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# 1.32 Final Gathering

Final gathering is a technique for estimating global illumination for a given point by either sampling a number of directions over the hemisphere over that point (such a set of sample is called a final gather point), or by averaging a number of nearby final gather points 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, and the global illumination at those points is then computed by the material shaders at these 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 gathers is sufficient.

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 globillum photon map and lower globillum accuracy is sufficient since each final gather averages over many values of indirect illumination.

Final gathering is `off` by default, but can be turned `on` in the options. Final gathering is also useful without photon tracing; this only takes first-bounce indirect light into account but often gives good results where complete physical accuracy is not required.

To change the number of rays shot in each final gather (and optionally the max 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` in the options. For example,

         finalgather accuracy 1000 1.5 0.25

increases the number of rays 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.

Next: 1.33 Volume Caustics Up: 1. Functionality Previous: 1.31 Global Illumination