Up to this point we have largely discussed the Phong lighting model.
The diffuse and specular terms for a single light are given by the
Section 10.1.1 discusses the use of sphere mapping to replace the OpenGL per-vertex specular illumination computation with one performed at each pixel. The specular contribution in the texture map is computed using the Phong formulation above. However, the Phong model can be substituted with other bi-directional reflectance functions to achieve other lighting effects. Since the texture coordinates are computed with a sphere mapping function, the resulting texture mapping operation accurately approximates view-dependent specular reflectance distributions.