[Impeller] Use the scissor to limit all draws by clip coverage. (#51698)

Attempts to improve flutter/flutter#145274.

Our new clipping technique paints walls on the depth buffer "in front" of the Entities that will be affected by said clips. So when an intersect clip is drawn (the common case), the clip will cover the whole framebuffer.

Depth is divvied up such that deeper clips get drawn _behind_ shallower clips, and so many clips actually don't end up drawing depth across the whole framebuffer. However, if the app does a lot of transitioning from a huge clips to a small clips, a lot of unnecessary depth churn occurs (very common in Flutter -- this happens with both the app bar and floating action button in the counter template, for example).

Since progressively deeper layers in the clip coverage stack always subset their parent layers, we can reduce waste for small intersect clips by setting the scissor to the clip coverage rect instead of drawing the clip to the whole screen.

Note that this change _does not_ help much with huge/fullscreen clips.

Also, we could potentially improve this further by computing much stricter bounds. Rather than just using clip coverage for the scissor, we could intersect it with the union of all draws affected by the clip at the cost of a bit more CPU churn per draw. I don't think that's enough juice for the squeeze though.

Before (`Play/AiksTest.CanRenderNestedClips/Metal`):

https://github.com/flutter/engine/assets/919017/7858400f-793a-4f7b-a0e4-fa3581198beb

After (`Play/AiksTest.CanRenderNestedClips/Metal`):

https://github.com/flutter/engine/assets/919017/b2f7c96d-a820-454d-91df-f5fae4976e91

Author: bdero

impeller/entity/entity_pass.cc

@@ -25,6 +25,7 @@
 #include "impeller/entity/inline_pass_context.h"
 #include "impeller/geometry/color.h"
 #include "impeller/geometry/rect.h"
+#include "impeller/geometry/size.h"
 #include "impeller/renderer/command_buffer.h"
 
 #ifdef IMPELLER_DEBUG
@@ -752,6 +753,25 @@ EntityPass::EntityResult EntityPass::GetEntityForElement(
   FML_UNREACHABLE();
 }
 
+static void SetClipScissor(std::optional<Rect> clip_coverage,
+                           RenderPass& pass,
+                           Point global_pass_position) {
+  if constexpr (!ContentContext::kEnableStencilThenCover) {
+    return;
+  }
+  // Set the scissor to the clip coverage area. We do this prior to rendering
+  // the clip itself and all its contents.
+  IRect scissor;
+  if (clip_coverage.has_value()) {
+    clip_coverage = clip_coverage->Shift(-global_pass_position);
+    scissor = IRect::RoundOut(clip_coverage.value());
+    // The scissor rect must not exceed the size of the render target.
+    scissor = scissor.Intersection(IRect::MakeSize(pass.GetRenderTargetSize()))
+                  .value_or(IRect());
+  }
+  pass.SetScissor(scissor);
+}
+
 bool EntityPass::RenderElement(Entity& element_entity,
                                size_t clip_depth_floor,
                                InlinePassContext& pass_context,
@@ -771,8 +791,10 @@ bool EntityPass::RenderElement(Entity& element_entity,
     // Restore any clips that were recorded before the backdrop filter was
     // applied.
     auto& replay_entities = clip_coverage_stack.GetReplayEntities();
-    for (const auto& entity : replay_entities) {
-      if (!entity.Render(renderer, *result.pass)) {
+    for (const auto& replay : replay_entities) {
+      SetClipScissor(clip_coverage_stack.CurrentClipCoverage(), *result.pass,
+                     global_pass_position);
+      if (!replay.entity.Render(renderer, *result.pass)) {
         VALIDATION_LOG << "Failed to render entity for clip restore.";
       }
     }
@@ -826,11 +848,18 @@ bool EntityPass::RenderElement(Entity& element_entity,
   element_entity.GetContents()->SetCoverageHint(
       Rect::Intersection(element_coverage_hint, current_clip_coverage));
 
-  if (!clip_coverage_stack.AppendClipCoverage(clip_coverage, element_entity,
-                                              clip_depth_floor,
-                                              global_pass_position)) {
-    // If the entity's coverage change did not change the clip coverage, we
-    // don't need to render it.
+  EntityPassClipStack::ClipStateResult clip_state_result =
+      clip_coverage_stack.ApplyClipState(clip_coverage, element_entity,
+                                         clip_depth_floor,
+                                         global_pass_position);
+
+  if (clip_state_result.clip_did_change) {
+    // We only need to update the pass scissor if the clip state has changed.
+    SetClipScissor(clip_coverage_stack.CurrentClipCoverage(), *result.pass,
+                   global_pass_position);
+  }
+
+  if (!clip_state_result.should_render) {
     return true;
   }
 

impeller/entity/entity_pass_clip_stack.cc

@@ -49,20 +49,23 @@ EntityPassClipStack::GetClipCoverageLayers() const {
   return subpass_state_.back().clip_coverage;
 }
 
-bool EntityPassClipStack::AppendClipCoverage(
-    Contents::ClipCoverage clip_coverage,
+EntityPassClipStack::ClipStateResult EntityPassClipStack::ApplyClipState(
+    Contents::ClipCoverage global_clip_coverage,
     Entity& entity,
     size_t clip_depth_floor,
     Point global_pass_position) {
+  ClipStateResult result = {.should_render = false, .clip_did_change = false};
+
   auto& subpass_state = GetCurrentSubpassState();
-  switch (clip_coverage.type) {
+  switch (global_clip_coverage.type) {
     case Contents::ClipCoverage::Type::kNoChange:
       break;
     case Contents::ClipCoverage::Type::kAppend: {
       auto op = CurrentClipCoverage();
       subpass_state.clip_coverage.push_back(
-          ClipCoverageLayer{.coverage = clip_coverage.coverage,
+          ClipCoverageLayer{.coverage = global_clip_coverage.coverage,
                             .clip_depth = entity.GetClipDepth() + 1});
+      result.clip_did_change = true;
 
       FML_DCHECK(subpass_state.clip_coverage.back().clip_depth ==
                  subpass_state.clip_coverage.front().clip_depth +
@@ -71,14 +74,14 @@ bool EntityPassClipStack::AppendClipCoverage(
       if (!op.has_value()) {
         // Running this append op won't impact the clip buffer because the
         // whole screen is already being clipped, so skip it.
-        return false;
+        return result;
       }
     } break;
     case Contents::ClipCoverage::Type::kRestore: {
       if (subpass_state.clip_coverage.back().clip_depth <=
           entity.GetClipDepth()) {
         // Drop clip restores that will do nothing.
-        return false;
+        return result;
       }
 
       auto restoration_index = entity.GetClipDepth() -
@@ -96,18 +99,19 @@ bool EntityPassClipStack::AppendClipCoverage(
         restore_coverage = restore_coverage->Shift(-global_pass_position);
       }
       subpass_state.clip_coverage.resize(restoration_index + 1);
+      result.clip_did_change = true;
 
       if constexpr (ContentContext::kEnableStencilThenCover) {
         // Skip all clip restores when stencil-then-cover is enabled.
         if (subpass_state.clip_coverage.back().coverage.has_value()) {
-          RecordEntity(entity, clip_coverage.type);
+          RecordEntity(entity, global_clip_coverage.type, Rect());
         }
-        return false;
+        return result;
       }
 
       if (!subpass_state.clip_coverage.back().coverage.has_value()) {
         // Running this restore op won't make anything renderable, so skip it.
-        return false;
+        return result;
       }
 
       auto restore_contents =
@@ -130,19 +134,23 @@ bool EntityPassClipStack::AppendClipCoverage(
 #endif
 
   entity.SetClipDepth(entity.GetClipDepth() - clip_depth_floor);
-  RecordEntity(entity, clip_coverage.type);
+  RecordEntity(entity, global_clip_coverage.type,
+               subpass_state.clip_coverage.back().coverage);
 
-  return true;
+  result.should_render = true;
+  return result;
 }
 
 void EntityPassClipStack::RecordEntity(const Entity& entity,
-                                       Contents::ClipCoverage::Type type) {
+                                       Contents::ClipCoverage::Type type,
+                                       std::optional<Rect> clip_coverage) {
   auto& subpass_state = GetCurrentSubpassState();
   switch (type) {
     case Contents::ClipCoverage::Type::kNoChange:
       return;
     case Contents::ClipCoverage::Type::kAppend:
-      subpass_state.rendered_clip_entities.push_back(entity.Clone());
+      subpass_state.rendered_clip_entities.push_back(
+          {.entity = entity.Clone(), .clip_coverage = clip_coverage});
       break;
     case Contents::ClipCoverage::Type::kRestore:
       if (!subpass_state.rendered_clip_entities.empty()) {
@@ -157,7 +165,8 @@ EntityPassClipStack::GetCurrentSubpassState() {
   return subpass_state_.back();
 }
 
-const std::vector<Entity>& EntityPassClipStack::GetReplayEntities() const {
+const std::vector<EntityPassClipStack::ReplayResult>&
+EntityPassClipStack::GetReplayEntities() const {
   return subpass_state_.back().rendered_clip_entities;
 }
 

impeller/entity/entity_pass_clip_stack.h

@@ -6,6 +6,8 @@
 #define FLUTTER_IMPELLER_ENTITY_ENTITY_PASS_CLIP_STACK_H_
 
 #include "impeller/entity/contents/contents.h"
+#include "impeller/entity/entity.h"
+#include "impeller/geometry/rect.h"
 
 namespace impeller {
 
@@ -21,6 +23,20 @@ struct ClipCoverageLayer {
 ///        stencil buffer is left in an identical state.
 class EntityPassClipStack {
  public:
+  struct ReplayResult {
+    Entity entity;
+    std::optional<Rect> clip_coverage;
+  };
+
+  struct ClipStateResult {
+    /// Whether or not the Entity should be rendered. If false, the Entity may
+    /// be safely skipped.
+    bool should_render = false;
+    /// Whether or not the current clip coverage changed during the call to
+    /// `ApplyClipState`.
+    bool clip_did_change = false;
+  };
+
   /// Create a new [EntityPassClipStack] with an initialized coverage rect.
   explicit EntityPassClipStack(const Rect& initial_coverage_rect);
 
@@ -34,24 +50,27 @@ class EntityPassClipStack {
 
   bool HasCoverage() const;
 
-  /// Returns true if entity should be rendered.
-  bool AppendClipCoverage(Contents::ClipCoverage clip_coverage,
-                          Entity& entity,
-                          size_t clip_depth_floor,
-                          Point global_pass_position);
+  /// @brief  Applies the current clip state to an Entity. If the given Entity
+  ///         is a clip operation, then the clip state is updated accordingly.
+  ClipStateResult ApplyClipState(Contents::ClipCoverage global_clip_coverage,
+                                 Entity& entity,
+                                 size_t clip_depth_floor,
+                                 Point global_pass_position);
 
   // Visible for testing.
-  void RecordEntity(const Entity& entity, Contents::ClipCoverage::Type type);
+  void RecordEntity(const Entity& entity,
+                    Contents::ClipCoverage::Type type,
+                    std::optional<Rect> clip_coverage);
 
   // Visible for testing.
-  const std::vector<Entity>& GetReplayEntities() const;
+  const std::vector<ReplayResult>& GetReplayEntities() const;
 
   // Visible for testing.
   const std::vector<ClipCoverageLayer> GetClipCoverageLayers() const;
 
  private:
   struct SubpassState {
-    std::vector<Entity> rendered_clip_entities;
+    std::vector<ReplayResult> rendered_clip_entities;
     std::vector<ClipCoverageLayer> clip_coverage;
   };
 

impeller/entity/entity_pass_unittests.cc

@@ -17,16 +17,26 @@ TEST(EntityPassClipStackTest, CanPushAndPopEntities) {
   EXPECT_TRUE(recorder.GetReplayEntities().empty());
 
   Entity entity;
-  recorder.RecordEntity(entity, Contents::ClipCoverage::Type::kAppend);
+  recorder.RecordEntity(entity, Contents::ClipCoverage::Type::kAppend,
+                        Rect::MakeLTRB(0, 0, 100, 100));
   EXPECT_EQ(recorder.GetReplayEntities().size(), 1u);
 
-  recorder.RecordEntity(entity, Contents::ClipCoverage::Type::kAppend);
+  recorder.RecordEntity(entity, Contents::ClipCoverage::Type::kAppend,
+                        Rect::MakeLTRB(0, 0, 50, 50));
   EXPECT_EQ(recorder.GetReplayEntities().size(), 2u);
-
-  recorder.RecordEntity(entity, Contents::ClipCoverage::Type::kRestore);
+  ASSERT_TRUE(recorder.GetReplayEntities()[0].clip_coverage.has_value());
+  ASSERT_TRUE(recorder.GetReplayEntities()[1].clip_coverage.has_value());
+  // NOLINTBEGIN(bugprone-unchecked-optional-access)
+  EXPECT_EQ(recorder.GetReplayEntities()[0].clip_coverage.value(),
+            Rect::MakeLTRB(0, 0, 100, 100));
+  EXPECT_EQ(recorder.GetReplayEntities()[1].clip_coverage.value(),
+            Rect::MakeLTRB(0, 0, 50, 50));
+  // NOLINTEND(bugprone-unchecked-optional-access)
+
+  recorder.RecordEntity(entity, Contents::ClipCoverage::Type::kRestore, Rect());
   EXPECT_EQ(recorder.GetReplayEntities().size(), 1u);
 
-  recorder.RecordEntity(entity, Contents::ClipCoverage::Type::kRestore);
+  recorder.RecordEntity(entity, Contents::ClipCoverage::Type::kRestore, Rect());
   EXPECT_TRUE(recorder.GetReplayEntities().empty());
 }
 
@@ -37,7 +47,7 @@ TEST(EntityPassClipStackTest, CanPopEntitiesSafely) {
   EXPECT_TRUE(recorder.GetReplayEntities().empty());
 
   Entity entity;
-  recorder.RecordEntity(entity, Contents::ClipCoverage::Type::kRestore);
+  recorder.RecordEntity(entity, Contents::ClipCoverage::Type::kRestore, Rect());
   EXPECT_TRUE(recorder.GetReplayEntities().empty());
 }
 
@@ -48,7 +58,8 @@ TEST(EntityPassClipStackTest, CanAppendNoChange) {
   EXPECT_TRUE(recorder.GetReplayEntities().empty());
 
   Entity entity;
-  recorder.RecordEntity(entity, Contents::ClipCoverage::Type::kNoChange);
+  recorder.RecordEntity(entity, Contents::ClipCoverage::Type::kNoChange,
+                        Rect());
   EXPECT_TRUE(recorder.GetReplayEntities().empty());
 }
 
@@ -61,12 +72,14 @@ TEST(EntityPassClipStackTest, AppendCoverageNoChange) {
   EXPECT_EQ(recorder.GetClipCoverageLayers()[0].clip_depth, 0u);
 
   Entity entity;
-  recorder.AppendClipCoverage(
+  EntityPassClipStack::ClipStateResult result = recorder.ApplyClipState(
       Contents::ClipCoverage{
           .type = Contents::ClipCoverage::Type::kNoChange,
           .coverage = std::nullopt,
       },
       entity, 0, Point(0, 0));
+  EXPECT_TRUE(result.should_render);
+  EXPECT_FALSE(result.clip_did_change);
 
   EXPECT_EQ(recorder.GetClipCoverageLayers()[0].coverage,
             Rect::MakeSize(Size::MakeWH(100, 100)));
@@ -82,12 +95,14 @@ TEST(EntityPassClipStackTest, AppendAndRestoreClipCoverage) {
   // Push a clip.
   Entity entity;
   entity.SetClipDepth(0);
-  recorder.AppendClipCoverage(
+  EntityPassClipStack::ClipStateResult result = recorder.ApplyClipState(
       Contents::ClipCoverage{
           .type = Contents::ClipCoverage::Type::kAppend,
           .coverage = Rect::MakeLTRB(50, 50, 55, 55),
       },
       entity, 0, Point(0, 0));
+  EXPECT_TRUE(result.should_render);
+  EXPECT_TRUE(result.clip_did_change);
 
   ASSERT_EQ(recorder.GetClipCoverageLayers().size(), 2u);
   EXPECT_EQ(recorder.GetClipCoverageLayers()[1].coverage,
@@ -97,7 +112,7 @@ TEST(EntityPassClipStackTest, AppendAndRestoreClipCoverage) {
 
   // Restore the clip.
   entity.SetClipDepth(0);
-  recorder.AppendClipCoverage(
+  recorder.ApplyClipState(
       Contents::ClipCoverage{
           .type = Contents::ClipCoverage::Type::kRestore,
           .coverage = Rect::MakeLTRB(50, 50, 55, 55),
@@ -120,12 +135,14 @@ TEST(EntityPassClipStackTest, UnbalancedRestore) {
   // Restore the clip.
   Entity entity;
   entity.SetClipDepth(0);
-  recorder.AppendClipCoverage(
+  EntityPassClipStack::ClipStateResult result = recorder.ApplyClipState(
       Contents::ClipCoverage{
           .type = Contents::ClipCoverage::Type::kRestore,
           .coverage = Rect::MakeLTRB(50, 50, 55, 55),
       },
       entity, 0, Point(0, 0));
+  EXPECT_FALSE(result.should_render);
+  EXPECT_FALSE(result.clip_did_change);
 
   ASSERT_EQ(recorder.GetClipCoverageLayers().size(), 1u);
   EXPECT_EQ(recorder.GetClipCoverageLayers()[0].coverage,
@@ -143,12 +160,16 @@ TEST(EntityPassClipStackTest, ClipAndRestoreWithSubpasses) {
   // Push a clip.
   Entity entity;
   entity.SetClipDepth(0u);
-  recorder.AppendClipCoverage(
-      Contents::ClipCoverage{
-          .type = Contents::ClipCoverage::Type::kAppend,
-          .coverage = Rect::MakeLTRB(50, 50, 55, 55),
-      },
-      entity, 0, Point(0, 0));
+  {
+    EntityPassClipStack::ClipStateResult result = recorder.ApplyClipState(
+        Contents::ClipCoverage{
+            .type = Contents::ClipCoverage::Type::kAppend,
+            .coverage = Rect::MakeLTRB(50, 50, 55, 55),
+        },
+        entity, 0, Point(0, 0));
+    EXPECT_TRUE(result.should_render);
+    EXPECT_TRUE(result.clip_did_change);
+  }
 
   ASSERT_EQ(recorder.GetClipCoverageLayers().size(), 2u);
   EXPECT_EQ(recorder.GetClipCoverageLayers()[1].coverage,
@@ -163,12 +184,16 @@ TEST(EntityPassClipStackTest, ClipAndRestoreWithSubpasses) {
             Rect::MakeLTRB(50, 50, 55, 55));
 
   entity.SetClipDepth(1);
-  recorder.AppendClipCoverage(
-      Contents::ClipCoverage{
-          .type = Contents::ClipCoverage::Type::kAppend,
-          .coverage = Rect::MakeLTRB(54, 54, 55, 55),
-      },
-      entity, 0, Point(0, 0));
+  {
+    EntityPassClipStack::ClipStateResult result = recorder.ApplyClipState(
+        Contents::ClipCoverage{
+            .type = Contents::ClipCoverage::Type::kAppend,
+            .coverage = Rect::MakeLTRB(54, 54, 55, 55),
+        },
+        entity, 0, Point(0, 0));
+    EXPECT_TRUE(result.should_render);
+    EXPECT_TRUE(result.clip_did_change);
+  }
 
   EXPECT_EQ(recorder.GetClipCoverageLayers()[1].coverage,
             Rect::MakeLTRB(54, 54, 55, 55));