conditionally save stencil

impeller/aiks/canvas.cc

@@ -379,6 +379,7 @@ void Canvas::ClipGeometry(std::unique_ptr<Geometry> geometry,
   entity.SetClipDepth(GetClipDepth());
 
   GetCurrentPass().AddEntity(entity);
+  GetCurrentPass().ContainsClip();
 
   ++xformation_stack_.back().clip_depth;
   xformation_stack_.back().contains_clips = true;

impeller/entity/entity_pass.cc

@@ -359,7 +359,7 @@ bool EntityPass::Render(ContentContext& renderer,
   // there's no need to set up a stencil attachment on the root render target.
   if (!supports_onscreen_backdrop_reads && reads_from_onscreen_backdrop) {
     auto offscreen_target = CreateRenderTarget(
-        renderer, root_render_target.GetRenderTargetSize(), true,
+        renderer, root_render_target.GetRenderTargetSize(), clips_applied_,
         GetClearColor(render_target.GetRenderTargetSize()));
 
     if (!OnRender(renderer,  // renderer
@@ -500,7 +500,6 @@ EntityPass::EntityResult EntityPass::GetEntityForElement(
   //--------------------------------------------------------------------------
   /// Setup entity element.
   ///
-
   if (const auto& entity = std::get_if<Entity>(&element)) {
     element_entity = *entity;
     element_entity.SetCapture(capture.CreateChild("Entity"));
@@ -512,16 +511,15 @@ EntityPass::EntityResult EntityPass::GetEntityForElement(
           Matrix::MakeTranslation(Vector3(-global_pass_position)) *
           element_entity.GetTransformation());
     }
+    return EntityPass::EntityResult::Success(element_entity);
   }
 
   //--------------------------------------------------------------------------
   /// Setup subpass element.
   ///
-
-  else if (const auto& subpass_ptr =
-               std::get_if<std::unique_ptr<EntityPass>>(&element)) {
+  if (const auto& subpass_ptr =
+          std::get_if<std::unique_ptr<EntityPass>>(&element)) {
     auto subpass = subpass_ptr->get();
-
     if (subpass->delegate_->CanElide()) {
       return EntityPass::EntityResult::Skip();
     }
@@ -691,11 +689,10 @@ EntityPass::EntityResult EntityPass::GetEntityForElement(
     element_entity.SetTransformation(subpass_texture_capture.AddMatrix(
         "Transform", Matrix::MakeTranslation(Vector3(subpass_coverage->origin -
                                                      global_pass_position))));
-  } else {
-    FML_UNREACHABLE();
-  }
 
-  return EntityPass::EntityResult::Success(element_entity);
+    return EntityPass::EntityResult::Success(element_entity);
+  }
+  FML_UNREACHABLE();
 }
 
 bool EntityPass::RenderElement(Entity& element_entity,
@@ -854,8 +851,9 @@ bool EntityPass::OnRender(
   TRACE_EVENT0("impeller", "EntityPass::OnRender");
 
   auto context = renderer.GetContext();
-  InlinePassContext pass_context(
-      context, pass_target, GetTotalPassReads(renderer), collapsed_parent_pass);
+  InlinePassContext pass_context(context, pass_target,
+                                 GetTotalPassReads(renderer), clips_applied_,
+                                 collapsed_parent_pass);
   if (!pass_context.IsValid()) {
     VALIDATION_LOG << SPrintF("Pass context invalid (Depth=%d)", pass_depth);
     return false;

impeller/entity/entity_pass.h

@@ -162,6 +162,10 @@ class EntityPass {
   std::optional<Rect> GetElementsCoverage(
       std::optional<Rect> coverage_limit) const;
 
+  void ContainsClip() {
+    clips_applied_ = true;
+  }
+
  private:
   struct EntityResult {
     enum Status {
@@ -297,6 +301,12 @@ class EntityPass {
   uint32_t advanced_blend_reads_from_pass_texture_ = 0;
   uint32_t backdrop_filter_reads_from_pass_texture_ = 0;
 
+  /// The number of clips applied in the current entity pass.
+  ///
+  /// If this value is zero, then the stencil buffer does not have to be stored
+  /// between backdrop filters.
+  bool clips_applied_ = false;
+
   uint32_t GetTotalPassReads(ContentContext& renderer) const;
 
   BackdropFilterProc backdrop_filter_proc_ = nullptr;

impeller/entity/inline_pass_context.cc

@@ -18,10 +18,12 @@ InlinePassContext::InlinePassContext(
     std::shared_ptr<Context> context,
     EntityPassTarget& pass_target,
     uint32_t pass_texture_reads,
+    bool contains_clips,
     std::optional<RenderPassResult> collapsed_parent_pass)
     : context_(std::move(context)),
       pass_target_(pass_target),
       total_pass_reads_(pass_texture_reads),
+      contains_clips_(contains_clips),
       is_collapsed_(collapsed_parent_pass.has_value()) {
   if (collapsed_parent_pass.has_value()) {
     pass_ = collapsed_parent_pass.value().pass;
@@ -142,15 +144,15 @@ InlinePassContext::RenderPassResult InlinePassContext::GetRenderPass(
 
   // Only clear the stencil if this is the very first pass of the
   // layer.
-  stencil->load_action =
-      pass_count_ > 0 ? LoadAction::kLoad : LoadAction::kClear;
+  stencil->load_action = (pass_count_ > 0 && contains_clips_)
+                             ? LoadAction::kLoad
+                             : LoadAction::kClear;
   // If we're on the last pass of the layer, there's no need to store the
   // stencil because nothing needs to read it.
-  stencil->store_action = pass_count_ == total_pass_reads_
+  stencil->store_action = (pass_count_ == total_pass_reads_ || !contains_clips_)
                               ? StoreAction::kDontCare
                               : StoreAction::kStore;
   pass_target_.target_.SetStencilAttachment(stencil.value());
-
   pass_target_.target_.SetColorAttachment(color0, 0);
 
   pass_ = command_buffer_->CreateRenderPass(pass_target_.GetRenderTarget());

impeller/entity/inline_pass_context.h

@@ -22,6 +22,7 @@ class InlinePassContext {
       std::shared_ptr<Context> context,
       EntityPassTarget& pass_target,
       uint32_t pass_texture_reads,
+      bool contains_clips,
       std::optional<RenderPassResult> collapsed_parent_pass = std::nullopt);
   ~InlinePassContext();
 
@@ -41,6 +42,7 @@ class InlinePassContext {
   std::shared_ptr<RenderPass> pass_;
   uint32_t pass_count_ = 0;
   uint32_t total_pass_reads_ = 0;
+  bool contains_clips_ = false;
   // Whether this context is collapsed into a parent entity pass.
   bool is_collapsed_ = false;