[Mesa-dev] [PATCH 1/2] radeonsi: simplify gl_FragCoord behavior

[Marek Olšák]

On Fri, Jan 8, 2016 at 12:35 PM, Axel Davy <axel.davy at ens.fr> wrote:
> On 08/01/2016 02:30, Marek Olšák wrote:
>>
>> From: Marek Olšák <marek.olsak at amd.com>
>>
>> It will become a system value, not an input.
>> ---
>>   src/gallium/drivers/radeonsi/si_state_shaders.c | 45
>> ++++++++++++-------------
>>   1 file changed, 22 insertions(+), 23 deletions(-)
>>
>> diff --git a/src/gallium/drivers/radeonsi/si_state_shaders.c
>> b/src/gallium/drivers/radeonsi/si_state_shaders.c
>> index 64adf69..460dda5 100644
>> --- a/src/gallium/drivers/radeonsi/si_state_shaders.c
>> +++ b/src/gallium/drivers/radeonsi/si_state_shaders.c
>> @@ -399,30 +399,29 @@ static void si_shader_ps(struct si_shader *shader)
>>         if (!pm4)
>>                 return;
>>   -     for (i = 0; i < info->num_inputs; i++) {
>> -               switch (info->input_semantic_name[i]) {
>> -               case TGSI_SEMANTIC_POSITION:
>> -                       /* SPI_BARYC_CNTL.POS_FLOAT_LOCATION
>> -                        * Possible vaules:
>> -                        * 0 -> Position = pixel center (default)
>> -                        * 1 -> Position = pixel centroid
>> -                        * 2 -> Position = at sample position
>> -                        */
>> -                       switch (info->input_interpolate_loc[i]) {
>> -                       case TGSI_INTERPOLATE_LOC_CENTROID:
>> -                               spi_baryc_cntl |=
>> S_0286E0_POS_FLOAT_LOCATION(1);
>> -                               break;
>> -                       case TGSI_INTERPOLATE_LOC_SAMPLE:
>> -                               spi_baryc_cntl |=
>> S_0286E0_POS_FLOAT_LOCATION(2);
>> -                               break;
>> -                       }
>> +       /* SPI_BARYC_CNTL.POS_FLOAT_LOCATION
>> +        * Possible vaules:
>> +        * 0 -> Position = pixel center
>> +        * 1 -> Position = pixel centroid
>> +        * 2 -> Position = at sample position
>> +        *
>> +        * From GLSL 4.5 specification, section 7.1:
>> +        *   "The variable gl_FragCoord is available as an input variable
>> from
>> +        *    within fragment shaders and it holds the window relative
>> coordinates
>> +        *    (x, y, z, 1/w) values for the fragment. If multi-sampling,
>> this
>> +        *    value can be for any location within the pixel, or one of
>> the
>> +        *    fragment samples. The use of centroid does not further
>> restrict
>> +        *    this value to be inside the current primitive."
>> +        *
>> +        * Meaning that centroid has no effect and we can return anything
>> within
>> +        * the pixel. Thus, return the value at sample position, because
>> that's
>> +        * the most accurate one shaders can get.
>> +        */
>> +       spi_baryc_cntl |= S_0286E0_POS_FLOAT_LOCATION(2);
>
>
> When combined with POS_FLOAT_ULC, in which situation could we see
> a difference in the POSITION value ?

No MSAA: the value is at the pixel center
MSAA, no sample shading: the value is at the sample position of any
visible sample (compatible with "centroid")
MSAA sample shading: the value is at the sample position of the computed sample

As far as I know, POS_FLOAT_ULC only offsets the value by a constant (0.5).

Marek