>> I thought the entire definition of AO is that it's a rough 
>> approximation to true GI that's useful because it renders really fast?
> 
> Indeed.  Raytracers typically cast out a few rays (the more the smoother 
> the result) to see if they hit anything within a certain distance.  GPU 
> methods typically just compare the depths of the surrounding pixels (so 
> it's essentially done in 2D).  Obviously for a GPU this is much faster 
> as it doesn't need to access the scene data (it might not even exist in 
> most cases!).

Ah, wait... Apparently I'm confusing SSAO with normal AO. (The former 
being commonly run on the GPU, the latter requiring an actual ray 
tracer.) I wasn't aware that Blender is doing true AO. I assumed it was 
just faking it, GPU-style.

>> (I idea why AA would be better...)
> 
> AFAIK standard GPU rendering uses a very limited AA algorithm (usually 
> fixed to a certain number of sub-pixels and some maximum like 8x), a 
> raytracer usually can use dynamic AA depth and as many rays as are 
> necessary.

This puzzles me. I'm not disputing you're wrong - usually there's some 
option to set the number of subsamples taken - but since a GPU can 
*only* draw straight lines, you'd think they could just use the 
closed-form formulas for doing mathematically perfect AA on polygon 
edges. It takes about 3 float-ops. No subsamples required.

Weird...

>> I guess I usually think of the main advantage of ray-tracing as being 
>> the ability to render things that aren't triangles.
> 
> I think there are some raytracers that can *only* render triangles.

Apparently I'm looking at one.

> I 
> guess it makes them simpler and they can be highly optimised for huge 
> lists of triangles.  Then you can make some layer that converts higher 
> order shapes to pixel sized triangles on the fly to feed the raytracer 
> if needed.  IIRC this is how Pixar's raytracer works.

That's what I heard. I also hear that only about 0.1% of the stuff Pixar 
does actually involves any ray tracing, with is kind of unbelievable...

I still find it rather hard to believe that you can take a complex shape 
such as the surface of a water splash and automatically tesselate it. 
But apparently somebody has figured out how to do this.

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