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Progressive Radiosity Variants.

Several variations on the basic progressive radiosity algorithm have
been developed, in an effort to find the optimal method for producing
the most pleasing results with minimum cost.  Each variant calculates
the form factors from a point on one surface to all other surfaces. 

The 'gathering' variant collects light energy from all other surfaces in
the environment, attenuated by the calculated form factors, and
updates the 'base' surface.  The 'base' surface is arbitrarily chosen. 
As in the 'gathering' variant, the 'base' surface is arbitrary.

The 'shooting' variant distributes light energy from the 'base' surface
to all other surfaces in the environment, attenuated by the calculated
form factors.  

The "shooting and sorting" variant first calculates the surface with the
greatest amount of unshot light energy, then uses this surface as the
'base' surface in the "shooting" variant.

In addition to these, an initial 'ambient' term can be approximated for
the environment and adjusted at each iteration, and is gradually
replaced by the true ambient contribution to the rendered image. The
'shooting and sorting' method is the most desirable, as it finds the
surface with the greatest potential contribution to the intensity solution
and updates all other surfaces in the environment with its energy.