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

[Olivier Galibert]

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.


> > Second question: how many samples should be shaded, and for which
> > coordinates?  What is the impact of depth testing failure?
> 
> As many as the user requested via glMinSampleShading, and the sample
> locations to choose seem to be up to the implementation.

Do you know what's usually expected?  Center of collapsed samples, one
of the samples, center of the pixel?


> > Third question: what happens when a variable has a "sample" qualifier
> > in the fragment shader?  Or "centroid"?
> 
> "When interpolating variables declared using sample in when MULTISAMPLE
> is enabled, the fragment shader will be invoked separately for each (!)
> covered sample and the variable will be sampled at the corresponding
> sample point."

So a "sample" anywhere means full supersampling, ok.

Thanks,

  OG.