>> I was thinking more along the lines of each surface possibly having a 
>> different code path for computing illumination. Rays in the same 
>> bundle can't take different code paths.
> 
> Sure they can, you just have an "if" in the shader code based on some 
> flag in the ray data.

I was under the impression that all cores in the bunch would have to 
take the same branch of the if - you can't have half go one way and half 
go the other. (That's what makes it a SIMD architecture.)

I was also under the impression you can have predicated instructions, 
however. (I.e., a single instruction that executes or doesn't based on a 
condition. But not a whole alternate execution path.)

> Some things I think the CPU still needs to do, like preparing the ray 
> data to put on the GPU at each step. It is quite hard to write a shader 
> that does anything other than return one "output value" per "pixel", ie 
> you can't write a shader that returns a veriable number of rays, the 
> only thing you can do is return a flag set that means "spawn another ray".

> I was thinking more about how to store the scene on the GPU efficiently, 
> if you just have a triangle list it is relatively simple.

Again, depends on whether you're writing a shader, or using a GPGPU system.

>> The only real problem is going to be trying to run something like 
>> isosurfaces, which use adaptive space division; a GPU probably won't 
>> like that.
> 
> Nah it's no problem :-)  Modern GPUs don't have any limit on loops or 
> number of instructions or anything like that.

But they don't allow recursion. That's the issue.

>> Also depends on whether you're trying to write your code as a shaper, 
>> or use a real GPGPU system. (Such as the OpenCL in the title.)
> 
> I think something like a raytracer will be too complicated for a 
> computer to try and figure out how to put it on the GPU the best way, 
> needs to be hand coded by an intelligent person :-)

...and?

All GPGPU does is allow you to run programs on a GPU which don't 
necessarily have to be shaped like shaders. You still have to write the 
code yourself. It's just that you don't have to pretend it's 
graphics-related even if it isn't.

> Damn.  If you think up *anything* you think is new, you can be 
> guaranteed that someone else somewhere has already thought of it :-)  
> The internet just makes it easy to find that person!

I've had the same problem with mathematics my entire life.

Do you know how hard it is to draw a cube made of 8 cubes and measure 
all their sides? Do you know how long it takes to expand (x+y)^9 
manually, by hand? Do you have any idea how long it takes to figure out 
what the pattern is and where it's coming from?

...and then I discover some entry-level textbook that tells me how some 
dead guy already figured all this out several centuries ago. AND FOR 
ARBITRARY EXPONENTS!! >_<

Why do I bother?

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