Ambient Occlusion

Ambient Occlusion

Often abbreviated AO.

Some implementations work somewhat like wikipedia describes it but not A:M.[1]

In A:M, AO is a darkening trick only. From a shading point on a surface, AO shoots many rays in quasi-random directions over the surface and counts the percentage of rays that reach the background. If no rays reach the background, then the occlusion is 100% and if all rays reach the background, then the occlusion is 0%.

So rays that reach the background do not contribute any light. They just do not contribute occlusion. Nothing in AO contributes light but rather, wherever there is light, AO has the potential to darken it. The AO darkens whatever illumination was computed on a surface. So A point with occlusion of 100% will get black and a point with occlusion of 0% will keep its computed illumination.

Technically, AO is not a shadow either. Shadows are the result of specific light not reaching some area of the scene. Shadows do not darken some areas of the scene. Whatever light was already computed from some light stays there when shadowed from another light. While AO does darkens part of the scene.

Mixing AO with Sub-Surface Scattering

AO will likely not work very well with SSS because they tend to compete one against the other. SSS tries to diffuse light in small parts of the geometry but usually, those small geometries are also those which have the most occlusion. Typical example is an ear.[2]