[Impeller] Provide the clear color to an advanced blend if it was optimized out

impeller/aiks/aiks_unittests.cc

@@ -4249,5 +4249,17 @@ TEST_P(AiksTest, MaskBlurWithZeroSigmaIsSkipped) {
   ASSERT_TRUE(OpenPlaygroundHere(canvas.EndRecordingAsPicture()));
 }
 
+TEST_P(AiksTest, AdvancedBlendWithClearColorOptimization) {
+  Canvas canvas;
+  canvas.DrawPaint(
+      {.color = {1.0, 1.0, 0.0, 1.0}, .blend_mode = BlendMode::kSource});
+
+  canvas.DrawRect(
+      Rect::MakeXYWH(0, 0, 200, 300),
+      {.color = {1.0, 0.0, 1.0, 1.0}, .blend_mode = BlendMode::kMultiply});
+
+  ASSERT_TRUE(OpenPlaygroundHere(canvas.EndRecordingAsPicture()));
+}
+
 }  // namespace testing
 }  // namespace impeller

impeller/entity/entity_pass.cc

@@ -19,6 +19,7 @@
 #include "impeller/entity/contents/filters/color_filter_contents.h"
 #include "impeller/entity/contents/filters/inputs/filter_input.h"
 #include "impeller/entity/contents/framebuffer_blend_contents.h"
+#include "impeller/entity/contents/solid_color_contents.h"
 #include "impeller/entity/contents/texture_contents.h"
 #include "impeller/entity/entity.h"
 #include "impeller/entity/inline_pass_context.h"
@@ -877,6 +878,7 @@ bool EntityPass::OnRender(
                                     !backdrop_filter_proc_;
   for (const auto& element : elements_) {
     // Skip elements that are incorporated into the clear color.
+    bool was_collapsing_clear_colors = is_collapsing_clear_colors;
     if (is_collapsing_clear_colors) {
       auto [entity_color, _] =
           ElementAsBackgroundColor(element, clear_color_size);
@@ -950,9 +952,20 @@ bool EntityPass::OnRender(
         FilterInput::Vector inputs = {
             FilterInput::Make(texture, result.entity.GetTransform().Invert()),
             FilterInput::Make(result.entity.GetContents())};
+        std::optional<Color> foreground_color;
+        std::optional<Rect> coverage;
+        if (was_collapsing_clear_colors) {
+          // If all previous elements were skipped due to clear color
+          // optimization, then provide the clear color as the foreground of the
+          // advanced blend.
+          foreground_color = GetClearColor(clear_color_size);
+          coverage = Rect::MakeSize(clear_color_size);
+        } else {
+          coverage = result.entity.GetCoverage();
+        }
         auto contents = ColorFilterContents::MakeBlend(
-            result.entity.GetBlendMode(), inputs);
-        contents->SetCoverageHint(result.entity.GetCoverage());
+            result.entity.GetBlendMode(), inputs, foreground_color);
+        contents->SetCoverageHint(coverage);
         result.entity.SetContents(std::move(contents));
         result.entity.SetBlendMode(BlendMode::kSource);
       }

impeller/entity/entity_unittests.cc

@@ -2443,6 +2443,14 @@ TEST_P(EntityTest, AdvancedBlendCoverageHintIsNotResetByEntityPass) {
                     "for advanced blends.";
   }
 
+  auto background_contents = std::make_shared<SolidColorContents>();
+  background_contents->SetGeometry(
+      Geometry::MakeRect(Rect::MakeXYWH(200, 200, 200, 200)));
+  background_contents->SetColor(Color::Blue());
+  Entity background_entity;
+  background_entity.SetTransform(Matrix::MakeScale(Vector3(2, 2, 1)));
+  background_entity.SetContents(background_contents);
+
   auto contents = std::make_shared<SolidColorContents>();
   contents->SetGeometry(Geometry::MakeRect(Rect::MakeXYWH(100, 100, 100, 100)));
   contents->SetColor(Color::Red());
@@ -2468,6 +2476,7 @@ TEST_P(EntityTest, AdvancedBlendCoverageHintIsNotResetByEntityPass) {
       RenderTarget::kDefaultColorAttachmentConfig, stencil_config);
   auto content_context = ContentContext(
       GetContext(), TypographerContextSkia::Make(), test_allocator);
+  pass->AddEntity(std::move(background_entity));
   pass->AddEntity(std::move(entity));
 
   EXPECT_TRUE(pass->Render(content_context, rt));