[Mesa-dev] RFC: ARB_sample_shading in gallium

[Ilia Mirkin]

On Fri, Mar 28, 2014 at 5:47 PM, Roland Scheidegger <sroland at vmware.com> wrote:
> Am 28.03.2014 22:18, schrieb Ilia Mirkin:
>> Hey guys,
>>
>> I was thinking of taking a shot at implementing ARB_sample_shading for
>> nv50 (well, nva3-nva8) this weekend. One of the issues is that it's
>> not implemented in gallium at all right now, so I need to pipe it
>> through somehow. I believe that the only piece of data that needs to
>> be piped through is the value returned by
>> _mesa_get_min_invocations_per_fragment, which is a function of the fp,
>> the drawbuffer, and the MS state. When that value is > 1, sample
>> shading is effectively enabled. (I guess even when it's == 1, things
>> like gl_SampleID still need to work, perhaps it's worth adding a
>> separate enabled bit too.)
>>
>> Should this single integer get its own set_* callback, similar to
>> set_sample_mask, or should it be included somewhere, e.g.
>> pipe_framebuffer_state? Or even added to the set_sample_mask call?
>>
>
> Would something like in d3d10.1 work where you simply say that inputs
> are interpolated at sample frequency? That way you can also have some
> inputs which are not interpolated at sample frequency (I thought there's
> opengl functionality for this too somewhere - even if not I'd really
> like to have that functionality in gallium). It would just need new
> interpolation mode enums.
> Though I guess this does not fully cover ARB_sample_shading - this
> extension allows you for instance to have msaa 4x, but run fs at 2x (I
> could be wrong but I don't think you can do that in d3d, I don't know if
> hw can do it presumably some can otherwise it wouldn't be in the
> extension, though it is definitely worded in a way that makes it
> possible to just run at full sample frequency).

I have 0 familiarity with d3d, but it does indeed seem like part of
the point of ARB_sample_shading is to run on less than 100% of the
samples. This appears to be supported by NVA3+ hardware based on our
current docs in rnndb, although the current piglit tests don't really
exercise all the functionality. [I haven't checked, but I assume NVC0+
as well.] Although only 1/2/4/8 are supported, based on those docs
(e.g. you can't tell it to run on 5 samples).

An alternative to passing in the result of
_mesa_get_min_invocations_per_fragment is to just pass the percentage
(which, I guess for D3D10.1 would either be 0 or 100?), and redoing
the calculation inside of gallium based on the same criteria.

  -ilia