[Impeller] distinguish between no clear color and transparent black clear color.

Author: jonahwilliams

impeller/aiks/aiks_unittests.cc

@@ -4555,5 +4555,24 @@ TEST_P(AiksTest, GaussianBlurWithoutDecalSupport) {
   ASSERT_TRUE(OpenPlaygroundHere(canvas.EndRecordingAsPicture()));
 }
 
+TEST_P(AiksTest, SubpassWithClearColorOptimization) {
+  Canvas canvas;
+
+  // Use a non-srcOver blend mode to ensure that we don't detect this as an
+  // opacity peephole optimization.
+  canvas.SaveLayer(
+      {.color = Color::Blue().WithAlpha(0.5), .blend_mode = BlendMode::kSource},
+      Rect::MakeLTRB(0, 0, 200, 200));
+  canvas.DrawPaint(
+      {.color = Color::BlackTransparent(), .blend_mode = BlendMode::kSource});
+  canvas.Restore();
+
+  canvas.SaveLayer(
+      {.color = Color::Blue(), .blend_mode = BlendMode::kDestinationOver});
+  canvas.Restore();
+
+  ASSERT_TRUE(OpenPlaygroundHere(canvas.EndRecordingAsPicture()));
+}
+
 }  // namespace testing
 }  // namespace impeller

impeller/aiks/paint_pass_delegate.cc

@@ -10,7 +10,6 @@
 #include "impeller/entity/contents/texture_contents.h"
 #include "impeller/entity/entity_pass.h"
 #include "impeller/geometry/color.h"
-#include "impeller/geometry/path_builder.h"
 
 namespace impeller {
 

impeller/entity/entity_pass.cc

@@ -342,7 +342,8 @@ bool EntityPass::Render(ContentContext& renderer,
   if (reads_from_onscreen_backdrop) {
     auto offscreen_target =
         CreateRenderTarget(renderer, root_render_target.GetRenderTargetSize(),
-                           GetClearColor(render_target.GetRenderTargetSize()));
+                           GetClearColor(render_target.GetRenderTargetSize())
+                               .value_or(Color::BlackTransparent()));
 
     if (!OnRender(renderer,  // renderer
                   capture,   // capture
@@ -447,7 +448,8 @@ bool EntityPass::Render(ContentContext& renderer,
   }
 
   // Set up the clear color of the root pass.
-  color0.clear_color = GetClearColor(render_target.GetRenderTargetSize());
+  color0.clear_color = GetClearColor(render_target.GetRenderTargetSize())
+                           .value_or(Color::BlackTransparent());
   root_render_target.SetColorAttachment(color0, 0);
 
   EntityPassTarget pass_target(
@@ -600,9 +602,10 @@ EntityPass::EntityResult EntityPass::GetEntityForElement(
     }
 
     auto subpass_target = CreateRenderTarget(
-        renderer,                               // renderer
-        subpass_size,                           // size
-        subpass->GetClearColor(subpass_size));  // clear_color
+        renderer,      // renderer
+        subpass_size,  // size
+        subpass->GetClearColor(subpass_size)
+            .value_or(Color::BlackTransparent()));  // clear_color
 
     if (!subpass_target.IsValid()) {
       VALIDATION_LOG << "Subpass render target is invalid.";
@@ -845,8 +848,7 @@ bool EntityPass::OnRender(
   }
   auto clear_color_size = pass_target.GetRenderTarget().GetRenderTargetSize();
 
-  if (!collapsed_parent_pass &&
-      !GetClearColor(clear_color_size).IsTransparent()) {
+  if (!collapsed_parent_pass && GetClearColor(clear_color_size).has_value()) {
     // Force the pass context to create at least one new pass if the clear color
     // is present.
     pass_context.GetRenderPass(pass_depth);
@@ -1139,21 +1141,25 @@ void EntityPass::SetBlendMode(BlendMode blend_mode) {
   flood_clip_ = Entity::IsBlendModeDestructive(blend_mode);
 }
 
-Color EntityPass::GetClearColor(ISize target_size) const {
-  Color result = Color::BlackTransparent();
+std::optional<Color> EntityPass::GetClearColor(ISize target_size) const {
   if (backdrop_filter_proc_) {
-    return result;
+    return std::nullopt;
   }
 
+  std::optional<Color> result = std::nullopt;
   for (const Element& element : elements_) {
     auto [entity_color, blend_mode] =
         ElementAsBackgroundColor(element, target_size);
     if (!entity_color.has_value()) {
       break;
     }
-    result = result.Blend(entity_color.value(), blend_mode);
+    result = result.value_or(Color::BlackTransparent())
+                 .Blend(entity_color.value(), blend_mode);
+  }
+  if (result.has_value()) {
+    return result->Premultiply();
   }
-  return result.Premultiply();
+  return result;
 }
 
 void EntityPass::SetBackdropFilter(BackdropFilterProc proc) {

impeller/entity/entity_pass.h

@@ -136,7 +136,11 @@ class EntityPass {
 
   void SetBlendMode(BlendMode blend_mode);
 
-  Color GetClearColor(ISize size = ISize::Infinite()) const;
+  /// @brief Return the clear color of the pass entities.
+  ///
+  /// If the returned value is std::nullopt, the clear color can be assumed to
+  /// be transparent black.
+  std::optional<Color> GetClearColor(ISize size = ISize::Infinite()) const;
 
   void SetBackdropFilter(BackdropFilterProc proc);