reflectance model can be computed in the vertex or the fragment shader.
it is computed in the vertex shader, one is said to be implementing per-vertex
it is computed in the fragment shader, one is said to be implementing per-fragment
many scenes, there are fewer vertices than fragments to be processed. Therefore, per-vertex lighting is typically
in per-vertex lighting, the lighting is only evaluated at the vertices and
interpolated along the primitive. In
this case, the tessellation of the surface affects the quality of the shading.