From 2682bdb32269a6a4e301384489944c02498d486c Mon Sep 17 00:00:00 2001
From: Brandon DeRosier <bdero@google.com>
Date: Thu, 22 Sep 2022 00:19:37 -0700
Subject: [PATCH] Factor destination alpha into color blend output
 (flutter/engine#36338)

---
 engine/src/flutter/impeller/entity/entity.cc                  | 3 ---
 engine/src/flutter/impeller/entity/entity.h                   | 4 ++--
 engine/src/flutter/impeller/entity/entity_unittests.cc        | 1 +
 .../impeller/entity/shaders/blending/advanced_blend.glsl      | 2 +-
 4 files changed, 4 insertions(+), 6 deletions(-)

diff --git a/engine/src/flutter/impeller/entity/entity.cc b/engine/src/flutter/impeller/entity/entity.cc
index 680dd6364a2..c7c054293f1 100644
--- a/engine/src/flutter/impeller/entity/entity.cc
+++ b/engine/src/flutter/impeller/entity/entity.cc
@@ -16,9 +16,6 @@
 
 namespace impeller {
 
-const BlendMode Entity::kLastPipelineBlendMode = BlendMode::kModulate;
-const BlendMode Entity::kLastAdvancedBlendMode = BlendMode::kLuminosity;
-
 Entity::Entity() = default;
 
 Entity::~Entity() = default;
diff --git a/engine/src/flutter/impeller/entity/entity.h b/engine/src/flutter/impeller/entity/entity.h
index 14481b29cfb..02a99015b04 100644
--- a/engine/src/flutter/impeller/entity/entity.h
+++ b/engine/src/flutter/impeller/entity/entity.h
@@ -19,8 +19,8 @@ class RenderPass;
 
 class Entity {
  public:
-  static const BlendMode kLastPipelineBlendMode;
-  static const BlendMode kLastAdvancedBlendMode;
+  static constexpr BlendMode kLastPipelineBlendMode = BlendMode::kModulate;
+  static constexpr BlendMode kLastAdvancedBlendMode = BlendMode::kLuminosity;
 
   /// An enum to define how to repeat, fold, or omit colors outside of the
   /// typically defined range of the source of the colors (such as the
diff --git a/engine/src/flutter/impeller/entity/entity_unittests.cc b/engine/src/flutter/impeller/entity/entity_unittests.cc
index ce7fdffa264..cd74a9889d7 100644
--- a/engine/src/flutter/impeller/entity/entity_unittests.cc
+++ b/engine/src/flutter/impeller/entity/entity_unittests.cc
@@ -662,6 +662,7 @@ TEST_P(EntityTest, BlendingModeOptions) {
     static_assert(b == BlendMode::kClear);  // Ensure the first item in
                                             // the switch is the first
                                             // item in the enum.
+    static_assert(Entity::kLastPipelineBlendMode == BlendMode::kModulate);
     switch (b) {
       case BlendMode::kClear:
         blend_mode_names.push_back("Clear");
diff --git a/engine/src/flutter/impeller/entity/shaders/blending/advanced_blend.glsl b/engine/src/flutter/impeller/entity/shaders/blending/advanced_blend.glsl
index 20d7b40a0ec..84e4781d99a 100644
--- a/engine/src/flutter/impeller/entity/shaders/blending/advanced_blend.glsl
+++ b/engine/src/flutter/impeller/entity/shaders/blending/advanced_blend.glsl
@@ -40,5 +40,5 @@ void main() {
 
   vec3 blended = Blend(dst.rgb, src.rgb);
 
-  frag_color = vec4(blended * src.a, src.a);
+  frag_color = vec4(blended, 1) * src.a * dst.a;
 }