[Mesa-dev] [PATCH 1/4] gallium: add pipe_surface::alpha_one field

[Nicolai Hähnle]

On 24.06.2016 16:40, Brian Paul wrote:
> On 06/24/2016 03:56 AM, Nicolai Hähnle wrote:
>> On 24.06.2016 04:07, Brian Paul wrote:
>>> If the user requests an RGB drawing surface but we actually create an
>>> RGBA surface, we need it to act as if A=1.  For blending, this means
>>> adjusting the blending terms to use 1/0 instead of
>>> DST_ALPHA/INV_DST_ALPHA.
>>> Drivers can use this flag to determine when that's needed.
>>
>> Having the bit is fine, but shouldn't there be a PIPE_CAP for it?
>
> No.  What would the state tracker do differently if there was a cap?
>
> If there's a driver currently simulating RGB with RGBA they're probably
> already doing blending wrong for the cases in question.  The new flags
> gives the driver a chance of fixing this.  It can be ignored otherwise.
 >
> And keep in mind that this is a corner case that should never come up in
> practice: if the app is blending into an RGB surface using blending
> terms that involve destination alpha, that's kind of senseless.
>
> The piglit fbo-blending-formats test hits this, however.

Those are passing with radeonsi.

I guess what happens is that RGB gets mapped to RGBX (as per Roland), 
and then we do have code in the driver that sets a DST_ALPHA_1 bit in 
that case which specifically tells the hardware to treat alpha as being 
always one. (So the bit would be trivial to implement for us, but now 
that I think about it again, it does seem a bit redundant with the RGBX 
formats.)

Nicolai

>
> -Brian
>
>
>>
>> Nicolai
>>
>>> A previous patch I posted last year did this entirely in the state
>>> tracker
>>> but it involved making blend state dependent on the framebuffer state.
>>> This approach avoids that dependency.
>>> ---
>>>   src/gallium/include/pipe/p_state.h | 1 +
>>>   1 file changed, 1 insertion(+)
>>>
>>> diff --git a/src/gallium/include/pipe/p_state.h
>>> b/src/gallium/include/pipe/p_state.h
>>> index 9c69355..8b0c3a2 100644
>>> --- a/src/gallium/include/pipe/p_state.h
>>> +++ b/src/gallium/include/pipe/p_state.h
>>> @@ -387,6 +387,7 @@ struct pipe_surface
>>>      unsigned height;              /**< logical height in pixels */
>>>
>>>      unsigned writable:1;          /**< writable shader resource */
>>> +   unsigned alpha_one:1;    /**< Should an RGBA surface should act
>>> like RGB? */
>>>
>>>      union {
>>>         struct {
>>>
>