[Mesa-dev] Mesa (gallium-index-bias): gallium: Add index bias parameter to draw_elements and friends.

[José Fonseca]

On Mon, 2010-04-19 at 10:57 -0700, Marek Olšák wrote:
> According to ARB_draw_elements_base_vertex, the index clamping should
> be done prior to adding the index offset and r500 seems to implement
> it the same way, which kinda makes the clamping useless here. 

Does it really change anything? Issue 8 says the follow is true:

  start <= indices[i] <= end

instead of 

  start <= indices[i] + basevertex <= end

If you want the produce the later with the former, then we could program
the hardware/gallium with

  MAX(start - basevertex, 0) <= indices[i] <= end - basevertex

and if (end - basevertex < 0) then just skip the draw, as all vertices
would be clamped.

Or am I missing something here?

But you raise a very good point regarding the minIndex/maxIndex
semantics that I was neglecting. I'll make sure the
documentation/implementation is consistent with the D3D APIs and GL
extension.

> To my knowledge there is no way to implement hw-accelerated, negative
> index offsets other than with the r500-specific register. We already
> do tons and tons of fallbacks and some crucial ones are still missing
> so I wouldn't like to implement more of them than is absolutely
> necessary.

I could turn your argument around: s/fallbacks/capabilities checks/.

Anyway, I don't care about this particular piece of functionality. It's
basic in D3D9/D3D10, but hardly relevant on GL.

Jose