The light rays from a distant light flow uniformly in space from one
direction. As a result, surfaces with the same orientation receive the same
amount of light independent of location. Surfaces with different orientations are
illuminated differently. Those facing toward the light source appear brightest,
while those facing away are completely un-illuminated by this light source
within the scene. In OpenGL, distant light sources are modeled with a color and
a direction. We saw an example of how to use a distant light when we were
illuminating our snowman in Example6-3.