Sphere mapping is an implementation of environment mapping .
When rendering, the light from the point is computed as a function of the outgoing direction and the environment map. In general, only one environment map point is used for each outgoing ray, resulting in a perfect specular reflection. In rendering, you often use a single environment map for an entire object by assuming that the single environment map is a reasonable approximation of the environment map which would be computed at each point on the object. This approximation is correct if the object is a sphere and the viewer and other objects in the scene are infinitely far away. In interactive polygonal rendering, make the additional assumption that the indices into the environment map may be computed at each vertex and linearly interpolated over each polygon.