To this point we have discussed how to compute an illumination model at a point on a surface. But, at which points on the surface is the illumination model applied? Where and how often it is applied has a noticable effect on the result.

Illuminating can be a costly process involving the computation of and normalizing of vectors to multiple light sources and the viewer.

For models defined by collections of polygonal facets or triangles:

• Each facet has a common surface normal
• If the light is directional then the diffuse contribution is constant across the facet
• If the eye is infinitely far away and the light is directional then the specular contribution is constant across the facet