[Mesa-dev] [PATCH 3/4] i965/blorp: Support overriding destination alpha to 1.0.

[Kenneth Graunke]

Currently, Blorp requires the source and destination formats to be
equal.  However, we'd really like to be able to blit between XRGB and
ARGB formats; our BLT engine paths have supported this for a long time.

For ARGB -> XRGB, nothing needs to occur: the missing alpha is already
interpreted as 1.0.  For XRGB -> ARGB, we need to smash the alpha
channel to 1.0 when writing the destination colors.  This is fairly
straightforward with blending.

For now, this code is never used, as the source and destination formats
still must be equal.  The next patch will relax that restriction.

Signed-off-by: Kenneth Graunke <kenneth at whitecape.org>
---
 src/mesa/drivers/dri/i965/gen6_blorp.cpp | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/src/mesa/drivers/dri/i965/gen6_blorp.cpp b/src/mesa/drivers/dri/i965/gen6_blorp.cpp
index eb61898..3834ae2 100644
--- a/src/mesa/drivers/dri/i965/gen6_blorp.cpp
+++ b/src/mesa/drivers/dri/i965/gen6_blorp.cpp
@@ -283,6 +283,25 @@ gen6_blorp_emit_blend_state(struct brw_context *brw,
    blend->blend1.write_disable_b = false;
    blend->blend1.write_disable_a = false;
 
+   /* When blitting from an XRGB source to a ARGB destination, we need to
+    * interpret the missing channel as 1.0.  Blending can do that for us:
+    * we simply use the RGB values from the fragment shader ("source RGB"),
+    * but smash the alpha channel to 1.
+    */
+   if (_mesa_get_format_bits(params->dst.mt->format, GL_ALPHA_BITS) > 0 &&
+       _mesa_get_format_bits(params->src.mt->format, GL_ALPHA_BITS) == 0) {
+      blend->blend0.blend_enable = 1;
+      blend->blend0.ia_blend_enable = 1;
+
+      blend->blend0.blend_func = BRW_BLENDFUNCTION_ADD;
+      blend->blend0.ia_blend_func = BRW_BLENDFUNCTION_ADD;
+
+      blend->blend0.source_blend_factor = BRW_BLENDFACTOR_SRC_COLOR;
+      blend->blend0.dest_blend_factor = BRW_BLENDFACTOR_ZERO;
+      blend->blend0.ia_source_blend_factor = BRW_BLENDFACTOR_ONE;
+      blend->blend0.ia_dest_blend_factor = BRW_BLENDFACTOR_ZERO;
+   }
+
    return cc_blend_state_offset;
 }
 
-- 
1.8.1.2