[Mesa-dev] [PATCH v2] i965/fs: Don't use the pixel interpolater for centroid interpolation

[Neil Roberts]

Chris Forbes <chrisf at ijw.co.nz> writes:

> Nitpicks aside, I don't think this is a great idea now that you've got
> the SKL PI working.

Can you explain why you don't think this is a good idea? Is it because
it is an optimisation for something that is not known to be a big use
case so carrying around the extra code just adds unnecessary maintenance
burden? I could agree with that so I'm happy to abandon the patch for
now if that's the general consensus.

> I also think it's broken -- you need to arrange to have the centroid
> barycentric coords delivered to the FS thread, which won't be
> happening if this is the *only* use of them. Masked in the tests,
> because they compare with a centroid-qualified input. [I'm assuming
> you don't always get these delivered to the FS in SKL, but no docs
> access...]

The changes to brw_compute_barycentric_interp_modes in the patch ensure
that the centroid barycentric coords are delivered whenever
interpolateAtCentroid is used in a shader. I don't think this is a
problem. At least it seems to work in a simple test without using a
separate varying with the centroid qualifier.

Regards,
- Neil