The simplest shading method applies only one illumination calculation for each primitive. This technique is called constant or flat shading. It is often used on polygonal primitives. Issues:

• For point light sources the direction to the light source varies over the facet
• For specular reflections the direction to the eye varies over the facet

None the less, often illumination is computed for only a single point on the facet. Which one? Usually the centroid. For a convex facet the centriod is given as follows: