[Nouveau] [RFC PATCH 00/11] Implement ARB_cull_distance

[Marek Olšák]

Another thing to consider is linking shaders that occur before the
rasterizer (e.g. any two shaders from VS->TCS->TES->GS). The maximum
number of written distances is still 8, but what happens if VS writes
1x clip and 7x cull and GS reads 8x clip and no cull? In this case
it's basically a generic varying. How is linking separate shaders
supposed to work with one combined clip-or-cull array? It doesn't seem
to be possible.

Marek

On Mon, May 25, 2015 at 5:07 PM, Ilia Mirkin <imirkin at alum.mit.edu> wrote:
> On Mon, May 25, 2015 at 9:40 AM, Tobias Klausmann
> <tobias.johannes.klausmann at mni.thm.de> wrote:
>>
>> On 25.05.2015 07:17, Dave Airlie wrote:
>>>
>>> On 25 May 2015 at 08:11, Marek Olšák <maraeo at gmail.com> wrote:
>>>>
>>>> It's the same on Radeon. There are 2x ClipOrCullDistance output
>>>> vectors and a mask saying it should clip or cull or do nothing.
>>>>
>>>> Marek
>>>>
>>> My thinking was gallium should have a single semantic and a mask in
>>> the shader definition maybe.
>>>
>>> though it doesn't solve the does nvidia do the right thing with
>>> cull[0] and clip[0], and what is the right thing.
>>>
>>> Dave.
>>
>>
>> I'm still convinced that both clip[0] and cull[0] should be possible. Plus i
>> have written a shader_test for this a while ago which you pushed to piglit
>> (fs-cull-and-clip-distance-different.shader_test). If i remember right
>> nvidia passed that test just fine.
>
> My take (and note that I last read the extension many months ago) is
> that you're supposed to figure out the max gl_ClipDistance[] written,
> and then write all your cull distances above that. So if you, e.g.,
> have something like
>
> gl_ClipDistance[5] = 1;
> gl_CullDistance[0] = 1;
>
> Then it would decide that there are 6 clip distances (or if there's an
> explicit out float gl_ClipDistance[n], then use that), and 1 cull
> distance. In the TGSI, I'm thinking this might look approximately like
>
> PROPERTY CULL_MASK (1<<6)
> DCL OUT[0], CLIPDIST[0]
> DCL OUT[1], CLIPDIST[1]
> MOV OUT[1].y, 1 (clip distance[5])
> MOV OUT[1].z, 1 (cull distance[0])
>
> Then basically you'd have
>
> (rast->clip_enable & shader->actual_clip_writes_mask) | cull_mask =
> the enabled distances
> cull_mask = cull mask
>
> This would work *very* well for nouveau, not sure how suitable it is
> for other hardware.
>
> Cheers,
>
>   -ilia