VCG ShadeVis computes a simple, static (but much more correct)
per-vertex ambient term. This effect, commonly known as ambient
occlusion, is aimed at providing more faithful shading for realtime
rendering. In practice, rather than considering the ambient lighting to
exist uniformly throughout a scene, this approach determines the ambient
brightness of each part of a surface to be proportional to the extent to
which the surface has "its outward view of its environment" free (i.e.,
"occluded") by other surfaces of the object. The most common technique
to compute ambient occlusion is based on shooting rays from each surface
element to evaluate the quantity of light that reaches it. VCG Shadevis
uses OpenGL to accelerate this process by simply rendering the scene
orthographically from a set of uniformly-distributed directions and
checking against the zbuffer whether each vertex is occluded.
