From 2cb41967e7340e2fb955ca103d9cdc1ac81fa27d Mon Sep 17 00:00:00 2001
From: gaaclarke <30870216+gaaclarke@users.noreply.github.com>
Date: Fri, 20 Oct 2023 16:40:06 -0700
Subject: [PATCH] [Impeller] increased padding for blurs (flutter/engine#47176)

fixes https://github.com/flutter/flutter/issues/119974

This duplicates the math found in skia at
https://skia.googlesource.com/skia/+/cf2131f85d57f938472e50b6643c3c2348709a55/gm/blurrect.cpp#484

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---
 .../src/flutter/impeller/aiks/aiks_unittests.cc  | 16 ++++++++++++++++
 .../entity/contents/solid_rrect_blur_contents.cc | 14 +++++++++++---
 2 files changed, 27 insertions(+), 3 deletions(-)

diff --git a/engine/src/flutter/impeller/aiks/aiks_unittests.cc b/engine/src/flutter/impeller/aiks/aiks_unittests.cc
index 3dda081b04..b0e4139f4e 100644
--- a/engine/src/flutter/impeller/aiks/aiks_unittests.cc
+++ b/engine/src/flutter/impeller/aiks/aiks_unittests.cc
@@ -3687,5 +3687,21 @@ TEST_P(AiksTest, ClearColorOptimizationWhenSubpassIsBiggerThanParentPass) {
   ASSERT_TRUE(OpenPlaygroundHere(canvas.EndRecordingAsPicture()));
 }
 
+TEST_P(AiksTest, BlurHasNoEdge) {
+  Canvas canvas;
+  canvas.Scale(GetContentScale());
+  canvas.DrawPaint({});
+  Paint blur = {
+      .color = Color::Green(),
+      .mask_blur_descriptor =
+          Paint::MaskBlurDescriptor{
+              .style = FilterContents::BlurStyle::kNormal,
+              .sigma = Sigma(47.6),
+          },
+  };
+  canvas.DrawRect(Rect{300, 300, 200, 200}, blur);
+  ASSERT_TRUE(OpenPlaygroundHere(canvas.EndRecordingAsPicture()));
+}
+
 }  // namespace testing
 }  // namespace impeller
diff --git a/engine/src/flutter/impeller/entity/contents/solid_rrect_blur_contents.cc b/engine/src/flutter/impeller/entity/contents/solid_rrect_blur_contents.cc
index 665cb599bc..e549856dc1 100644
--- a/engine/src/flutter/impeller/entity/contents/solid_rrect_blur_contents.cc
+++ b/engine/src/flutter/impeller/entity/contents/solid_rrect_blur_contents.cc
@@ -16,6 +16,14 @@
 
 namespace impeller {
 
+namespace {
+// Generous padding to make sure blurs with large sigmas are fully visible.
+// Used to expand the geometry around the rrect.
+Scalar PadForSigma(Scalar sigma) {
+  return sigma * 4.0;
+}
+}  // namespace
+
 SolidRRectBlurContents::SolidRRectBlurContents() = default;
 
 SolidRRectBlurContents::~SolidRRectBlurContents() = default;
@@ -44,7 +52,7 @@ std::optional<Rect> SolidRRectBlurContents::GetCoverage(
     return std::nullopt;
   }
 
-  Scalar radius = sigma_.sigma * 2;
+  Scalar radius = PadForSigma(sigma_.sigma);
 
   auto ltrb = rect_->GetLTRB();
   Rect bounds = Rect::MakeLTRB(ltrb[0] - radius, ltrb[1] - radius,
@@ -66,9 +74,9 @@ bool SolidRRectBlurContents::Render(const ContentContext& renderer,
 
   // Clamp the max kernel width/height to 1000.
   auto blur_sigma = std::min(sigma_.sigma, 250.0f);
-  // Increase quality by make the radius a bit bigger than the typical
+  // Increase quality by making the radius a bit bigger than the typical
   // sigma->radius conversion we use for slower blurs.
-  auto blur_radius = blur_sigma * 2;
+  auto blur_radius = PadForSigma(blur_sigma);
   auto positive_rect = rect_->GetPositive();
   {
     auto left = -blur_radius;