[Mesa-dev] [PATCH 2/3] mesa/st: add per sample shading state to fp key and set interpolation

[Ilia Mirkin]

On Sat, Jul 5, 2014 at 6:22 PM, Marek Olšák <maraeo at gmail.com> wrote:
> There is this vague statement in the sample shading spec:
>
>     When the sample shading fraction is 1.0, a separate set of colors and
>     other associated data are evaluated for each sample, each set of values
>     are evaluated at the sample location.
>
> I thought it was about interpolation, meaning that the fraction must
> 1.0 for the per-sample interpolation to be enabled, right?

I guess so. I'm pretty much just doing what the intel driver is
doing... see brw_wm.c.

/* Gets the minimum number of shader invocations per fragment.
 * This function is useful to determine if we need to do per
 * sample shading or per fragment shading.
 */
GLint
_mesa_get_min_invocations_per_fragment(struct gl_context *ctx,
                                       const struct gl_fragment_program *prog,
                                       bool ignore_sample_qualifier)

So I guess instead of > 1 this should be checking for ==
ctx->DrawBuffer->Visual.samples? (and > 1)

Although in that case, I _really_ don't get the point of having
non-1.0/0.0 fractions ever existing. Why would you want it to be
shaded for e.g. 4/8 samples if all the inputs are going to get
interpolated the same way?

  -ilia

>
> Marek
>
>
> On Sat, Jul 5, 2014 at 6:07 AM, Ilia Mirkin <imirkin at alum.mit.edu> wrote:
>> This enables a gallium driver not to care about the semantics of
>> ARB_sample_shading vs ARB_gpu_shader5 sample attributes. When
>> ARB_sample_shading-style sample shading is enabled, all of the fp inputs
>> are marked for interpolation at the sample location.
>>
>> Signed-off-by: Ilia Mirkin <imirkin at alum.mit.edu>
>> ---
>>
>> I _think_ I got this right... at least the tests still pass. But my
>> understanding of all the various update atoms, etc is really weak... please
>> read carefully :)
>>
>> Now that I understand all this interpolation stuff better, I see why it was
>> suggested I add proper per-sample interpolation first and the
>> ARB_sample_shading stuff second. Oh well...
>>
>>  src/mesa/state_tracker/st_atom_shader.c | 6 +++++-
>>  src/mesa/state_tracker/st_program.c     | 3 +++
>>  src/mesa/state_tracker/st_program.h     | 3 +++
>>  3 files changed, 11 insertions(+), 1 deletion(-)
>>
>> diff --git a/src/mesa/state_tracker/st_atom_shader.c b/src/mesa/state_tracker/st_atom_shader.c
>> index 67c6157..6515a98 100644
>> --- a/src/mesa/state_tracker/st_atom_shader.c
>> +++ b/src/mesa/state_tracker/st_atom_shader.c
>> @@ -89,6 +89,10 @@ update_fp( struct st_context *st )
>>     key.clamp_color = st->clamp_frag_color_in_shader &&
>>                       st->ctx->Color._ClampFragmentColor;
>>
>> +   /* Ignore sample qualifier while computing this flag. */
>> +   key.persample_shading =
>> +      _mesa_get_min_invocations_per_fragment(st->ctx, &stfp->Base, true) > 1;
>> +
>>     st->fp_variant = st_get_fp_variant(st, stfp, &key);
>>
>>     st_reference_fragprog(st, &st->fp, stfp);
>> @@ -108,7 +112,7 @@ update_fp( struct st_context *st )
>>  const struct st_tracked_state st_update_fp = {
>>     "st_update_fp",                                     /* name */
>>     {                                                   /* dirty */
>> -      _NEW_BUFFERS,                                    /* mesa */
>> +      _NEW_BUFFERS | _NEW_MULTISAMPLE,                 /* mesa */
>>        ST_NEW_FRAGMENT_PROGRAM                           /* st */
>>     },
>>     update_fp                                   /* update */
>> diff --git a/src/mesa/state_tracker/st_program.c b/src/mesa/state_tracker/st_program.c
>> index b603759..9d7b7c4 100644
>> --- a/src/mesa/state_tracker/st_program.c
>> +++ b/src/mesa/state_tracker/st_program.c
>> @@ -548,6 +548,9 @@ st_translate_fragment_program(struct st_context *st,
>>           else
>>              interpLocation[slot] = TGSI_INTERPOLATE_LOC_CENTER;
>>
>> +         if (key->persample_shading)
>> +            interpLocation[slot] = TGSI_INTERPOLATE_LOC_SAMPLE;
>> +
>>           switch (attr) {
>>           case VARYING_SLOT_POS:
>>              input_semantic_name[slot] = TGSI_SEMANTIC_POSITION;
>> diff --git a/src/mesa/state_tracker/st_program.h b/src/mesa/state_tracker/st_program.h
>> index ce9174f..9a5b6a8 100644
>> --- a/src/mesa/state_tracker/st_program.h
>> +++ b/src/mesa/state_tracker/st_program.h
>> @@ -58,6 +58,9 @@ struct st_fp_variant_key
>>
>>     /** for ARB_color_buffer_float */
>>     GLuint clamp_color:1;
>> +
>> +   /** for ARB_sample_shading */
>> +   GLuint persample_shading:1;
>>  };
>>
>>
>> --
>> 1.8.5.5
>>
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