Scale aa lines (#165917)

fixes https://github.com/flutter/flutter/issues/165905

Notice that the horizontal lines both have about 1 pixel of blurring for
both scales.

<img width="687" alt="Screenshot 2025-03-25 at 1 09 16 PM"
src="https://github.com/user-attachments/assets/b65dc1bf-6bf3-44d0-9605-95b51efc932c"
/>

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we expect every widget to implement].
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above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
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Fixes] where supported.
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engine/src/flutter/impeller/display_list/aiks_dl_path_unittests.cc

@@ -382,6 +382,41 @@ TEST_P(AiksTest, DrawLinesRenderCorrectly) {
   ASSERT_TRUE(OpenPlaygroundHere(builder.Build()));
 }
 
+// The goal of this test is to show that scaling the lines doesn't also scale
+// the antialiasing. The amount of blurring should be the same for both
+// horizontal lines.
+TEST_P(AiksTest, ScaleExperimentAntialiasLines) {
+  Scalar scale = 5.0;
+  Scalar line_width = 10.f;
+  auto callback = [&]() -> sk_sp<DisplayList> {
+    if (AiksTest::ImGuiBegin("Controls", nullptr,
+                             ImGuiWindowFlags_AlwaysAutoResize)) {
+      ImGui::SliderFloat("Scale", &scale, 0.001, 5);
+      ImGui::SliderFloat("Width", &line_width, 1, 20);
+
+      ImGui::End();
+    }
+    DisplayListBuilder builder;
+    builder.Scale(GetContentScale().x, GetContentScale().y);
+
+    DlPaint paint;
+    paint.setColor(DlColor::kGreenYellow());
+    paint.setStrokeWidth(line_width);
+
+    builder.DrawLine(DlPoint(100, 100), DlPoint(350, 100), paint);
+    builder.DrawLine(DlPoint(100, 100), DlPoint(350, 150), paint);
+
+    builder.Translate(100, 300);
+    builder.Scale(scale, scale);
+    builder.Translate(-100, -300);
+    builder.DrawLine(DlPoint(100, 300), DlPoint(350, 300), paint);
+    builder.DrawLine(DlPoint(100, 300), DlPoint(350, 450), paint);
+
+    return builder.Build();
+  };
+  ASSERT_TRUE(OpenPlaygroundHere(callback));
+}
+
 TEST_P(AiksTest, SimpleExperimentAntialiasLines) {
   DisplayListBuilder builder;
   builder.Scale(GetContentScale().x, GetContentScale().y);

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

@@ -24,7 +24,7 @@ using CreateGeometryCallback =
                                  const Geometry* geometry)>;
 
 const int32_t kCurveResolution = 32;
-const Scalar kSampleRadius = 0.5f;
+const float kSampleRadius = 1.f;
 
 struct LineInfo {
   Vector3 e0;
@@ -62,6 +62,8 @@ uint8_t DoubleToUint8(double x) {
 /// See also: CreateGradientTexture
 std::shared_ptr<Texture> CreateCurveTexture(
     Scalar width,
+    Scalar radius,
+    Scalar scale,
     const std::shared_ptr<impeller::Context>& context) {
   //
   impeller::TextureDescriptor texture_descriptor;
@@ -73,8 +75,8 @@ std::shared_ptr<Texture> CreateCurveTexture(
   curve_data.reserve(kCurveResolution);
   for (int i = 0; i < kCurveResolution; ++i) {
     double norm = (static_cast<double>(i) + 1.0) / 32.0;
-    double loc = norm * (kSampleRadius + width / 2.0);
-    double den = kSampleRadius * 2.0 + 1.0;
+    double loc = scale * norm * (radius + width / 2.0);
+    double den = radius * 2.0 + 1.0;
     curve_data.push_back(DoubleToUint8(loc / den));
   }
 
@@ -96,13 +98,14 @@ GeometryResult CreateGeometry(const ContentContext& renderer,
           corners, transform,
           /*extend_endpoints=*/line_geometry->GetCap() != Cap::kButt,
           line_geometry->GetP0(), line_geometry->GetP1(),
-          line_geometry->GetWidth() + kSampleRadius)) {
+          line_geometry->GetWidth() + kSampleRadius * 2.0)) {
     return kEmptyResult;
   }
 
   auto& host_buffer = renderer.GetTransientsBuffer();
 
   size_t count = 4;
+  Scalar scale = entity.GetTransform().GetMaxBasisLengthXY();
   LineInfo line_info =
       CalculateLineInfo(line_geometry->GetP0(), line_geometry->GetP1(),
                         line_geometry->GetWidth(), kSampleRadius);
@@ -121,8 +124,8 @@ GeometryResult CreateGeometry(const ContentContext& renderer,
         }
       });
 
-  std::shared_ptr<Texture> curve_texture =
-      CreateCurveTexture(line_geometry->GetWidth(), renderer.GetContext());
+  std::shared_ptr<Texture> curve_texture = CreateCurveTexture(
+      line_geometry->GetWidth(), kSampleRadius, scale, renderer.GetContext());
 
   SamplerDescriptor sampler_desc;
   sampler_desc.min_filter = MinMagFilter::kLinear;

engine/src/flutter/testing/impeller_golden_tests_output.txt

@@ -886,6 +886,7 @@ impeller_Play_AiksTest_SaveLayerDrawsBehindSubsequentEntities_Vulkan.png
 impeller_Play_AiksTest_SaveLayerFiltersScaleWithTransform_Metal.png
 impeller_Play_AiksTest_SaveLayerFiltersScaleWithTransform_OpenGLES.png
 impeller_Play_AiksTest_SaveLayerFiltersScaleWithTransform_Vulkan.png
+impeller_Play_AiksTest_ScaleExperimentAntialiasLines_Metal.png
 impeller_Play_AiksTest_ScaledK_Metal.png
 impeller_Play_AiksTest_ScaledK_OpenGLES.png
 impeller_Play_AiksTest_ScaledK_Vulkan.png