The three-part discretization process results in a set of subsurfaces used in the subsequent radiative transfer analysis. In most cases, any one subsurface of this system does not have an unobstructed view of all other subsurface. Subsurfaces may be facing away from each other or the line of sight between partially or fully occluded by other surfaces.
An array of occlusion factors is found that describes view that all subsurfaces have of all other subsurfaces. Ray-casting is used to find these factors. Somewhat like the occlusion process described above, an array of points is established on a subsurface; the density determined adaptively by the proximity of the other subsurface of the pair being considered. From each of these points, an angularly uniform spray of rays is established to the other subsurface. The fraction of all these rays that are not either fully occluded (by opaque surfaces) or partially occluded (by image-preserving transmissive surfaces) establishes the occlusion factor between the pair of subsurfaces.