Final Gathering. No, it's not the name of yet another horror
movie sequel (although Google isn't much help for figuring out
what it in fact is). It's a technique for effectively smoothing
noise in global illumination from Lambertian surfaces.
Basically, given a solution to global illumination (like
radiosity or a photon map) instead of looking up the diffuse
light value in the GI solution, you do one final bounce of light
by shooting rays out across the hemisphere above the point you
are rendering. These rays sample the secondary light that would
be hitting this point much like a distribution raytracer would
send out rays to sample the BRDF. In this case, a random
sampling of the Lambertian distribution is not the best bet
(according to Jensen anways). It is better to use a grid of
points placed across the hemisphere according to Lambert's
cosine law and then jitter these points slightly to ensure the
full hemisphere gets sampled. When these rays hit a surface a
distribution raytracer would send out more rays