Final gathering (FG) is a method of simulating indirect lighting: the light reflected onto an object from surrounding surfaces. To do this, the renderer calculates a grid of FG points by shooting rays from the camera into the scene. From the FG points, extra rays are shot into the scene in a hemispherical pattern. The renderer can then calculate the illumination that would be received at each FG point according to whether these new rays strike surrounding surfaces. It's a powerful tool for creating subtle, real-world lighting effects, but one that many beginners struggle to achieve good results with.
A popular use of FG is to light a scene using self-illuminating objects to bounce light into that scene, rather than any active lights. However, for this tutorial, we're going to use a slightly more conventional set-up: we're going to use FG to light a scene with one real light plus some reflectors. Since space is limited, we won't go too deeply into all the settings: instead, we'll add the reflector objects, assign some materials and finally set up FG in a very basic way. The result will be a rig that mimics a real photographer's studio set-up, suitable for use in 'virtual photography' renders of a type that might be used in advertising or illustration work.
