Factor destination alpha into color blend output (flutter#36338)

bdero · web-flow · commit 7a9f3c75ccb5 · 2022-09-22T00:19:37.000-07:00
diff --git a/impeller/entity/entity.cc b/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/impeller/entity/entity.h b/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/impeller/entity/entity_unittests.cc b/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/impeller/entity/shaders/blending/advanced_blend.glsl b/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;
 }

Original file line number	Diff line number	Diff line change
`@@ -40,5 +40,5 @@ void main() {`
`40`	`40`
`41`	`41`	`vec3 blended = Blend(dst.rgb, src.rgb);`
`42`	`42`
`43`		`- frag_color = vec4(blended * src.a, src.a);`
	`43`	`+ frag_color = vec4(blended, 1) * src.a * dst.a;`
`44`	`44`	`}`