The Global Illumination panel controls provide methods for rendering bounced light within an environment, including final gathering, caustics, and photons.
Render Setup Dialog > Global Illumination panel
Note: The Global Illumination panel appears only when mental ray is the active renderer.
Final Gathering (FG) Rollout (mental ray Renderer)
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 samples 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. In the former case, the hemisphere orientation is determined by the surface normal of the triangle on whose surface the point lies.
Reuse (FG and GI Disk Caching) Rollout (mental ray Renderer)
The Reuse rollout gathers together all controls for generating and using final gather map (FGM) and photon map (PMAP) files, and adds the ability to reduce or eliminate flickering in rendered animations by interpolating among final gather map files.