[Mesa-dev] [PATCH] st/mesa: Override blend factors involving alpha if it doesn't exist.

[Marek Olšák]

Yeah, this will be more complicated because it's per RT.

I suggest adding a PIPE_CAP for the hw capability to force DST_ALPHA
to 0, and applying this workaround only if the PIPE_CAP is 0.

Marek


On Sat, Aug 25, 2018 at 10:46 AM, Ilia Mirkin <imirkin at alum.mit.edu> wrote:
> You have to make sure to flip blend->independent_blend_enable to 1 in
> that case, and that will only work on some (well, most, but not all)
> hardware. Pre-something Tesla-era and I assume r600-era gpu's must
> have all rt's configured the same way.
>
> I'm also not 100% sure that it's OK to access the draw buffers in
> here, but Marek would know for sure -- he restructured the atoms logic
> a while back, and I haven't re-learned the new scheme.
>
>   -ilia
>
> On Sat, Aug 25, 2018 at 3:48 AM, Kenneth Graunke <kenneth at whitecape.org> wrote:
>> When faking an RGB format with an RGBA format, there may be a channel
>> of data containing garbage.  st/mesa already overrides texture swizzles
>> to replace the A channel with ONE.  This patch makes it override blend
>> factors to achieve a similar effect.
>>
>> It appears that st_update_blend is already called when the framebuffer
>> is changed, so it should be safe to look at the render target formats
>> without adding additional state dependencies.
>> ---
>>  src/mesa/state_tracker/st_atom_blend.c | 35 ++++++++++++++++++++++++++
>>  1 file changed, 35 insertions(+)
>>
>> diff --git a/src/mesa/state_tracker/st_atom_blend.c b/src/mesa/state_tracker/st_atom_blend.c
>> index 804de2f154f..3e6e4411107 100644
>> --- a/src/mesa/state_tracker/st_atom_blend.c
>> +++ b/src/mesa/state_tracker/st_atom_blend.c
>> @@ -41,6 +41,7 @@
>>
>>  #include "framebuffer.h"
>>  #include "main/blend.h"
>> +#include "main/glformats.h"
>>  #include "main/macros.h"
>>
>>  /**
>> @@ -142,6 +143,28 @@ blend_per_rt(const struct gl_context *ctx, unsigned num_cb)
>>     return GL_FALSE;
>>  }
>>
>> +/**
>> + * Modify blend function to force destination alpha to 1.0
>> + *
>> + * If \c function specifies a blend function that uses destination alpha,
>> + * replace it with a function that hard-wires destination alpha to 1.0.  This
>> + * is used when rendering to xRGB targets.
>> + */
>> +static enum pipe_blendfactor
>> +fix_xrgb_alpha(enum pipe_blendfactor factor)
>> +{
>> +   switch (factor) {
>> +   case PIPE_BLENDFACTOR_DST_ALPHA:
>> +      return PIPE_BLENDFACTOR_ONE;
>> +
>> +   case PIPE_BLENDFACTOR_INV_DST_ALPHA:
>> +   case PIPE_BLENDFACTOR_SRC_ALPHA_SATURATE:
>> +      return PIPE_BLENDFACTOR_ZERO;
>> +   default:
>> +      return factor;
>> +   }
>> +}
>> +
>>  void
>>  st_update_blend( struct st_context *st )
>>  {
>> @@ -210,6 +233,18 @@ st_update_blend( struct st_context *st )
>>              blend->rt[i].alpha_dst_factor =
>>                 translate_blend(ctx->Color.Blend[j].DstA);
>>           }
>> +
>> +         const struct gl_renderbuffer *rb =
>> +            ctx->DrawBuffer->_ColorDrawBuffers[i];
>> +
>> +         if (rb && !_mesa_base_format_has_channel(rb->_BaseFormat,
>> +                                                  GL_TEXTURE_ALPHA_TYPE)) {
>> +            struct pipe_rt_blend_state *rt = &blend->rt[i];
>> +            rt->rgb_src_factor = fix_xrgb_alpha(rt->rgb_src_factor);
>> +            rt->rgb_dst_factor = fix_xrgb_alpha(rt->rgb_dst_factor);
>> +            rt->alpha_src_factor = fix_xrgb_alpha(rt->alpha_src_factor);
>> +            rt->alpha_dst_factor = fix_xrgb_alpha(rt->alpha_dst_factor);
>> +         }
>>        }
>>     }
>>     else {
>> --
>> 2.18.0
>>
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