[Mesa-dev] [PATCH 1/4] i965/fs: Use sample interpolation for interpolateAtCentroid in persample mode

[Anuj Phogat]

On Wed, Sep 14, 2016 at 10:45 AM, Jason Ekstrand <jason at jlekstrand.net> wrote:
> From the ARB_gpu_shader5 spec:
>
>    The built-in functions interpolateAtCentroid() and interpolateAtSample()
>    will sample variables as though they were declared with the "centroid"
>    or "sample" qualifiers, respectively.
>
> When running with persample dispatch forced by the API, we interpolate
> anything that isn't flat as if it's qualified by "sample".  In order to
> keep interpolateAtCentroid() consistent with the "centroid" qualifier, we
> need to make interpolateAtCentroid() do sample interpolation instead.
> Nothing in the GLSL spec guarantees that the result of
> interpolateAtCentroid is uniform across samples in any way, so this is a
> perfectly fine thing to do.
>
This explanation sounds good to me. To be consistent with what
we do in case of per sample interpolation, shouldn't we do sample
interpolation in case of InterpolateAtOffset() too? This series
doesn't seem to include it.

> Fixes 8 of the new dEQP-VK.pipeline.multisample_interpolation.* Vulkan CTS
> tests that specifically validate consistency between the "sample" qualifier
> and interpolateAtSample()
>
> Signed-off-by: Jason Ekstrand <jason at jlekstrand.net>
> ---
>  src/mesa/drivers/dri/i965/brw_fs.cpp | 26 ++++++++++++++++++++++++++
>  1 file changed, 26 insertions(+)
>
> diff --git a/src/mesa/drivers/dri/i965/brw_fs.cpp b/src/mesa/drivers/dri/i965/brw_fs.cpp
> index 75642d3..9dbb699 100644
> --- a/src/mesa/drivers/dri/i965/brw_fs.cpp
> +++ b/src/mesa/drivers/dri/i965/brw_fs.cpp
> @@ -6497,6 +6497,32 @@ brw_nir_set_default_interpolation(const struct gen_device_info *devinfo,
>           var->data.sample = false;
>        }
>     }
> +
> +   if (per_sample_interpolation) {
> +      nir_foreach_block(block, nir_shader_get_entrypoint(nir)) {
> +         nir_foreach_instr(instr, block) {
> +            if (instr->type != nir_instr_type_intrinsic)
> +               continue;
> +
> +            nir_intrinsic_instr *intrin = nir_instr_as_intrinsic(instr);
> +            if (intrin->intrinsic != nir_intrinsic_interp_var_at_centroid)
> +               continue;
> +
> +            nir_variable *var = intrin->variables[0]->var;
> +            if (var->data.interpolation == INTERP_MODE_FLAT)
> +               continue;
> +
> +            /* The description of the interpolateAtCentroid intrinsic is that
> +             * it interpolates the variable as if it had the "centroid"
> +             * qualifier.  When executing with per_sample_interpolation, this
> +             * is equivalent to having the "sample" qualifier.  Just convert
> +             * it to a load_var instead.
> +             */
> +            assert(var->data.sample);
> +            intrin->intrinsic = nir_intrinsic_load_var;
> +         }
> +      }
> +   }
>  }
>
>  /**
> --
> 2.5.0.400.gff86faf
>
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