[Impeller] Keep track of a stencil depth floor for non-collapsed subpasses (flutter/engine#33473)

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

@@ -805,5 +805,29 @@ TEST_P(AiksTest, SiblingSaveLayerBoundsAreRespected) {
   ASSERT_TRUE(OpenPlaygroundHere(canvas.EndRecordingAsPicture()));
 }
 
+TEST_P(AiksTest, CanRenderClippedLayers) {
+  Canvas canvas;
+
+  canvas.DrawPaint({.color = Color::White()});
+
+  // Draw a green circle on the screen.
+  {
+    // Increase the clip depth for the savelayer to contend with.
+    canvas.ClipPath(PathBuilder{}.AddCircle({100, 100}, 50).TakePath());
+
+    canvas.SaveLayer({}, Rect::MakeXYWH(50, 50, 100, 100));
+
+    // Fill the layer with white.
+    canvas.DrawRect(Rect::MakeSize({400, 400}), {.color = Color::White()});
+    // Fill the layer with green, but do so with a color blend that can't be
+    // collapsed into the parent pass.
+    canvas.DrawRect(
+        Rect::MakeSize({400, 400}),
+        {.color = Color::Green(), .blend_mode = Entity::BlendMode::kColorBurn});
+  }
+
+  ASSERT_TRUE(OpenPlaygroundHere(canvas.EndRecordingAsPicture()));
+}
+
 }  // namespace testing
 }  // namespace impeller

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

@@ -266,7 +266,8 @@ size_t Canvas::GetStencilDepth() const {
 void Canvas::SaveLayer(Paint paint, std::optional<Rect> bounds) {
   Save(true, paint.blend_mode);
 
-  GetCurrentPass().SetDelegate(
+  auto& new_layer_pass = GetCurrentPass();
+  new_layer_pass.SetDelegate(
       std::make_unique<PaintPassDelegate>(paint, bounds));
 
   if (bounds.has_value()) {

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

@@ -180,7 +180,8 @@ bool EntityPass::Render(ContentContext& renderer,
 bool EntityPass::RenderInternal(ContentContext& renderer,
                                 RenderTarget render_target,
                                 Point position,
-                                uint32_t depth) const {
+                                uint32_t pass_depth,
+                                size_t stencil_depth_floor) const {
   TRACE_EVENT0("impeller", "EntityPass::Render");
 
   auto context = renderer.GetContext();
@@ -233,7 +234,7 @@ bool EntityPass::RenderInternal(ContentContext& renderer,
       if (subpass->delegate_->CanCollapseIntoParentPass()) {
         // Directly render into the parent target and move on.
         if (!subpass->RenderInternal(renderer, render_target, position,
-                                     depth)) {
+                                     pass_depth, stencil_depth_floor)) {
           return false;
         }
         continue;
@@ -286,13 +287,16 @@ bool EntityPass::RenderInternal(ContentContext& renderer,
         return false;
       }
 
+      // Stencil textures aren't shared between EntityPasses (as much of the
+      // time they are transient).
       if (!subpass->RenderInternal(renderer, subpass_target,
-                                   subpass_coverage->origin, ++depth)) {
+                                   subpass_coverage->origin, ++pass_depth,
+                                   subpass->stencil_depth_)) {
         return false;
       }
 
       element_entity.SetContents(std::move(offscreen_texture_contents));
-      element_entity.SetStencilDepth(stencil_depth_);
+      element_entity.SetStencilDepth(subpass->stencil_depth_);
       element_entity.SetBlendMode(subpass->blend_mode_);
       // Once we have filters being applied for SaveLayer, some special sauce
       // may be needed here (or in PaintPassDelegate) to ensure the filter
@@ -305,7 +309,7 @@ bool EntityPass::RenderInternal(ContentContext& renderer,
     }
 
     // =========================================================================
-    // Render the element ======================================================
+    // Configure the RenderPass ================================================
     // =========================================================================
 
     if (pass && element_entity.GetBlendMode() >
@@ -350,7 +354,7 @@ bool EntityPass::RenderInternal(ContentContext& renderer,
       }
 
       command_buffer->SetLabel(
-          "EntityPass Command Buffer: Depth=" + std::to_string(depth) +
+          "EntityPass Command Buffer: Depth=" + std::to_string(pass_depth) +
           " Count=" + std::to_string(pass_count));
 
       // Never clear the texture for subsequent passes.
@@ -373,12 +377,20 @@ bool EntityPass::RenderInternal(ContentContext& renderer,
         return false;
       }
 
-      pass->SetLabel("EntityPass Render Pass: Depth=" + std::to_string(depth) +
-                     " Count=" + std::to_string(pass_count));
+      pass->SetLabel(
+          "EntityPass Render Pass: Depth=" + std::to_string(pass_depth) +
+          " Count=" + std::to_string(pass_count));
 
       ++pass_count;
     }
 
+    // =========================================================================
+    // Render the element ======================================================
+    // =========================================================================
+
+    element_entity.SetStencilDepth(element_entity.GetStencilDepth() -
+                                   stencil_depth_floor);
+
     if (!element_entity.Render(renderer, *pass)) {
       return false;
     }

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

@@ -61,7 +61,8 @@ class EntityPass {
   bool RenderInternal(ContentContext& renderer,
                       RenderTarget render_target,
                       Point position,
-                      uint32_t depth) const;
+                      uint32_t pass_depth,
+                      size_t stencil_depth_floor = 0) const;
 
   std::vector<Element> elements_;