occluded: Occluded shading color. The default is white.
unoccluded: Unoccluded shading color. The default is black.
numSamples: Controls the number of occlusion samples per camera hit point. The default is 4.
distribution: Sample distribution can be uniform or cosine. The defaul is cosine.
cosineSpread: Sample spread of rays for cosine distribution. The defaul of 1 gives lambertian distribution.
falloff: Falloff values greater than zero create a softer look. The default is 0.
maxDistance: The max distance at which objects may occlude. The default of 0 is infinite.
direction: Trace rays in the direction of the surface normal (outside), inverse to the normal (inside), or in both directions. The default is outside.
biasNormal: Add a directional bias to the normal vector. The default is no bias.
traceSet: Restrict ray tracing to a subset of objects defined by one or more trace groups.