[Mesa-dev] [PATCH 1/2] st/mesa: adjust blending modes if we don't have destination alpha

[Ilia Mirkin]

On Tue, Apr 28, 2015 at 7:52 PM, Brian Paul <brianp at vmware.com> wrote:
> On 04/28/2015 04:35 PM, Ilia Mirkin wrote:
>>
>> On Tue, Apr 28, 2015 at 6:26 PM, Ilia Mirkin <imirkin at alum.mit.edu> wrote:
>>>
>>> Reviewed-by: Ilia Mirkin <imirkin at alum.mit.edu>
>>
>>
>> aaaactually....
>>
>>>
>>> Awesome! Now I can go remove this set of hacks from freedreno. And
>>> this fixes the same issue in nouveau. Thanks for doing it the real way
>>> :)
>>>
>>> On Tue, Apr 28, 2015 at 6:16 PM, Brian Paul <brianp at vmware.com> wrote:
>>>>
>>>> If the user requested a GL_RGB texture but the driver actually allocated
>>>> an RGBA texture, the alpha values in the texture may not be defined.
>>>>
>>>> If we later bind the texture as a color target and try to blend into
>>>> it with GL_DST_ALPHA or GL_ONE_MINUS_DST_ALPHA we may blend with
>>>> undefined alpha values when, in fact, the dest alpha value should be
>>>> one.
>>>> So replace GL_DST_ALPHA/GL_ONE_MINUS_DST_ALPHA with GL_ONE/GL_ZERO.
>>>>
>>>> Fixes the piglit fbo-blending-formats test for some GL_RGB formats
>>>> with the VMware driver.  Also tested with llvmpipe.
>>>> ---
>>>>   src/mesa/state_tracker/st_atom_blend.c | 38
>>>> +++++++++++++++++++++++++---------
>>>>   1 file changed, 28 insertions(+), 10 deletions(-)
>>>>
>>>> diff --git a/src/mesa/state_tracker/st_atom_blend.c
>>>> b/src/mesa/state_tracker/st_atom_blend.c
>>>> index 6bb4077..30bff7a 100644
>>>> --- a/src/mesa/state_tracker/st_atom_blend.c
>>>> +++ b/src/mesa/state_tracker/st_atom_blend.c
>>>> @@ -44,10 +44,21 @@
>>>>   /**
>>>>    * Convert GLenum blend tokens to pipe tokens.
>>>>    * Both blend factors and blend funcs are accepted.
>>>> + * \param destBaseFormat  the base format of the render target, such as
>>>> + *                        GL_RGBA, GL_RGB, GL_RED, GL_ALPHA, etc.
>>>>    */
>>>>   static GLuint
>>>> -translate_blend(GLenum blend)
>>>> +translate_blend(GLenum blend, GLenum destBaseFormat)
>>>>   {
>>>> +   /* If we don't have destination alpha and the blend factor is either
>>>> +    * GL_DST_ALPHA or GL_ONE_MINUS_DST_ALPHA then we use
>>>> +    * PIPE_BLENDFACTOR_ONE or _ZERO instead.
>>>> +    */
>>>> +   const bool haveDstA = (destBaseFormat == GL_RGBA ||
>>>> +                          destBaseFormat == GL_ALPHA ||
>>>> +                          destBaseFormat == GL_INTENSITY ||
>>>> +                          destBaseFormat == GL_LUMINANCE_ALPHA);
>>>> +
>>>>      switch (blend) {
>>>>      /* blend functions */
>>>>      case GL_FUNC_ADD:
>>>> @@ -69,7 +80,7 @@ translate_blend(GLenum blend)
>>>>      case GL_SRC_ALPHA:
>>>>         return PIPE_BLENDFACTOR_SRC_ALPHA;
>>>>      case GL_DST_ALPHA:
>>>> -      return PIPE_BLENDFACTOR_DST_ALPHA;
>>>> +      return haveDstA ? PIPE_BLENDFACTOR_DST_ALPHA :
>>>> PIPE_BLENDFACTOR_ONE;
>>>>      case GL_DST_COLOR:
>>>>         return PIPE_BLENDFACTOR_DST_COLOR;
>>>>      case GL_SRC_ALPHA_SATURATE:
>>>> @@ -91,7 +102,7 @@ translate_blend(GLenum blend)
>>>>      case GL_ONE_MINUS_DST_COLOR:
>>>>         return PIPE_BLENDFACTOR_INV_DST_COLOR;
>>>>      case GL_ONE_MINUS_DST_ALPHA:
>>>> -      return PIPE_BLENDFACTOR_INV_DST_ALPHA;
>>>> +      return haveDstA ? PIPE_BLENDFACTOR_INV_DST_ALPHA :
>>>> PIPE_BLENDFACTOR_ZERO;
>>>>      case GL_ONE_MINUS_CONSTANT_COLOR:
>>>>         return PIPE_BLENDFACTOR_INV_CONST_COLOR;
>>>>      case GL_ONE_MINUS_CONSTANT_ALPHA:
>>>> @@ -208,14 +219,21 @@ update_blend( struct st_context *st )
>>>>      else if (ctx->Color.BlendEnabled) {
>>>>         /* blending enabled */
>>>>         for (i = 0, j = 0; i < num_state; i++) {
>>>> +         const struct gl_renderbuffer *rb;
>>>> +         GLenum baseFormat;
>>>>
>>>>            blend->rt[i].blend_enable = (ctx->Color.BlendEnabled >> i) &
>>>> 0x1;
>>>>
>>>>            if (ctx->Extensions.ARB_draw_buffers_blend)
>>>>               j = i;
>>>>
>>>> +         /* _NEW_BUFFERS */
>>>> +         /* Get the base format of the render target */
>>>> +         rb = ctx->DrawBuffer->_ColorDrawBuffers[j];
>>
>>
>> That's the wrong render target, no? You need the i'th render target.
>
>
> Ah, I think you're right.
>
> BTW, I think there's more i/j mix-ups in this code (independent of this
> patch).  I'll send a separate patch for that after I check the specs.
>
>
>> And what happens if I'm not using independent blend but one of the
>> RT's is RGB while the other is RGBA?
>
>
> Yeah, there's no simple fix for that, AFAIK.

Well, presumably if the driver supports independent blend, you could
turn that on? If the driver doesn't do independent blend, then you're
SOL, but you should at least leave the DST_ALPHA setting alone...