Ian Romanick wrote:
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> I've been digging through the softpipe code in gallium for the past few
> days. I think I'm starting to "get it". I do have one question about
> the way quads are processed.
>
> It looks like the triangle setup code produces a row of quads and passes
> them off, one at a time, to the fragment shader. The fragment shader
> then does the attribute interpolation and passes the fragments to the
> real shader. Is that correct?
>
> It seems like instead we'd want the setup code to produce large groups
> of quads with all of attributes pre-interpolated. Each quad would be
> tagged with some ordering information. The shader code could then
> process quads however it wanted as longs as the tagged ordering
> information was respected. This would allow a few things:
>
> 1. Group quad processing to better use the tile cache.
>
> 2. Trivially distribute quad processing to multiple CPU cores (not just
> on Cell, either).
>
> 3. If the shader doesn't modify Z values, early Z culling could be
> performed.
>
> I don't see any easy way to achieve this with the current code. Am I
> missing something, or is this a place where I could jump in and help?
For multiprocessing I think we'll do things at the granularity of tiles,
not quads. That is, a processor will get a batch of triangles and a
tile position, then render those triangles into that tile. Roughly:
render_batch(triangles, tilex, tiley)
{
get_tile_color_and_z(tilex, tiley);
set_scissor_to_tile_bounds();
render_triangles_into_tile();
put_tile_color_and_z(tilex, tiley);
}
As for attribute interpolation, that's something that can be done with a
few extra instructions prefixed onto the fragment shader.
Eventually, we'd like the whole quad pipeline (shading, z/stencil,
blend) to be done with dynamically generated code. Then the triangle
rasterizer itself, etc.
-Brian
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