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

[Tobias Klausmann]

On 27.05.2015 18:28, Marek Olšák wrote:
> 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?

i think this should be rejected anyway (in the glsl?!), constraining vs 
output to be the same as gs input where the last definition is the valid 
one....

>   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.

Ah btw, if we follow Brian Paul, overlapping indexes are fine! (and it 
is way more intuitive to use for a shader developer)

>> 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