[Impeller] Remove the use of a subpixel minimum stroke size for MSAA (#56223)

Based on #55230

Fixes flutter/flutter#156438

Author: jason-simmons

impeller/entity/geometry/circle_geometry.cc

@@ -42,11 +42,9 @@ GeometryResult CircleGeometry::GetPositionBuffer(const ContentContext& renderer,
                                                  RenderPass& pass) const {
   auto& transform = entity.GetTransform();
 
-  Scalar half_width = stroke_width_ < 0
-                          ? 0.0
-                          : LineGeometry::ComputePixelHalfWidth(
-                                transform, stroke_width_,
-                                pass.GetSampleCount() == SampleCount::kCount4);
+  Scalar half_width = stroke_width_ < 0 ? 0.0
+                                        : LineGeometry::ComputePixelHalfWidth(
+                                              transform, stroke_width_);
 
   const std::shared_ptr<Tessellator>& tessellator = renderer.GetTessellator();
 

impeller/entity/geometry/geometry.cc

@@ -133,9 +133,7 @@ bool Geometry::CanApplyMaskFilter() const {
 Scalar Geometry::ComputeStrokeAlphaCoverage(const Matrix& transform,
                                             Scalar stroke_width) {
   Scalar scaled_stroke_width = transform.GetMaxBasisLengthXY() * stroke_width;
-  // If the stroke width is 0 or greater than kMinStrokeSizeMSAA, don't apply
-  // any additional alpha. This is intended to match Skia behavior.
-  if (scaled_stroke_width == 0.0 || scaled_stroke_width >= kMinStrokeSizeMSAA) {
+  if (scaled_stroke_width == 0.0 || scaled_stroke_width >= kMinStrokeSize) {
     return 1.0;
   }
   // This scalling is eyeballed from Skia.

impeller/entity/geometry/geometry.h

@@ -16,11 +16,6 @@ namespace impeller {
 
 class Tessellator;
 
-/// @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;
 
 struct GeometryResult {

impeller/entity/geometry/line_geometry.cc

@@ -15,21 +15,19 @@ LineGeometry::LineGeometry(Point p0, Point p1, Scalar width, Cap cap)
 LineGeometry::~LineGeometry() = default;
 
 Scalar LineGeometry::ComputePixelHalfWidth(const Matrix& transform,
-                                           Scalar width,
-                                           bool msaa) {
+                                           Scalar width) {
   Scalar max_basis = transform.GetMaxBasisLengthXY();
   if (max_basis == 0) {
     return {};
   }
 
-  Scalar min_size = (msaa ? kMinStrokeSize : kMinStrokeSizeMSAA) / max_basis;
+  Scalar min_size = kMinStrokeSize / max_basis;
   return std::max(width, min_size) * 0.5f;
 }
 
 Vector2 LineGeometry::ComputeAlongVector(const Matrix& transform,
-                                         bool allow_zero_length,
-                                         bool msaa) const {
-  Scalar stroke_half_width = ComputePixelHalfWidth(transform, width_, msaa);
+                                         bool allow_zero_length) const {
+  Scalar stroke_half_width = ComputePixelHalfWidth(transform, width_);
   if (stroke_half_width < kEhCloseEnough) {
     return {};
   }
@@ -49,9 +47,8 @@ Vector2 LineGeometry::ComputeAlongVector(const Matrix& transform,
 
 bool LineGeometry::ComputeCorners(Point corners[4],
                                   const Matrix& transform,
-                                  bool extend_endpoints,
-                                  bool msaa) const {
-  auto along = ComputeAlongVector(transform, extend_endpoints, msaa);
+                                  bool extend_endpoints) const {
+  auto along = ComputeAlongVector(transform, extend_endpoints);
   if (along.IsZero()) {
     return false;
   }
@@ -80,8 +77,7 @@ GeometryResult LineGeometry::GetPositionBuffer(const ContentContext& renderer,
   using VT = SolidFillVertexShader::PerVertexData;
 
   auto& transform = entity.GetTransform();
-  auto radius = ComputePixelHalfWidth(
-      transform, width_, pass.GetSampleCount() == SampleCount::kCount4);
+  auto radius = ComputePixelHalfWidth(transform, width_);
 
   if (cap_ == Cap::kRound) {
     std::shared_ptr<Tessellator> tessellator = renderer.GetTessellator();
@@ -90,8 +86,7 @@ GeometryResult LineGeometry::GetPositionBuffer(const ContentContext& renderer,
   }
 
   Point corners[4];
-  if (!ComputeCorners(corners, transform, cap_ == Cap::kSquare,
-                      pass.GetSampleCount() == SampleCount::kCount4)) {
+  if (!ComputeCorners(corners, transform, cap_ == Cap::kSquare)) {
     return kEmptyResult;
   }
 
@@ -123,7 +118,7 @@ GeometryResult LineGeometry::GetPositionBuffer(const ContentContext& renderer,
 std::optional<Rect> LineGeometry::GetCoverage(const Matrix& transform) const {
   Point corners[4];
   // Note: MSAA boolean doesn't matter for coverage computation.
-  if (!ComputeCorners(corners, transform, cap_ != Cap::kButt, /*msaa=*/false)) {
+  if (!ComputeCorners(corners, transform, cap_ != Cap::kButt)) {
     return {};
   }
 

impeller/entity/geometry/line_geometry.h

@@ -15,9 +15,7 @@ class LineGeometry final : public Geometry {
 
   ~LineGeometry() override;
 
-  static Scalar ComputePixelHalfWidth(const Matrix& transform,
-                                      Scalar width,
-                                      bool msaa);
+  static Scalar ComputePixelHalfWidth(const Matrix& transform, Scalar width);
 
   // |Geometry|
   bool CoversArea(const Matrix& transform, const Rect& rect) const override;
@@ -44,12 +42,10 @@ class LineGeometry final : public Geometry {
   // @return true if the transform and width were not degenerate
   bool ComputeCorners(Point corners[4],
                       const Matrix& transform,
-                      bool extend_endpoints,
-                      bool msaa) const;
+                      bool extend_endpoints) const;
 
   Vector2 ComputeAlongVector(const Matrix& transform,
-                             bool allow_zero_length,
-                             bool msaa) const;
+                             bool allow_zero_length) const;
 
   // |Geometry|
   GeometryResult GetPositionBuffer(const ContentContext& renderer,

impeller/entity/geometry/stroke_path_geometry.cc

@@ -574,10 +574,7 @@ GeometryResult StrokePathGeometry::GetPositionBuffer(
     return {};
   }
 
-  Scalar min_size =
-      (pass.GetSampleCount() == SampleCount::kCount4 ? kMinStrokeSizeMSAA
-                                                     : kMinStrokeSize) /
-      max_basis;
+  Scalar min_size = kMinStrokeSize / max_basis;
   Scalar stroke_width = std::max(stroke_width_, min_size);
 
   auto& host_buffer = renderer.GetTransientsBuffer();