Ambient occlusion is a technique which adds visual realism to the image without being physically correct. The ambient occlusion result can be used to darken concave areas, which human eye perceives as indirect illumination shadows, or contact shadows. The advantage of ambient occlusion is its computational speed. As it may be computed with very short rays and as no additional shader evaluations are required, the performance may be significantly higher than for final gathering.
Ambient occlusion is enabled by default. However, no actual computations will happen until requested by further settings or by shaders. The ambient occlusion cache is set off by default. In this mode, only shaders which call for ambient occlusion values will initiate computation on demand. If no such shaders exist in the scene, there is no overhead compared scenes rendered with ambient occlusion turned off. In case the ambient occlusion caching is enabled then mental ray will perform some computations before rendering starts, to fill the cache.
The global defaults for ambient occlusion settings can be specified as scene options or on the mental ray command line. Most of these values can be overwritten by a shader.
The ambient occlusion caching may be enabled for the current
rendering to gain overall speed. In this case, several preprocess passes
will be computed. In the first pass, some ambient occlusion points are created
on a coarse grid. Subsequent passes refine the grid adaptively. The density
of the grid is determined by the
"ambient occlusion cache density" setting which
gives the upper bound to the number of ambient occlusion
points per pixel. During tile rendering, ambient occlusion values are
interpolated from several ambient occlusion points closest to the lookup
location. The number of points to be used for interpolation is given by the
"ambient occlusion cache points" setting, and
defaults to 64.
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