Where do we Illuminate ?...
From Illumination (a costly process), lighting model to...
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.
... Shading or Surface rendering algorithm
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