[Impeller] Fix new convex path shadow generation in perspective

flar · flar · commit 40c779fb293e · 2026-03-03T14:14:42.000-08:00
diff --git a/engine/src/flutter/impeller/display_list/aiks_dl_shadow_unittests.cc b/engine/src/flutter/impeller/display_list/aiks_dl_shadow_unittests.cc
@@ -930,5 +930,89 @@ TEST_P(AiksTest,
   ASSERT_TRUE(OpenPlaygroundHere(dl));
 }
 
+TEST_P(AiksTest, CanDrawPerspectiveConvexShadow) {
+  DisplayListBuilder builder;
+  builder.Clear(DlColor::kWhite());
+  builder.Scale(GetContentScale().x, GetContentScale().y);
+
+  // The rect path should be optimized to a rect-specific shadow shader.
+  DlPath rect_path = DlPath::MakeRectLTRB(-50, -50, 50, 75);
+
+  // The pentagon path should go through the general convex path shadow
+  // tessellator.
+  DlPathBuilder path_builder;
+  path_builder.MoveTo(DlPoint(-50, -50));
+  path_builder.LineTo(DlPoint(50, -50));
+  path_builder.LineTo(DlPoint(50, 50));
+  path_builder.LineTo(DlPoint(0, 75));
+  path_builder.LineTo(DlPoint(-50, 50));
+  path_builder.Close();
+  DlPath pentagon_path = path_builder.TakePath();
+
+  Matrix simple_y_rotate_matrix = Matrix::MakeRotationY(Degrees(45));
+
+  Matrix perspective_matrix;
+  perspective_matrix.e[2][3] = 0.0015f;
+  perspective_matrix = perspective_matrix * simple_y_rotate_matrix;
+
+  auto draw_paths_and_shadows = [&builder](const DlPath& path,
+                                           const DlColor& color,
+                                           const Matrix& matrix) {
+    builder.Save();
+
+    DlPaint paint;
+    paint.setColor(color.withAlphaF(0.25f));
+
+    DlPaint shadow_paint;
+    shadow_paint.setMaskFilter(
+        DlBlurMaskFilter::Make(DlBlurStyle::kNormal, 10.0f));
+
+    builder.Save();
+    builder.Transform(matrix);
+    builder.DrawPath(path, paint);
+    builder.Restore();
+
+    builder.Translate(150, 0);
+
+    builder.Save();
+    builder.Transform(matrix);
+    builder.DrawPath(path, shadow_paint);
+    builder.Restore();
+
+    builder.Translate(150, 0);
+
+    builder.Save();
+    builder.Transform(matrix);
+    builder.DrawPath(path, shadow_paint);
+    builder.DrawPath(path, paint);
+    builder.Restore();
+
+    builder.Restore();
+  };
+
+  auto draw_test = [&](const DlPath& path) {
+    builder.Save();
+
+    builder.Translate(0, 75);
+    builder.DrawPath(path, DlPaint(DlColor::kPurple()));
+
+    builder.Translate(150, -75);
+    draw_paths_and_shadows(path, DlColor::kGreen(), simple_y_rotate_matrix);
+    builder.Translate(0, 175);
+    draw_paths_and_shadows(path, DlColor::kBlue(), perspective_matrix);
+
+    builder.Restore();
+  };
+
+  builder.Translate(100, 100);
+
+  draw_test(rect_path);
+  builder.Translate(0, 350);
+  draw_test(pentagon_path);
+
+  auto dl = builder.Build();
+  ASSERT_TRUE(OpenPlaygroundHere(dl));
+}
+
 }  // namespace testing
 }  // namespace impeller
diff --git a/engine/src/flutter/impeller/entity/geometry/shadow_path_geometry.cc b/engine/src/flutter/impeller/entity/geometry/shadow_path_geometry.cc
@@ -494,9 +494,17 @@ PolygonInfo::PolygonInfo(Scalar occluder_height)
 
 const std::shared_ptr<ShadowVertices> PolygonInfo::CalculateConvexShadowMesh(
     const impeller::PathSource& source,
-    const Matrix& matrix,
+    const Matrix& incoming_matrix,
     const Tessellator::Trigs& trigs) {
-  if (!matrix.IsInvertible()) {
+  // We are operating in "2D device space" with respect to scale and there
+  // is no need to involve translations or Z or perspective in the various
+  // calculations, so we extract the incoming matrix's 2D Bases and just
+  // use those measurements to guide our tessellation.
+  Scalar x_scale = incoming_matrix.GetBasisX().GetLength();
+  Scalar y_scale = incoming_matrix.GetBasisY().GetLength();
+  Matrix matrix = Matrix::MakeScale({x_scale, y_scale, 1.0f});
+
+  if (!incoming_matrix.IsInvertible() || !matrix.IsInvertible()) {
     return ShadowVertices::kEmpty;
   }
 
