OpenGL Programming Guide

Chapter 6Lighting

The Mathematics of Lighting

Advanced

This section presents the equations used by OpenGL to perform lighting calculations to determine colors when in RGBA mode. (You can find the corresponding calculations for color-index mode in "The Mathematics of Color-Index Mode Lighting." ) You don't need to read this section if you're willing to experiment to obtain the lighting conditions you want. Even after reading this section, you'll probably have to experiment, but you'll have a better idea of how the values of parameters affect a vertex's color. Remember that if lighting is not enabled, the color of a vertex is simply the current color; if it is enabled, the lighting computations described here are carried out in eye coordinates.

In the following equations, mathematical operations are performed separately on the R, G, and B components. Thus, for example, when three terms are shown as added together, the R values, the G values, and the B values for each term are separately added to form the final RGB color (R1+R2+R3, G1+G2+G3, B1+B2+B3). When three terms are multiplied, the calculation is (R1R2R3, G1G2G3, B1B2B3). (Remember that the final A or alpha component at a vertex is equal to the material's diffuse alpha value at that vertex.)

The color produced by lighting a vertex is computed as follows:

vertex color = the material emission at that vertex + the global ambient light scaled by the material's ambient property at that vertex + the ambient, diffuse, and specular contributions from all the light sources, properly attenuated

After lighting calculations are performed, the color values are clamped (in RGBA mode) to the range [0,1].

Note that OpenGL's lighting calculations don't take into account the possibility of one object blocking light from another, so shadows aren't automatically created. (See "Shadows" for a technique to create shadows.) Also keep in mind that with OpenGL, illuminated objects don't radiate light onto other objects.

Material Emission
Scaled Global Ambient Light
Contributions from Light Sources
Putting It All Together

OpenGL Programming Guide