Ambient occlusion

Ambient occlusion is a technique which adds visual realism to the image, though by its nature the rendered image is not physically correct. The ambient occlusion can be added to darken concave areas, which human eye perceives as indirect illumination 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 set on by default. However, the ambient occlusion caching is set off. If caching is disabled, each ambient occlusion lookup is created on demand. If shaders do not call ambient occlusion in the scene, there is no overhead compared to the scenes rendered with ambient occlusion turned off (unless the ambient occlusion caching is enabled, in which case some unnecessary precomputing passes would be done).

The number of ambient occlusion rays used for computation of each ambient occlusion value is given by the "ambient occlusion rays" string option. That value can be overwritten by a shader. The default value is 256.

To speedup rendering, ambient occlusion caching may be enabled with the "ambient occlusion cache" string option. If caching is turned on, several preprocess passes are 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 identified by the "ambient occlusion cache density" string option which gives the upper bound to the number of ambient occlusion points per pixel.

During tile rendering, ambient occlusion is interpolated from several ambient occlusion points closest to the lookup location. The number of points used for interpolation is given by the "ambient occlusion cache points" string option. The default value is 64.

Ambient occlusion caching is disabled by default.