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

[Ilia Mirkin]

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