Another coverage fix for border mask blur (flutter#158)

Author: bdero

impeller/entity/contents/filters/border_mask_blur_filter_contents.cc

@@ -117,13 +117,10 @@ std::optional<Rect> BorderMaskBlurFilterContents::GetFilterCoverage(
   if (!coverage.has_value()) {
     return std::nullopt;
   }
-
+  auto transform = inputs[0]->GetTransform(entity);
   auto transformed_blur_vector =
-      inputs[0]
-          ->GetTransform(entity)
-          .TransformDirection(
-              Vector2(Radius{sigma_x_}.radius, Radius{sigma_y_}.radius))
-          .Abs();
+      transform.TransformDirection(Vector2(Radius{sigma_x_}.radius, 0)).Abs() +
+      transform.TransformDirection(Vector2(0, Radius{sigma_y_}.radius)).Abs();
   auto extent = coverage->size + transformed_blur_vector * 2;
   return Rect(coverage->origin - transformed_blur_vector,
               Size(extent.x, extent.y));

impeller/entity/entity_unittests.cc

@@ -766,12 +766,12 @@ TEST_P(EntityTest, GaussianBlurFilter) {
     // Renders a red "cover" rectangle that shows the original position of the
     // unfiltered input.
     Entity cover_entity;
-    cover_entity.SetContents(
-        SolidColorContents::Make(PathBuilder{}
-                                     .AddRect(Rect(-Point(bridge->GetSize()) / 2,
-                                                   Size(bridge->GetSize())))
-                                     .TakePath(),
-                                 cover_color));
+    cover_entity.SetContents(SolidColorContents::Make(
+        PathBuilder{}
+            .AddRect(
+                Rect(-Point(bridge->GetSize()) / 2, Size(bridge->GetSize())))
+            .TakePath(),
+        cover_color));
     cover_entity.SetTransformation(ctm);
 
     cover_entity.Render(context, pass);
@@ -853,5 +853,33 @@ TEST_P(EntityTest, SolidStrokeCoverageIsCorrect) {
   }
 }
 
+TEST_P(EntityTest, BorderMaskBlurCoverageIsCorrect) {
+  auto fill = std::make_shared<SolidColorContents>();
+  fill->SetPath(
+      PathBuilder{}.AddRect(Rect::MakeXYWH(0, 0, 300, 400)).TakePath());
+  fill->SetColor(Color::CornflowerBlue());
+  auto border_mask_blur = FilterContents::MakeBorderMaskBlur(
+      FilterInput::Make(fill), FilterContents::Radius{3},
+      FilterContents::Radius{4});
+
+  {
+    Entity e;
+    e.SetTransformation(Matrix());
+    auto actual = border_mask_blur->GetCoverage(e);
+    auto expected = Rect::MakeXYWH(-3, -4, 306, 408);
+    ASSERT_TRUE(actual.has_value());
+    ASSERT_RECT_NEAR(actual.value(), expected);
+  }
+
+  {
+    Entity e;
+    e.SetTransformation(Matrix::MakeRotationZ(Radians{kPi / 4}));
+    auto actual = border_mask_blur->GetCoverage(e);
+    auto expected = Rect::MakeXYWH(-287.792, -4.94975, 504.874, 504.874);
+    ASSERT_TRUE(actual.has_value());
+    ASSERT_RECT_NEAR(actual.value(), expected);
+  }
+}
+
 }  // namespace testing
 }  // namespace impeller