Computing a single new entry requires work assuming efficient visibility checking. Therefore it takes a lot of work to get to a reasonably converged solution. There are too many insignificant transfers of energy when doing a gather at each element. We want to reorder the compuation so that significant transfers are accounted for first. Rather than gathering energy at each reciever, shoot from each source.
Surfaces can gather energy after they have shot.
You can never have shot all of the energy in the environment. When energy is shot to a patch, both the total radiance and the unshot radiance are incremented by the amount of radiance striking it. Patches are sorted by energy, not radiance, because a very tiny, yet bright surface will have less effect on an environment than a very large yet less bright surface. As the solution progresses, the overall intensity of the environment increases. Implicit Matrices A matrix as a set of links.