[Impeller] rrect_blur: scale max radius clamp by transform (#161238)

migrated PR https://github.com/flutter/engine/pull/54350

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engine/src/flutter/impeller/display_list/dl_golden_blur_unittests.cc

@@ -154,7 +154,6 @@ TEST_P(DlGoldenTest, ShimmerTest) {
 
 TEST_P(DlGoldenTest, StrokedRRectFastBlur) {
   impeller::Point content_scale = GetContentScale();
-
   DlRect rect = DlRect::MakeXYWH(50, 50, 100, 100);
   DlRoundRect rrect = DlRoundRect::MakeRectRadius(rect, 10.0f);
   DlPaint fill = DlPaint().setColor(DlColor::kBlue());
@@ -176,5 +175,37 @@ TEST_P(DlGoldenTest, StrokedRRectFastBlur) {
   ASSERT_TRUE(OpenPlaygroundHere(builder.Build()));
 }
 
+// Top left and bottom right circles are expected to be comparable (not exactly
+// equal).
+// See also: https://github.com/flutter/flutter/issues/152778
+TEST_P(DlGoldenTest, LargeDownscaleRrect) {
+  impeller::Point content_scale = GetContentScale();
+  auto draw = [&](DlCanvas* canvas, const std::vector<sk_sp<DlImage>>& images) {
+    canvas->Scale(content_scale.x, content_scale.y);
+    canvas->DrawColor(DlColor(0xff111111));
+    {
+      canvas->Save();
+      canvas->Scale(0.25, 0.25);
+      DlPaint paint;
+      paint.setColor(DlColor::kYellow());
+      paint.setMaskFilter(
+          DlBlurMaskFilter::Make(DlBlurStyle::kNormal, /*sigma=*/1000));
+      canvas->DrawCircle(SkPoint::Make(0, 0), 1200, paint);
+      canvas->Restore();
+    }
+
+    DlPaint paint;
+    paint.setColor(DlColor::kYellow());
+    paint.setMaskFilter(
+        DlBlurMaskFilter::Make(DlBlurStyle::kNormal, /*sigma=*/250));
+    canvas->DrawCircle(SkPoint::Make(1024, 768), 300, paint);
+  };
+
+  DisplayListBuilder builder;
+  draw(&builder, /*images=*/{});
+
+  ASSERT_TRUE(OpenPlaygroundHere(builder.Build()));
+}
+
 }  // namespace testing
 }  // namespace flutter

engine/src/flutter/impeller/entity/contents/solid_rrect_blur_contents.cc

@@ -128,10 +128,15 @@ bool SolidRRectBlurContents::Render(const ContentContext& renderer,
   using VS = RRectBlurPipeline::VertexShader;
   using FS = RRectBlurPipeline::FragmentShader;
 
-  // Clamp the max kernel width/height to 1000 to limit the extent
+  Matrix basis_invert = entity.GetTransform().Basis().Invert();
+  Vector2 max_sigmas =
+      Vector2((basis_invert * Vector2(500.f, 0.f)).GetLength(),
+              (basis_invert * Vector2(0.f, 500.f)).GetLength());
+  Scalar max_sigma = std::min(max_sigmas.x, max_sigmas.y);
+  // Clamp the max kernel width/height to 1000 (@ 2x) to limit the extent
   // of the blur and to kEhCloseEnough to prevent NaN calculations
-  // trying to evaluate a Guassian distribution with a sigma of 0.
-  Scalar blur_sigma = std::clamp(sigma_.sigma, kEhCloseEnough, 250.0f);
+  // trying to evaluate a Gaussian distribution with a sigma of 0.
+  auto blur_sigma = std::clamp(sigma_.sigma, kEhCloseEnough, max_sigma);
   // Increase quality by making the radius a bit bigger than the typical
   // sigma->radius conversion we use for slower blurs.
   Scalar blur_radius = PadForSigma(blur_sigma);

engine/src/flutter/testing/impeller_golden_tests_output.txt

@@ -988,6 +988,9 @@ impeller_Play_DlGoldenTest_GaussianVsRRectBlurScaled_Vulkan.png
 impeller_Play_DlGoldenTest_GaussianVsRRectBlur_Metal.png
 impeller_Play_DlGoldenTest_GaussianVsRRectBlur_OpenGLES.png
 impeller_Play_DlGoldenTest_GaussianVsRRectBlur_Vulkan.png
+impeller_Play_DlGoldenTest_LargeDownscaleRrect_Metal.png
+impeller_Play_DlGoldenTest_LargeDownscaleRrect_OpenGLES.png
+impeller_Play_DlGoldenTest_LargeDownscaleRrect_Vulkan.png
 impeller_Play_DlGoldenTest_SaveLayerAtFractionalValue_Metal.png
 impeller_Play_DlGoldenTest_SaveLayerAtFractionalValue_OpenGLES.png
 impeller_Play_DlGoldenTest_SaveLayerAtFractionalValue_Vulkan.png