[Algorithms] why not geometry?

[Jeff R. <je...@gm...>]

So, say hypothetically that a person wanted to render a tree trunk. The
typical approach is to use solid triangles for the trunk and a few major
branches and use alpha-tested or blended geometry for the smaller branches
and twigs, the goal being of course to reduce the triangle count to get good
performance. One then ends up rendering for example something like 1-2k
triangles instead of the 50k or more that might be required to render the
'full' tree. Seems logical.

But my question is - is this still wise? And if so, why?

Consider:

- A typical vertex shader often compiles into about as many instructions as
a fragment shader nowadays (for us in the 50-100 range). I would expect then
that from a pure computation standpoint computing a single vertex would be
roughly as fast as a single pixel on a modern GPU, since vertices and
fragments now share ALU in most cases.

- Geometry in this case saves fill rate. Even if the tradeoff for vertices
to fragments isnt 1:1, drawing your tree branches with geometry will avoid
the massive overdraw that large alpha-tested triangles can incur. Even
proper sorting won't save all those fragments that get discarded by the
alpha test.

- Geometry looks better! You get antialiasing, proper depth buffer
interaction, parallax, etc. all of which are trickier to attain in full for
impostor geometry.

- The advent of the 'geometry shader' makes replicating parts of your data
stream more feasible. I could see a situation of local geometry instancing
where a large branch has the same 'twig' present in several different
positions/orientations/scales along its length. This means you allow maybe
100 tris in your vertex buffer for a given twig, and the geometry shader
replicates it at draw time using a constant table to maybe 16 instances or
so to fill out your tree. Then you'd get a very large number of triangles
generated by a single draw call that only passes in a fraction of the data.

I suppose maybe the memory problems alone could be what hurt vertex
processing so much, given there's so much data flying around. Plus I've
heard that geometry shaders aren't even very fast yet.

Any thoughts? Has anyone gone down this road - using lots of triangles in
place of flat textures?


Jeff Russell