[Impeller] EntityPass should cover the whole screen for some blend modes (flutter/engine#35157)

engine/src/flutter/impeller/aiks/aiks_unittests.cc

@@ -252,6 +252,31 @@ TEST_P(AiksTest, CanRenderRadialGradient) {
   ASSERT_TRUE(OpenPlaygroundHere(canvas.EndRecordingAsPicture()));
 }
 
+TEST_P(AiksTest, BlendModeShouldCoverWholeScreen) {
+  Canvas canvas;
+  Paint paint;
+
+  paint.color = Color::Red();
+  canvas.DrawPaint(paint);
+
+  paint.blend_mode = Entity::BlendMode::kSourceOver;
+  canvas.SaveLayer(paint);
+
+  paint.color = Color::White();
+  canvas.DrawRect({100, 100, 400, 400}, paint);
+
+  paint.blend_mode = Entity::BlendMode::kSource;
+  canvas.SaveLayer(paint);
+
+  paint.color = Color::Blue();
+  canvas.DrawRect({200, 200, 200, 200}, paint);
+
+  canvas.Restore();
+  canvas.Restore();
+
+  ASSERT_TRUE(OpenPlaygroundHere(canvas.EndRecordingAsPicture()));
+}
+
 TEST_P(AiksTest, CanRenderGroupOpacity) {
   Canvas canvas;
 

engine/src/flutter/impeller/entity/entity.cc

@@ -42,6 +42,9 @@ std::optional<Rect> Entity::GetCoverage() const {
 }
 
 bool Entity::ShouldRender(const ISize& target_size) const {
+  if (BlendModeShouldCoverWholeScreen(blend_mode_)) {
+    return true;
+  }
   return contents_->ShouldRender(*this, target_size);
 }
 
@@ -73,6 +76,23 @@ Entity::BlendMode Entity::GetBlendMode() const {
   return blend_mode_;
 }
 
+bool Entity::BlendModeShouldCoverWholeScreen(BlendMode blend_mode) {
+  switch (blend_mode) {
+    case BlendMode::kClear:
+    case BlendMode::kSource:
+    case BlendMode::kSourceIn:
+    case BlendMode::kDestinationIn:
+    case BlendMode::kSourceOut:
+    case BlendMode::kDestinationOut:
+    case BlendMode::kDestinationATop:
+    case BlendMode::kXor:
+    case BlendMode::kModulate:
+      return true;
+    default:
+      return false;
+  }
+}
+
 bool Entity::Render(const ContentContext& renderer,
                     RenderPass& parent_pass) const {
   if (!contents_) {

engine/src/flutter/impeller/entity/entity.h

@@ -121,6 +121,8 @@ class Entity {
 
   bool Render(const ContentContext& renderer, RenderPass& parent_pass) const;
 
+  static bool BlendModeShouldCoverWholeScreen(BlendMode blend_mode);
+
  private:
   Matrix transformation_;
   std::shared_ptr<Contents> contents_;

engine/src/flutter/impeller/entity/entity_pass.cc

@@ -252,7 +252,10 @@ EntityPass::EntityResult EntityPass::GetEntityForElement(
 
     auto subpass_coverage =
         GetSubpassCoverage(*subpass, Rect::MakeSize(root_pass_size));
-
+    if (subpass->cover_whole_screen) {
+      subpass_coverage = Rect(
+          position, Size(pass_context.GetRenderTarget().GetRenderTargetSize()));
+    }
     if (backdrop_contents) {
       auto backdrop_coverage = backdrop_contents->GetCoverage(Entity{});
       if (backdrop_coverage.has_value()) {
@@ -501,6 +504,7 @@ void EntityPass::SetStencilDepth(size_t stencil_depth) {
 
 void EntityPass::SetBlendMode(Entity::BlendMode blend_mode) {
   blend_mode_ = blend_mode;
+  cover_whole_screen = Entity::BlendModeShouldCoverWholeScreen(blend_mode);
 }
 
 void EntityPass::SetBackdropFilter(std::optional<BackdropFilterProc> proc) {

engine/src/flutter/impeller/entity/entity_pass.h

@@ -114,7 +114,7 @@ class EntityPass {
   Matrix xformation_;
   size_t stencil_depth_ = 0u;
   Entity::BlendMode blend_mode_ = Entity::BlendMode::kSourceOver;
-
+  bool cover_whole_screen = false;
   /// This flag is set to `true` whenever an entity is added to the pass that
   /// requires reading the pass texture during rendering. This can happen in the
   /// following scenarios: