[Impeller] make strokes slightly lighter. (#53067)

We can set the minimum stroke width to 0.5 to guarantee at least one hit of 4x MSAA coverage. This doesn't fix stroke fidelity issues but it does make it a bit better.

flutter/flutter#144313

Author: jonahwilliams

impeller/aiks/aiks_path_unittests.cc

@@ -48,6 +48,18 @@ TEST_P(AiksTest, CanRenderThickCurvedStrokes) {
   ASSERT_TRUE(OpenPlaygroundHere(canvas.EndRecordingAsPicture()));
 }
 
+TEST_P(AiksTest, CanRenderThinCurvedStrokes) {
+  Canvas canvas;
+  Paint paint;
+  paint.color = Color::Red();
+  // Impeller doesn't support hairlines yet, but size this guarantees
+  // the smallest possible stroke width.
+  paint.stroke_width = 0.01;
+  paint.style = Paint::Style::kStroke;
+  canvas.DrawPath(PathBuilder{}.AddCircle({100, 100}, 50).TakePath(), paint);
+  ASSERT_TRUE(OpenPlaygroundHere(canvas.EndRecordingAsPicture()));
+}
+
 TEST_P(AiksTest, CanRenderStrokePathThatEndsAtSharpTurn) {
   Canvas canvas;
 

impeller/entity/geometry/stroke_path_geometry.cc

@@ -15,6 +15,13 @@ using VS = SolidFillVertexShader;
 
 namespace {
 
+/// @brief The minimum stroke size can be less than one physical pixel because
+///        of MSAA, but no less that half a physical pixel otherwise we might
+///        not hit one of the sample positions.
+static constexpr Scalar kMinStrokeSizeMSAA = 0.5f;
+
+static constexpr Scalar kMinStrokeSize = 1.0f;
+
 template <typename VertexWriter>
 using CapProc = std::function<void(VertexWriter& vtx_builder,
                                    const Point& position,
@@ -530,7 +537,10 @@ GeometryResult StrokePathGeometry::GetPositionBuffer(
     return {};
   }
 
-  Scalar min_size = 1.0f / sqrt(std::abs(determinant));
+  Scalar min_size =
+      (pass.GetSampleCount() == SampleCount::kCount4 ? kMinStrokeSizeMSAA
+                                                     : kMinStrokeSize) /
+      sqrt(std::abs(determinant));
   Scalar stroke_width = std::max(stroke_width_, min_size);
 
   auto& host_buffer = renderer.GetTransientsBuffer();
@@ -584,7 +594,8 @@ std::optional<Rect> StrokePathGeometry::GetCoverage(
   if (determinant == 0) {
     return std::nullopt;
   }
-  Scalar min_size = 1.0f / sqrt(std::abs(determinant));
+  // Use the most conervative coverage setting.
+  Scalar min_size = kMinStrokeSize / sqrt(std::abs(determinant));
   max_radius *= std::max(stroke_width_, min_size);
   return path_bounds->Expand(max_radius).TransformBounds(transform);
 }

testing/impeller_golden_tests_output.txt

@@ -460,6 +460,9 @@ impeller_Play_AiksTest_CanRenderTextWithLargePerspectiveTransform_Vulkan.png
 impeller_Play_AiksTest_CanRenderThickCurvedStrokes_Metal.png
 impeller_Play_AiksTest_CanRenderThickCurvedStrokes_OpenGLES.png
 impeller_Play_AiksTest_CanRenderThickCurvedStrokes_Vulkan.png
+impeller_Play_AiksTest_CanRenderThinCurvedStrokes_Metal.png
+impeller_Play_AiksTest_CanRenderThinCurvedStrokes_OpenGLES.png
+impeller_Play_AiksTest_CanRenderThinCurvedStrokes_Vulkan.png
 impeller_Play_AiksTest_CanRenderTiledTextureClampWithTranslate_Metal.png
 impeller_Play_AiksTest_CanRenderTiledTextureClampWithTranslate_OpenGLES.png
 impeller_Play_AiksTest_CanRenderTiledTextureClampWithTranslate_Vulkan.png