[Impeller] scales blur coverage to match rendered output (flutter/engine#47621)

in pursuit of: https://github.com/flutter/flutter/issues/131580

I'm starting to write tests for the new blur, but I wanted to make tests for the old blur to make sure I understood the coverage rules.  I decided to check this in separately while I work on the other blur.

Even though the goal is to delete this class eventually.  Most of this work transfers over to the new blur.

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

engine/src/flutter/ci/licenses_golden/excluded_files

@@ -135,6 +135,7 @@
 ../../../flutter/impeller/display_list/skia_conversions_unittests.cc
 ../../../flutter/impeller/docs
 ../../../flutter/impeller/entity/contents/checkerboard_contents_unittests.cc
+../../../flutter/impeller/entity/contents/filters/directional_gaussian_blur_filter_contents_unittests.cc
 ../../../flutter/impeller/entity/contents/filters/inputs/filter_input_unittests.cc
 ../../../flutter/impeller/entity/contents/test
 ../../../flutter/impeller/entity/contents/vertices_contents_unittests.cc

engine/src/flutter/impeller/entity/BUILD.gn

@@ -280,6 +280,7 @@ impeller_component("entity_unittests") {
 
   sources = [
     "contents/checkerboard_contents_unittests.cc",
+    "contents/filters/directional_gaussian_blur_filter_contents_unittests.cc",
     "contents/filters/inputs/filter_input_unittests.cc",
     "contents/vertices_contents_unittests.cc",
     "entity_pass_target_unittests.cc",

engine/src/flutter/impeller/entity/contents/filters/directional_gaussian_blur_filter_contents.cc

@@ -313,7 +313,9 @@ std::optional<Rect> DirectionalGaussianBlurFilterContents::GetFilterCoverage(
 
   auto transform = inputs[0]->GetTransform(entity) * effect_transform.Basis();
   auto transformed_blur_vector =
-      transform.TransformDirection(blur_direction_ * Radius{blur_sigma_}.radius)
+      transform
+          .TransformDirection(blur_direction_ *
+                              Radius{ScaleSigma(blur_sigma_)}.radius)
           .Abs();
   return coverage->Expand(transformed_blur_vector);
 }

engine/src/flutter/impeller/entity/contents/filters/directional_gaussian_blur_filter_contents_unittests.cc

@@ -0,0 +1,129 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "flutter/testing/testing.h"
+#include "gmock/gmock.h"
+#include "impeller/entity/contents/content_context.h"
+#include "impeller/entity/contents/filters/directional_gaussian_blur_filter_contents.h"
+#include "impeller/entity/entity_playground.h"
+#include "impeller/renderer/testing/mocks.h"
+
+namespace impeller {
+namespace testing {
+
+using ::testing::Return;
+
+namespace {
+
+Scalar CalculateSigmaForBlurRadius(Scalar blur_radius) {
+  // See Sigma.h
+  return (blur_radius / kKernelRadiusPerSigma) + 0.5;
+}
+}  // namespace
+
+class DirectionalGaussianBlurFilterContentsTest : public EntityPlayground {
+ public:
+  // Stubs in the minimal support to make rendering pass.
+  void SetupMinimalMockContext() {
+    // This mocking code was removed since it wasn't strictly needed yet. If it
+    // is needed you can find it here:
+    // https://gist.github.com/gaaclarke/c2f6bf5fc6ecb10678da03789abc5843.
+  }
+};
+
+INSTANTIATE_PLAYGROUND_SUITE(DirectionalGaussianBlurFilterContentsTest);
+
+TEST_P(DirectionalGaussianBlurFilterContentsTest, CoverageWithEffectTransform) {
+  TextureDescriptor desc = {
+      .format = PixelFormat::kB8G8R8A8UNormInt,
+      .size = ISize(100, 100),
+  };
+  Scalar sigma_radius_1 = CalculateSigmaForBlurRadius(1.0);
+  auto contents = std::make_unique<DirectionalGaussianBlurFilterContents>();
+  contents->SetSigma(Sigma{sigma_radius_1});
+  contents->SetDirection({1.0, 0.0});
+  std::shared_ptr<Texture> texture =
+      GetContentContext()->GetContext()->GetResourceAllocator()->CreateTexture(
+          desc);
+  FilterInput::Vector inputs = {FilterInput::Make(texture)};
+  Entity entity;
+  entity.SetTransformation(Matrix::MakeTranslation({100, 100, 0}));
+  std::optional<Rect> coverage = contents->GetFilterCoverage(
+      inputs, entity, /*effect_transform=*/Matrix::MakeScale({2.0, 2.0, 1.0}));
+  EXPECT_TRUE(coverage.has_value());
+  if (coverage.has_value()) {
+    EXPECT_NEAR(coverage->GetLeft(), 100 - 2,
+                0.5);  // Higher tolerance for sigma scaling.
+    EXPECT_NEAR(coverage->GetTop(), 100, 0.01);
+    EXPECT_NEAR(coverage->GetRight(), 200 + 2,
+                0.5);  // Higher tolerance for sigma scaling.
+    EXPECT_NEAR(coverage->GetBottom(), 200, 0.01);
+  }
+}
+
+TEST(DirectionalGaussianBlurFilterContentsTest, FilterSourceCoverage) {
+  Scalar sigma_radius_1 = CalculateSigmaForBlurRadius(1.0);
+  auto contents = std::make_unique<DirectionalGaussianBlurFilterContents>();
+  contents->SetSigma(Sigma{sigma_radius_1});
+  contents->SetDirection({1.0, 0.0});
+  std::optional<Rect> coverage = contents->GetFilterSourceCoverage(
+      /*effect_transform=*/Matrix::MakeScale({2.0, 2.0, 1.0}),
+      /*output_limit=*/Rect::MakeLTRB(100, 100, 200, 200));
+  ASSERT_EQ(coverage, Rect::MakeLTRB(100 - 2, 100, 200 + 2, 200));
+}
+
+TEST_P(DirectionalGaussianBlurFilterContentsTest, RenderNoCoverage) {
+  Scalar sigma_radius_1 = CalculateSigmaForBlurRadius(1.0);
+  auto contents = std::make_unique<DirectionalGaussianBlurFilterContents>();
+  contents->SetSigma(Sigma{sigma_radius_1});
+  contents->SetDirection({1.0, 0.0});
+  std::shared_ptr<ContentContext> renderer = GetContentContext();
+  Entity entity;
+  Rect coverage_hint = Rect::MakeLTRB(0, 0, 0, 0);
+  std::optional<Entity> result =
+      contents->GetEntity(*renderer, entity, coverage_hint);
+  ASSERT_FALSE(result.has_value());
+}
+
+TEST_P(DirectionalGaussianBlurFilterContentsTest,
+       RenderCoverageMatchesGetCoverage) {
+  TextureDescriptor desc = {
+      .format = PixelFormat::kB8G8R8A8UNormInt,
+      .size = ISize(100, 100),
+  };
+  std::shared_ptr<Texture> texture =
+      GetContentContext()->GetContext()->GetResourceAllocator()->CreateTexture(
+          desc);
+  Scalar sigma_radius_1 = CalculateSigmaForBlurRadius(1.0);
+  auto contents = std::make_unique<DirectionalGaussianBlurFilterContents>();
+  contents->SetSigma(Sigma{sigma_radius_1});
+  contents->SetDirection({1.0, 0.0});
+  contents->SetInputs({FilterInput::Make(texture)});
+  std::shared_ptr<ContentContext> renderer = GetContentContext();
+
+  Entity entity;
+  std::optional<Entity> result =
+      contents->GetEntity(*renderer, entity, /*coverage_hint=*/{});
+  EXPECT_TRUE(result.has_value());
+  if (result.has_value()) {
+    EXPECT_EQ(result.value().GetBlendMode(), BlendMode::kSourceOver);
+    std::optional<Rect> result_coverage = result.value().GetCoverage();
+    std::optional<Rect> contents_coverage = contents->GetCoverage(entity);
+    EXPECT_TRUE(result_coverage.has_value());
+    EXPECT_TRUE(contents_coverage.has_value());
+    if (result_coverage.has_value() && contents_coverage.has_value()) {
+      EXPECT_NEAR(result_coverage.value().GetLeft(),
+                  contents_coverage.value().GetLeft(), kEhCloseEnough);
+      EXPECT_NEAR(result_coverage.value().GetTop(),
+                  contents_coverage.value().GetTop(), kEhCloseEnough);
+      EXPECT_NEAR(result_coverage.value().GetRight(),
+                  contents_coverage.value().GetRight(), kEhCloseEnough);
+      EXPECT_NEAR(result_coverage.value().GetBottom(),
+                  contents_coverage.value().GetBottom(), kEhCloseEnough);
+    }
+  }
+}
+
+}  // namespace testing
+}  // namespace impeller