[Mesa-dev] Clarifications w.r.t MSAA

[Christoph Bumiller]

On 06/12/2012 06:52 PM, Jerome Glisse wrote:
> On Tue, Jun 12, 2012 at 8:39 AM, Christoph Bumiller
> <e0425955 at student.tuwien.ac.at> wrote:
>> On 06/12/2012 02:25 PM, Olivier Galibert wrote:
>>> On Tue, Jun 12, 2012 at 01:50:08PM +0200, Christoph Bumiller wrote:
>>>>> First question: how many depths should be computed, and for which
>>>>> coordinates? Which of these values is associated with which sample?
>>>>
>>>> One for each sample point. The depth buffer will be multisampled as well.
>>>> Coverage sampling (CSAA) where you have extra coverage samples that do
>>>> NOT (necessarily) correspond to color sample locations are not covered
>>>> by the GL spec, it's vendor-specific.
>>>
>>> Ok.  So that means that if the shader writes z, you have to do full
>>> supersampling then.
>>>
>>
>> No, I don't think that's the case. You get per-sample depth values if
>> you use fixed-pipe depth, but shader-computed depth should simply be
>> replicated (to all samples covered by the shader invocation), like color
>> outputs.
> 
> I don't think thats how it wors, each sample will have its color and
> depth value no matter if fixed pipeline or not. When resolving the

Sorry, "fixed-pipe" was misleading, I meant the z-value from the
rasterizer (which can be regarded as fixed functionality), not "without
(custom) shaders".

If the shader is only invoked once for each fragment (i.e.
MinSampleShading == 1), all the samples that belong to that fragment
will share the same color and depth values.

> msaa surface, you only use the sample that cover the surface to make
> the average.
> 
> Anyway that's my understanding.
> 
> Cheers,
> Jerome
>