OpenGL Programming Guide

Chapter 6Lighting

Real-World and OpenGL Lighting

When you look at a physical surface, your eye's perception of the color depends on the distribution of photon energies that arrive and trigger your cone cells, as described in "Color Perception." Those photons come from a light source or combination of sources, some of which are absorbed and some of which are reflected by the surface. In addition, different surfaces may have very different properties - some are shiny, and preferentially reflect light in certain directions, while others scatter incoming light equally in all directions. Most surfaces are somewhere in between.

OpenGL approximates light and lighting as if light can be broken into red, green, and blue components. Thus, the color of light sources is characterized by the amount of red, green, and blue light they emit, and the material of surfaces is characterized by the percentage of the incoming red, green, and blue components that are reflected in various directions. The OpenGL lighting equations are just an approximation, but one that works fairly well and can be computed relatively quickly. If you desire a more accurate (or just different) lighting model, you have to do your own calculations in software. Such software can be enormously complex, as a few hours of reading any optics textbook should convince you.

In the OpenGL lighting model, the light in a scene comes from several light sources that can individually be turned on and off. Some light comes from a particular direction or position, and some light is generally scattered about the scene. For example, when you turn on a light bulb in a room, most of the light comes from the bulb, but some light comes after bouncing off one, two, three, or more walls. This bounced light (called ambient) is assumed to be so scattered that there is no way to tell its original direction, but it disappears if a particular light source is turned off.

Finally, there might be a general ambient light in the scene that comes from no particular source, as if it had been scattered so many times that its original source is impossible to determine.

In the OpenGL model, the light sources have an effect only when there are surfaces that absorb and reflect light. Each surface is assumed to be composed of a material with various properties. A material might emit its own light (like headlights on an automobile), it might scatter some incoming light in all directions, and it might reflect some portion of the incoming light in a preferential direction like a mirror or shiny surface.

The OpenGL lighting model considers the lighting to be divided into four independent components: emitted, ambient, diffuse, and specular. All four components are computed independently, and then added together.

Emitted, Ambient, Diffuse, and Specular Light
Material Colors
RGB Values for Lights and Materials

OpenGL Programming Guide