0.0, 0.0), or black. The color ramp builds smoothly up to the diffuse material color at index 47 (when i = 31), which is assigned the pure magenta RGB value (1.0, 0.0, 1.0). The second loop builds the ramp between the magenta diffuse color and the white (1.0, 1.0, 1.0) specular color (index 79). Figure J-20 shows the result of using this color ramp with a single lighted sphere.

The Mathematics of Color-Index Mode Lighting

OpenGL Programming Guide