Autodesk 12812-051462-9011 User Guide 3 - Page 1041

expensive to compute for every illuminated point. In the former case, the hemisphere orientation is determined by the surface normal of the triangle on whose surface the point lies. For diffuse scenes, final gathering often improves the quality of the global illumination solution. Without final gathering, the global illumination on a diffuse surface is computed by estimating the photon density (and energy) near that point. With final gathering, many new rays are sent out to sample the hemisphere above the point to determine the incident illumination. Some of these rays strike diffuse surfaces, and the global illumination at those points is then computed by the material shaders at these point, using illumination from the photon map, if available, and from other material properties. Other rays strike specular surfaces and do not contribute to the final gather color (since that type of light transport is a secondary caustic). Tracing many rays (each with a photon map lookup) is very time-consuming, so it is done only when necessary. In most cases, interpolation and extrapolation from previous nearby final gathers is sufficient. Final gathering is also useful without photon tracing; this takes only first-bounce indirect light into account but often gives good results where complete physical accuracy is not required. Interior rendered with final gathering only Final gathering is useful in scenes with slow variation in the indirect illumination, such as purely diffuse scenes. For such scenes, final gathering eliminates photon map artifacts such as low-frequency noise and dark corners. With final gathering, fewer photons are needed in the photon map and, 6296 | Chapter 20 Rendering