Better clipRect culling accuracy under scaling transforms (flutter/engine#53508)

The clipRect culling in DisplayListBuilder can sometimes fail to cull a clip due to round-off error in the math and this case happens to trigger when trying to cull against the initial frame bounds of a Pixel 6a with its uncommon 2.625 DPR as a scaling transform. An optimization in the culling tests for axis-aligned transforms (very common in Flutter) happens to do the right thing for this odd case, likely due to less math to incur round-off errors.

engine/src/flutter/display_list/display_list_unittests.cc

@@ -4420,7 +4420,9 @@ class ClipExpector : public virtual DlOpReceiver,
 
   template <typename T>
   void check(T shape, ClipOp clip_op, bool is_aa) {
-    ASSERT_LT(index_, clip_expectations_.size()) << label();
+    ASSERT_LT(index_, clip_expectations_.size())
+        << label() << std::endl
+        << "extra clip shape = " << shape;
     auto expected = clip_expectations_[index_];
     EXPECT_EQ(expected.clip_op, clip_op) << label();
     EXPECT_EQ(expected.is_aa, is_aa) << label();
@@ -4443,6 +4445,30 @@ class ClipExpector : public virtual DlOpReceiver,
   }
 };
 
+TEST_F(DisplayListTest, ClipRectCullingPixel6a) {
+  // These particular values create bit errors if we use the path that
+  // tests for inclusion in local space, but work OK if we use a forward
+  // path that tests for inclusion in device space, due to the fact that
+  // the extra matrix inversion is just enough math to cause the transform
+  // to place the local space cull corners just outside the original rect.
+  // The test in device space only works under a simple scale, such as we
+  // use for DPR adjustments (and which are not always inversion friendly).
+
+  auto frame = SkRect::MakeLTRB(0.0f, 0.0f, 1080.0f, 2400.0f);
+  DlScalar DPR = 2.625f;
+  auto clip = SkRect::MakeLTRB(0.0f, 0.0f, 1080.0f / DPR, 2400.0f / DPR);
+
+  DisplayListBuilder cull_builder;
+  cull_builder.ClipRect(frame, ClipOp::kIntersect, false);
+  cull_builder.Scale(DPR, DPR);
+  cull_builder.ClipRect(clip, ClipOp::kIntersect, false);
+  auto cull_dl = cull_builder.Build();
+
+  CLIP_EXPECTOR(expector);
+  expector.addExpectation(frame, ClipOp::kIntersect, false);
+  cull_dl->Dispatch(expector);
+}
+
 TEST_F(DisplayListTest, ClipRectCulling) {
   auto clip = SkRect::MakeLTRB(10.0f, 10.0f, 20.0f, 20.0f);
 

engine/src/flutter/display_list/utils/dl_matrix_clip_tracker.cc

@@ -239,6 +239,12 @@ bool DisplayListMatrixClipState::rect_covers_cull(const DlRect& content) const {
   if (cull_rect_.IsEmpty()) {
     return true;
   }
+  if (matrix_.IsAligned2D()) {
+    // This transform-to-device calculation is faster and more accurate
+    // for rect-to-rect aligned transformations, but not accurate under
+    // (non-quadrant) rotations and skews.
+    return content.TransformAndClipBounds(matrix_).Contains(cull_rect_);
+  }
   DlPoint corners[4];
   if (!getLocalCullCorners(corners)) {
     return false;

engine/src/flutter/display_list/utils/dl_matrix_clip_tracker_unittests.cc

@@ -779,6 +779,33 @@ TEST(DisplayListMatrixClipState, RectCoverage) {
   EXPECT_FALSE(state.rect_covers_cull(rect.Expand(0.0f, 0.0f, 0.0f, -0.1f)));
 }
 
+TEST(DisplayListMatrixClipState, RectCoverageAccuracy) {
+  // These particular values create bit errors if we use the path that
+  // tests for inclusion in local space, but work OK if we use a forward
+  // path that tests for inclusion in device space, due to the fact that
+  // the extra matrix inversion is just enough math to cause the transform
+  // to place the local space cull corners just outside the original rect.
+  // The test in device space only works under a simple scale, such as we
+  // use for DPR adjustments (and which are not always inversion friendly).
+
+  DlRect cull = DlRect::MakeLTRB(0.0f, 0.0f, 1080.0f, 2400.0f);
+  DlScalar DPR = 2.625;
+  DlRect rect = DlRect::MakeLTRB(0.0f, 0.0f, 1080.0f / DPR, 2400.0f / DPR);
+
+  DisplayListMatrixClipState state(cull);
+  state.scale(DPR, DPR);
+
+  EXPECT_TRUE(state.rect_covers_cull(rect));
+  EXPECT_TRUE(state.rect_covers_cull(rect.Expand(0.1f, 0.0f, 0.0f, 0.0f)));
+  EXPECT_TRUE(state.rect_covers_cull(rect.Expand(0.0f, 0.1f, 0.0f, 0.0f)));
+  EXPECT_TRUE(state.rect_covers_cull(rect.Expand(0.0f, 0.0f, 0.1f, 0.0f)));
+  EXPECT_TRUE(state.rect_covers_cull(rect.Expand(0.0f, 0.0f, 0.0f, 0.1f)));
+  EXPECT_FALSE(state.rect_covers_cull(rect.Expand(-0.1f, 0.0f, 0.0f, 0.0f)));
+  EXPECT_FALSE(state.rect_covers_cull(rect.Expand(0.0f, -0.1f, 0.0f, 0.0f)));
+  EXPECT_FALSE(state.rect_covers_cull(rect.Expand(0.0f, 0.0f, -0.1f, 0.0f)));
+  EXPECT_FALSE(state.rect_covers_cull(rect.Expand(0.0f, 0.0f, 0.0f, -0.1f)));
+}
+
 TEST(DisplayListMatrixClipState, RectCoverageUnderScale) {
   DlRect rect = DlRect::MakeLTRB(100.0f, 100.0f, 200.0f, 200.0f);
   DisplayListMatrixClipState state(rect);