[Mesa-dev] [PATCH 19/53] st/nine: Clamp color inputs for ps <= 2.0 at ps level instead of vs

[Marek Olšák]

BTW, colors should indeed be clamped for SM 2.0 in the VS. The reason
is that old ps2.0 hardware used the R8G8B8A8_UNORM format for color
varyings, meaning that all colors were always clamped when they left
the vertex shader and this behavior couldn't be disabled. r300-r400
behave this way, so they don't need any clamping. There are 2 CAPs for
this:

PIPE_CAP_VERTEX_COLOR_UNCLAMPED
PIPE_CAP_VERTEX_COLOR_CLAMPED

If just PIPE_CAP_VERTEX_COLOR_CLAMPED is advertised, the colors are
clamped after the vertex shader and this cannot be changed.
(r300-r400)

If both CAPs are advertised, you can choose if you want clamping or
not by setting pipe_rasterizer_state::clamp_vertex_color. (r500)

If just PIPE_CAP_VERTEX_COLOR_UNCLAMPED is advertised, the colors are
not clamped and this cannot be changed. (r600 and later chips)

Marek

On Sun, Jan 11, 2015 at 10:30 AM, Axel Davy <axel.davy at ens.fr> wrote:
> On 07/01/2015 17:36, Axel Davy wrote :
>>
>> Nine code was clamping color outputs for vs < 3,
>> but msdn docs says it is done in the ps.
>> Wine seems to clamp them at the vs level.
>>
>> It makes more sense to clamp at vs level for performance,
>> but according to doc, ps 2.x shouldn't see clamping.
>>
>>
> We have found a game using ps 2.x that didn't work well anymore
> with this patch, so likely the documentation is wrong about ps 2.x
> behaviour.
>
> Since it is disallowed to use vs <= 2 with ps 3 and vs 3 with ps <= 2
> (except for software renderer),
> then it is better for performance to clamp color outputs at vs <= 2 level,
> which is what was doing
> previous code.
>
> So this patch is dropped.
>
> Axel Davy
>
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