Final gathering is a technique for estimating global illumination for a given point by either sampling a number of directions in the hemisphere over that point (such a sample set is called a final gather point), or by averaging a number of final gather points nearby since final gather points are too expensive to compute for every illuminated point.
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 hit diffuse surfaces; the global illumination at those points is then computed by the material shaders at those sample point, using illumination from the globillum photon map if available and other material properties. Other rays hit 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 only done when necessary - in most cases, interpolation and extrapolation from previous nearby final gatherings is sufficient.
Final gathering is useful in scenes where the indirect illumination various slowly, such as purely diffuse scenes. For such scenes, final gathering eliminates photon map artifacts such as low frequency noise and dark corners. Fewer photons are needed in the globillum photon map and lower globillum accuracy is sufficient since each final gather averages over many values of indirect illumination.
In film production work, final gathering is often used rather than photon mapping, except for caustics. Final gathering by default supports a single bounce only. Multiple-bounce effects tend to have far less impact on the final image than the first bounce; these are performed by photons by default and by final gathering only if the shaders adjusts the trace depth. Although physical correctness is lost, this is often "good enough" for film production, and final gathering is easier to control than photons emanating from distant light sources. However, for accurate indoor illumination simulations and other CAD-related applications, photon mapping is preferred.
Final gathering is set off by default, but can be turned on in the options. To change the number of rays shot in each final gather (and optionally the maximum distance at which a final gathering result can be used for interpolation and the minimum distance at which it must be used), specify a finalgather accuracy setting in the options. For example,
finalgather accuracy 1000 1.5 0.25
increases the number of rays and reduces noise in scenes with complex illumination and geometry. The default number of rays is 1000. The default maximum distance depends on the scene extent; decreasing it will reduce noise but increase render time. The default minimum distance is 10% of the maximum distance.
mental ray 3.4 has changed the final gathering algorithm. It normally achieves better quality in less time with approximately one-half the number of rays. It is necessary to modify the accuracy settings though, for example changing 1000 to 500 in the preceeding example, or rendering times will increase.
mental ray offers the following final gathering modes:
The mode can be selected by a
scene option or on the
mental ray command line.
The default is "3.4" compatibility mode.
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