10.8.1.2 Metals

Metals are conductive and have free electrons. As a result, metals are opaque and tend to be very reflective, and their ambient, diffuse, and specular colors tend to be the same. How the free electrons are excited by light at different wavelengths determines the color of the metal. Materials like steel and nickel have nearly the same response over all visible wavelengths, resulting in a grayish reflection. Copper and gold, on the other hand, reflect long wavelengths more strongly than short ones, giving them their reddish and yellowish colors.

The color of light reflected from metals is also a function of incident and exiting light directions. This can't be modeled accurately with the OpenGL lighting model, compromising the metallic look of objects. However, a modified form of environment mapping (such as the OpenGL sphere mapping) can be used to approximate the proper visual effect.