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.
There are some things you should keep in mind when working with FG. Everything in the scene, from materials, light settings and your environment, is sampled. Since version 5, XSI has included different visibility options for objects to include or exclude them from the FG calculation. Increasing the accuracy of FG or using more bounces will lead to more realistic results, but also increase the render time and in some cases will brighten up the scene. You can adjust the overall brightness of the FG with the Multiplier value. Another important use of FG in XSI is in image-based lighting. Although the tutorial does not cover this technique, you can explore it for yourself by taking a look at pepper_IBL.scn in the Scenes directory of the project included in the support files.