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> 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!).
> As far as I know, I'm not using any ray tracer features, and yet the final
> render looks different than the edit window. (If only by being more
> grainy...)
If it looks grainy then you've probably got AO turned on with a low number
of rays.
Note that in the edit window there are several different options for the
display mode (wireframe, shaded, textures etc).
> Well, I suppose. (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.
> 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. 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.
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