>> If your work packages are large enough, then these are no issues. But 
>> in a raytracer, be prepared for rather small work packages.
> 
> And that's another problem.  While a GPU has *hundreds* of cores that 
> all work in parallel, in raytracing you're unlikely to encounter more 
> than a few dozen rays which require the same test.  It's just not 
> efficient to solve this problem with GPUs.

I'm not sure I agree with these statements.

Tracing a scene involves shooting thousands, or even tens of thousands 
of rays at the top-level geometry. While there may be some objects which 
are quite small, most objects will be hit by hundreds and hundreds of rays.

Now, whether it's efficient to run the entire rendering engine on the 
GPU is debatable, but it would seem to be that it should be possible to 
achieve *some* speedup by running at least part of the tracing process 
on the GPU. (The only question is bandwidth.)

> Note that, while GPGPU has come a long way in recent years, the only way 
> to make it more useful for POV is to make it more like a CPU and less 
> like a GPU.  While GPUs in general may go this route in the future (Tim 
> Sweeney thinks so), really GPUs will focus first and foremost on what 
> games need... and games don't need raytracing yet.

More to the point, if a GPU was more like a CPU, it wouldn't be faster 
than a CPU.

-- 
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*

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