[Mesa-dev] [PATCH 2/2] i965/msaa: Implement GL_SAMPLE_ALPHA_TO_{COVERAGE, ONE}.

[Anuj Phogat]

On Fri, Jun 22, 2012 at 10:02 PM, Paul Berry <stereotype441 at gmail.com> wrote:
> This patch enables the multisampling parameters
> GL_SAMPLE_ALPHA_TO_COVERAGE and GL_SAMPLE_ALPHA_TO_ONE, which allow
> the fragment shader's alpha output to be converted into a sample
> coverage mask and ignored for blending.  i965 supports these
> parameters through the BLEND_STATE structure.
>
> The GL spec allows, but does not require, the implementation to dither
> the conversion from alpha to a sample coverage mask, so that alpha
> values that aren't a multiple of 1/num_samples result in the correct
> proportion of samples being lit.  A bit exists in the BLEND_STATE
> structure to enable this functionality, but according to the hardware
> docs it must be disabled on Sandy Bridge (see the Sandy Bridge PRM,
> Vol2, Part1, p379: AlphaToCoverage Dither Enable).  So it is enabled
> for Gen7 only.
>
> Fixes piglit tests
> "EXT_framebuffer_multisample/sample-alpha-to-{coverage,one} {2,4}".
> ---
>  src/mesa/drivers/dri/i965/gen6_cc.c |   10 +++++++++-
>  1 files changed, 9 insertions(+), 1 deletions(-)
>
> diff --git a/src/mesa/drivers/dri/i965/gen6_cc.c b/src/mesa/drivers/dri/i965/gen6_cc.c
> index b4c5329..e0523ef 100644
> --- a/src/mesa/drivers/dri/i965/gen6_cc.c
> +++ b/src/mesa/drivers/dri/i965/gen6_cc.c
> @@ -161,6 +161,13 @@ gen6_upload_blend_state(struct brw_context *brw)
>       blend[b].blend1.write_disable_g = !ctx->Color.ColorMask[b][1];
>       blend[b].blend1.write_disable_b = !ctx->Color.ColorMask[b][2];
>       blend[b].blend1.write_disable_a = !ctx->Color.ColorMask[b][3];
> +
> +      /* _NEW_MULTISAMPLE */
> +      blend[b].blend1.alpha_to_coverage =
> +         ctx->Multisample._Enabled && ctx->Multisample.SampleAlphaToCoverage;
> +      blend[b].blend1.alpha_to_one =
> +         ctx->Multisample._Enabled && ctx->Multisample.SampleAlphaToOne;
> +      blend[b].blend1.alpha_to_coverage_dither = (brw->intel.gen >= 7);
>    }
>
>    brw->state.dirty.cache |= CACHE_NEW_BLEND_STATE;
> @@ -169,7 +176,8 @@ gen6_upload_blend_state(struct brw_context *brw)
>  const struct brw_tracked_state gen6_blend_state = {
>    .dirty = {
>       .mesa = (_NEW_COLOR |
> -              _NEW_BUFFERS),
> +               _NEW_BUFFERS |
> +               _NEW_MULTISAMPLE),
>       .brw = BRW_NEW_BATCH,
>       .cache = 0,
>    },
> --
> 1.7.7.6
>

Verified that following piglit tests passes on Sandybridge with these
patches: sample-coverage, sample-alpha-to-coverage and
sample-alpha-to-one
This series is:
Reviewed-by: Anuj Phogat <anuj.phogat at gmail.com>