Orchid XP v8 wrote:
>>> 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.

Let's say a scene rendered for display on a 1080p TV has approximately 
300 objects in it, each one covering a similar portion of the display 
(for simplicity's sake).

That's a total of 1,920x1,080=2,073,600 pixels, or 6,912 for each 
object.  Admittedly, this is more than my statement of "a few dozen" :) 
  However, each level of recursion will split the groups rays - let's 
say into 4 groups (I honestly think they would be grouped in more, 
smaller, groups, but this will make the math easier).  After only 4 
levels of recursion (a value entirely common in scenes) the rays are 
bundled in groups of 25 (assuming an even distribution, of course).

With a smaller image, more objects, and less tight grouping, you'd get 
such small groups after only one or two levels of recursion.

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

True ;)

...Chambers

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