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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.