> 1) Recursion.  As clipka (Christian?) wrote, it is absolutely essential 
> for POV.
>
> 2) Data parallelization versus code parallelization (this is related to 
> the first, but is not strictly the same).
>
> The ray tracing algorithm follows drastically different code branches on a 
> single set of data, based on recursion (reflections & refractions), as 
> well as the other various computations needed (texture calculation, light 
> source occlusion, etc) which almost all need access to the entire scene.

Often you need to drastically change your algorithm to get it running well 
on a GPU.  It's quite trivial to rewrite the raytracing algorithm to work on 
masses of rays in parallel with no branching or ray spawning, and then do 
the branching/spawning for the rays all in one go.  After this you can 
efficiently use the GPU and CPU in parallel.

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