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

[Marek Olšák]

On Wed, Apr 29, 2015 at 12:44 PM, Jose Fonseca <jfonseca at vmware.com> wrote:
> I think there are two different things here: one is the driver internally
> fakes BGRX with BGRA, and obviously it's the pipe driver that needs to fix
> up alpha channel blending to simulate it's one.
>
> The other is the state tracker is faking BGRX with BGRA, and in that case,
> it's the state tracker that needs to do the fix up (as the pipe has no way
> to know if that's intentional or not).

That's a fair point. On the other hand, I don't see why drivers can't
support BGRX even if they can force A=1 for texturing only.

>
> Or are you saying that state trackers should never attempt to fulfill BGRX
> formats with BGRA, and that every driver needs to workaround this
> internally?

Not necessarily.

>
> I don't fill too strongly about this, but it wouldn't sound consistent.
> Nowadays have a pipe cap for every single little thing out there.  I don't
> see a reason to treat render target formats differently. Otherwise something
> that can be easily done once in the state tracker now needs to be replicated
> in every driver.
>
>
> But you have a point about _NEW_BUFFERS.  It could be avoided.  We could
> introduce a new flag _NEW_BUFFERS_EMULATED, set when fbo's with emulated
> formats (e.g., backing BGRX with BGRA), so that drivers that support (or
> workaround) these things internally don't .  On the other hand, _NEW_BUFFERS
> probably doesn't change that frequently, blend state probably changes much
> more often.

A new CAP would work. There is an easier way to do it though, see the
attached patch (assuming blend_force_dst_alpha_to_one is set
correctly).

Marek
-------------- next part --------------
diff --git a/src/mesa/state_tracker/st_atom_blend.c b/src/mesa/state_tracker/st_atom_blend.c
index 6bb4077..0d1542f 100644
--- a/src/mesa/state_tracker/st_atom_blend.c
+++ b/src/mesa/state_tracker/st_atom_blend.c
@@ -286,7 +286,7 @@ const struct st_tracked_state st_update_blend = {
    "st_update_blend",					/* name */
    {							/* dirty */
       (_NEW_COLOR | _NEW_MULTISAMPLE),  /* XXX _NEW_BLEND someday? */	/* mesa */
-      0,						/* st */
+      ST_NEW_BLEND,					/* st */
    },
    update_blend,					/* update */
 };
diff --git a/src/mesa/state_tracker/st_context.c b/src/mesa/state_tracker/st_context.c
index 5fe132a..d7ebe22 100644
--- a/src/mesa/state_tracker/st_context.c
+++ b/src/mesa/state_tracker/st_context.c
@@ -95,6 +95,10 @@ void st_invalidate_state(struct gl_context * ctx, GLuint new_state)
       st->dirty.st |= ST_NEW_VERTEX_PROGRAM;
    }
 
+   if (st->blend_force_dst_alpha_to_one && (new_state & _NEW_BUFFERS)) {
+      st->dirty.st |= ST_NEW_BLEND;
+   }
+
    st->dirty.mesa |= new_state;
    st->dirty.st |= ST_NEW_MESA;
 
diff --git a/src/mesa/state_tracker/st_context.h b/src/mesa/state_tracker/st_context.h
index 8a9504b..a9c861d 100644
--- a/src/mesa/state_tracker/st_context.h
+++ b/src/mesa/state_tracker/st_context.h
@@ -53,7 +53,7 @@ struct u_upload_mgr;
 #define ST_NEW_FRAGMENT_PROGRAM        (1 << 1)
 #define ST_NEW_VERTEX_PROGRAM          (1 << 2)
 #define ST_NEW_FRAMEBUFFER             (1 << 3)
-/* gap, re-use it */
+#define ST_NEW_BLEND                   (1 << 4)
 #define ST_NEW_GEOMETRY_PROGRAM        (1 << 5)
 #define ST_NEW_VERTEX_ARRAYS           (1 << 6)
 #define ST_NEW_RASTERIZER              (1 << 7)
@@ -89,6 +89,7 @@ struct st_context
    struct draw_stage *rastpos_stage;  /**< For glRasterPos */
    GLboolean clamp_frag_color_in_shader;
    GLboolean clamp_vert_color_in_shader;
+   GLboolean blend_force_dst_alpha_to_one;
    boolean has_stencil_export; /**< can do shader stencil export? */
    boolean has_time_elapsed;
    boolean has_shader_model3;