[Impeller] Use minimal coverage for stencil restores after overdraw p…

…revention (#39358)

impeller/entity/contents/clip_contents.cc

@@ -136,6 +136,11 @@ ClipRestoreContents::ClipRestoreContents() = default;
 
 ClipRestoreContents::~ClipRestoreContents() = default;
 
+void ClipRestoreContents::SetRestoreCoverage(
+    std::optional<Rect> restore_coverage) {
+  restore_coverage_ = restore_coverage;
+}
+
 std::optional<Rect> ClipRestoreContents::GetCoverage(
     const Entity& entity) const {
   return std::nullopt;
@@ -164,19 +169,20 @@ bool ClipRestoreContents::Render(const ContentContext& renderer,
   auto options = OptionsFromPassAndEntity(pass, entity);
   options.stencil_compare = CompareFunction::kLess;
   options.stencil_operation = StencilOperation::kSetToReferenceValue;
+  options.primitive_type = PrimitiveType::kTriangleStrip;
   cmd.pipeline = renderer.GetClipPipeline(options);
   cmd.stencil_reference = entity.GetStencilDepth();
 
-  // Create a rect that covers the whole render target.
-  auto size = pass.GetRenderTargetSize();
+  // Create a rect that covers either the given restore area, or the whole
+  // render target texture.
+  auto ltrb = restore_coverage_.value_or(Rect(Size(pass.GetRenderTargetSize())))
+                  .GetLTRB();
   VertexBufferBuilder<VS::PerVertexData> vtx_builder;
   vtx_builder.AddVertices({
-      {Point(0.0, 0.0)},
-      {Point(size.width, 0.0)},
-      {Point(size.width, size.height)},
-      {Point(0.0, 0.0)},
-      {Point(size.width, size.height)},
-      {Point(0.0, size.height)},
+      {Point(ltrb[0], ltrb[1])},
+      {Point(ltrb[2], ltrb[1])},
+      {Point(ltrb[0], ltrb[3])},
+      {Point(ltrb[2], ltrb[3])},
   });
   cmd.BindVertices(vtx_builder.CreateVertexBuffer(pass.GetTransientsBuffer()));
 

impeller/entity/contents/clip_contents.h

@@ -55,6 +55,12 @@ class ClipRestoreContents final : public Contents {
 
   ~ClipRestoreContents();
 
+  /// @brief  The area on the pass texture where this clip restore will be
+  ///         applied. If unset, the entire pass texture will be restored.
+  ///
+  /// @note   This rectangle is not transformed by the entity's transformation.
+  void SetRestoreCoverage(std::optional<Rect> coverage);
+
   // |Contents|
   std::optional<Rect> GetCoverage(const Entity& entity) const override;
 
@@ -73,6 +79,8 @@ class ClipRestoreContents final : public Contents {
               RenderPass& pass) const override;
 
  private:
+  std::optional<Rect> restore_coverage_;
+
   FML_DISALLOW_COPY_AND_ASSIGN(ClipRestoreContents);
 };
 

impeller/entity/contents/linear_gradient_contents.cc

@@ -111,7 +111,9 @@ bool LinearGradientContents::RenderTexture(const ContentContext& renderer,
   }
 
   if (geometry_result.prevent_overdraw) {
-    return ClipRestoreContents().Render(renderer, entity, pass);
+    auto restore = ClipRestoreContents();
+    restore.SetRestoreCoverage(GetCoverage(entity));
+    return restore.Render(renderer, entity, pass);
   }
   return true;
 }
@@ -165,7 +167,9 @@ bool LinearGradientContents::RenderSSBO(const ContentContext& renderer,
   }
 
   if (geometry_result.prevent_overdraw) {
-    return ClipRestoreContents().Render(renderer, entity, pass);
+    auto restore = ClipRestoreContents();
+    restore.SetRestoreCoverage(GetCoverage(entity));
+    return restore.Render(renderer, entity, pass);
   }
   return true;
 }

impeller/entity/contents/radial_gradient_contents.cc

@@ -103,7 +103,9 @@ bool RadialGradientContents::RenderSSBO(const ContentContext& renderer,
   }
 
   if (geometry_result.prevent_overdraw) {
-    return ClipRestoreContents().Render(renderer, entity, pass);
+    auto restore = ClipRestoreContents();
+    restore.SetRestoreCoverage(GetCoverage(entity));
+    return restore.Render(renderer, entity, pass);
   }
   return true;
 }
@@ -166,7 +168,9 @@ bool RadialGradientContents::RenderTexture(const ContentContext& renderer,
   }
 
   if (geometry_result.prevent_overdraw) {
-    return ClipRestoreContents().Render(renderer, entity, pass);
+    auto restore = ClipRestoreContents();
+    restore.SetRestoreCoverage(GetCoverage(entity));
+    return restore.Render(renderer, entity, pass);
   }
   return true;
 }

impeller/entity/contents/runtime_effect_contents.cc

@@ -213,7 +213,9 @@ bool RuntimeEffectContents::Render(const ContentContext& renderer,
   pass.AddCommand(std::move(cmd));
 
   if (geometry_result.prevent_overdraw) {
-    return ClipRestoreContents().Render(renderer, entity, pass);
+    auto restore = ClipRestoreContents();
+    restore.SetRestoreCoverage(GetCoverage(entity));
+    return restore.Render(renderer, entity, pass);
   }
   return true;
 }

impeller/entity/contents/solid_color_contents.cc

@@ -83,7 +83,9 @@ bool SolidColorContents::Render(const ContentContext& renderer,
   }
 
   if (geometry_result.prevent_overdraw) {
-    return ClipRestoreContents().Render(renderer, entity, pass);
+    auto restore = ClipRestoreContents();
+    restore.SetRestoreCoverage(GetCoverage(entity));
+    return restore.Render(renderer, entity, pass);
   }
   return true;
 }

impeller/entity/contents/sweep_gradient_contents.cc

@@ -109,7 +109,9 @@ bool SweepGradientContents::RenderSSBO(const ContentContext& renderer,
   }
 
   if (geometry_result.prevent_overdraw) {
-    return ClipRestoreContents().Render(renderer, entity, pass);
+    auto restore = ClipRestoreContents();
+    restore.SetRestoreCoverage(GetCoverage(entity));
+    return restore.Render(renderer, entity, pass);
   }
   return true;
 }
@@ -173,7 +175,9 @@ bool SweepGradientContents::RenderTexture(const ContentContext& renderer,
   }
 
   if (geometry_result.prevent_overdraw) {
-    return ClipRestoreContents().Render(renderer, entity, pass);
+    auto restore = ClipRestoreContents();
+    restore.SetRestoreCoverage(GetCoverage(entity));
+    return restore.Render(renderer, entity, pass);
   }
   return true;
 }

impeller/entity/contents/tiled_texture_contents.cc

@@ -89,7 +89,9 @@ bool TiledTextureContents::Render(const ContentContext& renderer,
   }
 
   if (geometry_result.prevent_overdraw) {
-    return ClipRestoreContents().Render(renderer, entity, pass);
+    auto restore = ClipRestoreContents();
+    restore.SetRestoreCoverage(GetCoverage(entity));
+    return restore.Render(renderer, entity, pass);
   }
   return true;
 }