[Mesa-dev] [PATCH 02/12] i965: Program 16x MSAA sample positions.

[Ben Widawsky]

On Thu, Sep 17, 2015 at 05:00:04PM +0100, Neil Roberts wrote:
> This is the standard pattern used by the other 3D graphics API.
> 
> BDW has slots for these values, but they aren't actually used until
> SKL. Even though the documentation for BDW says they must be zero, it
> doesn't seem to cause any harm to program them anyway.
> 
> The comment above for the 8x sample positions says that the hardware
> implements centroid interpolation by picking the centre-most sample
> that is inside the primitive. That implies that it might be worthwhile
> to pick a pattern that includes 0.5,0.5. However by experimentation
> this doesn't seem to actually be the case. With the sample positions
> in this patch, if I modify the piglit test below so that it instead
> reports the centroid position, it reports 0.492188,0.421875 which
> doesn't match any of the positions. If I modify the sample positions
> so that they include one at exactly 0.5,0.5 it doesn't help and it
> reports another position which is even further from the center for
> some reason.
> 
> arb_gpu_shader5-interpolateAtSample-different
> 
> Kenneth Graunke experimented with some other patterns that have a
> higher standard deviation but I think after some discussion it was
> decided that it would be better to pick the same pattern as the other
> graphics API in case there are games that rely on this pattern.
> 
> (Based on a patch by Kenneth Graunke)
> 
> Cc: Kenneth Graunke <kenneth at whitecape.org>

Reviewed-by: Ben Widawsky <ben at bwidawsk.net>