[Impeller] make strokes < 0.5 physical pixels visually thinner.

impeller/entity/contents/conical_gradient_contents.cc

@@ -73,13 +73,14 @@ bool ConicalGradientContents::RenderSSBO(const ContentContext& renderer,
       };
   return ColorSourceContents::DrawGeometry<VS>(
       renderer, entity, pass, pipeline_callback, frame_info,
-      [this, &renderer](RenderPass& pass) {
+      [this, &renderer, &entity](RenderPass& pass) {
         FS::FragInfo frag_info;
         frag_info.center = center_;
         frag_info.radius = radius_;
         frag_info.tile_mode = static_cast<Scalar>(tile_mode_);
         frag_info.decal_border_color = decal_border_color_;
-        frag_info.alpha = GetOpacityFactor();
+        frag_info.alpha =
+            GetOpacityFactor() * GetGeometry()->ComputeAlphaCoverage(entity);
         if (focus_) {
           frag_info.focus = focus_.value();
           frag_info.focus_radius = focus_radius_;
@@ -127,15 +128,16 @@ bool ConicalGradientContents::RenderTexture(const ContentContext& renderer,
       };
   return ColorSourceContents::DrawGeometry<VS>(
       renderer, entity, pass, pipeline_callback, frame_info,
-      [this, &renderer, &gradient_texture](RenderPass& pass) {
+      [this, &renderer, &gradient_texture, &entity](RenderPass& pass) {
         FS::FragInfo frag_info;
         frag_info.center = center_;
         frag_info.radius = radius_;
         frag_info.tile_mode = static_cast<Scalar>(tile_mode_);
         frag_info.decal_border_color = decal_border_color_;
         frag_info.texture_sampler_y_coord_scale =
             gradient_texture->GetYCoordScale();
-        frag_info.alpha = GetOpacityFactor();
+        frag_info.alpha =
+            GetOpacityFactor() * GetGeometry()->ComputeAlphaCoverage(entity);
         frag_info.half_texel =
             Vector2(0.5 / gradient_texture->GetSize().width,
                     0.5 / gradient_texture->GetSize().height);

impeller/entity/contents/linear_gradient_contents.cc

@@ -77,7 +77,7 @@ bool LinearGradientContents::RenderTexture(const ContentContext& renderer,
       };
   return ColorSourceContents::DrawGeometry<VS>(
       renderer, entity, pass, pipeline_callback, frame_info,
-      [this, &renderer](RenderPass& pass) {
+      [this, &renderer, &entity](RenderPass& pass) {
         auto gradient_data = CreateGradientBuffer(colors_, stops_);
         auto gradient_texture =
             CreateGradientTexture(gradient_data, renderer.GetContext());
@@ -92,7 +92,9 @@ bool LinearGradientContents::RenderTexture(const ContentContext& renderer,
         frag_info.decal_border_color = decal_border_color_;
         frag_info.texture_sampler_y_coord_scale =
             gradient_texture->GetYCoordScale();
-        frag_info.alpha = GetOpacityFactor();
+        frag_info.alpha =
+            GetOpacityFactor() * GetGeometry()->ComputeAlphaCoverage(entity);
+        ;
         frag_info.half_texel =
             Vector2(0.5 / gradient_texture->GetSize().width,
                     0.5 / gradient_texture->GetSize().height);
@@ -137,13 +139,14 @@ bool LinearGradientContents::RenderSSBO(const ContentContext& renderer,
       };
   return ColorSourceContents::DrawGeometry<VS>(
       renderer, entity, pass, pipeline_callback, frame_info,
-      [this, &renderer](RenderPass& pass) {
+      [this, &renderer, &entity](RenderPass& pass) {
         FS::FragInfo frag_info;
         frag_info.start_point = start_point_;
         frag_info.end_point = end_point_;
         frag_info.tile_mode = static_cast<Scalar>(tile_mode_);
         frag_info.decal_border_color = decal_border_color_;
-        frag_info.alpha = GetOpacityFactor();
+        frag_info.alpha =
+            GetOpacityFactor() * GetGeometry()->ComputeAlphaCoverage(entity);
         frag_info.start_to_end = end_point_ - start_point_;
         frag_info.inverse_dot_start_to_end =
             CalculateInverseDotStartToEnd(start_point_, end_point_);

impeller/entity/contents/radial_gradient_contents.cc

@@ -79,13 +79,14 @@ bool RadialGradientContents::RenderSSBO(const ContentContext& renderer,
       };
   return ColorSourceContents::DrawGeometry<VS>(
       renderer, entity, pass, pipeline_callback, frame_info,
-      [this, &renderer](RenderPass& pass) {
+      [this, &renderer, &entity](RenderPass& pass) {
         FS::FragInfo frag_info;
         frag_info.center = center_;
         frag_info.radius = radius_;
         frag_info.tile_mode = static_cast<Scalar>(tile_mode_);
         frag_info.decal_border_color = decal_border_color_;
-        frag_info.alpha = GetOpacityFactor();
+        frag_info.alpha =
+            GetOpacityFactor() * GetGeometry()->ComputeAlphaCoverage(entity);
 
         auto& host_buffer = renderer.GetTransientsBuffer();
         auto colors = CreateGradientColors(colors_, stops_);
@@ -126,15 +127,16 @@ bool RadialGradientContents::RenderTexture(const ContentContext& renderer,
       };
   return ColorSourceContents::DrawGeometry<VS>(
       renderer, entity, pass, pipeline_callback, frame_info,
-      [this, &renderer, &gradient_texture](RenderPass& pass) {
+      [this, &renderer, &gradient_texture, &entity](RenderPass& pass) {
         FS::FragInfo frag_info;
         frag_info.center = center_;
         frag_info.radius = radius_;
         frag_info.tile_mode = static_cast<Scalar>(tile_mode_);
         frag_info.decal_border_color = decal_border_color_;
         frag_info.texture_sampler_y_coord_scale =
             gradient_texture->GetYCoordScale();
-        frag_info.alpha = GetOpacityFactor();
+        frag_info.alpha =
+            GetOpacityFactor() * GetGeometry()->ComputeAlphaCoverage(entity);
         frag_info.half_texel =
             Vector2(0.5 / gradient_texture->GetSize().width,
                     0.5 / gradient_texture->GetSize().height);

impeller/entity/contents/solid_color_contents.cc

@@ -51,7 +51,8 @@ bool SolidColorContents::Render(const ContentContext& renderer,
   using VS = SolidFillPipeline::VertexShader;
 
   VS::FrameInfo frame_info;
-  frame_info.color = GetColor().Premultiply();
+  frame_info.color =
+      GetColor().Premultiply() * GetGeometry()->ComputeAlphaCoverage(entity);
 
   PipelineBuilderCallback pipeline_callback =
       [&renderer](ContentContextOptions options) {

impeller/entity/contents/sweep_gradient_contents.cc

@@ -87,14 +87,15 @@ bool SweepGradientContents::RenderSSBO(const ContentContext& renderer,
       };
   return ColorSourceContents::DrawGeometry<VS>(
       renderer, entity, pass, pipeline_callback, frame_info,
-      [this, &renderer](RenderPass& pass) {
+      [this, &renderer, &entity](RenderPass& pass) {
         FS::FragInfo frag_info;
         frag_info.center = center_;
         frag_info.bias = bias_;
         frag_info.scale = scale_;
         frag_info.tile_mode = static_cast<Scalar>(tile_mode_);
         frag_info.decal_border_color = decal_border_color_;
-        frag_info.alpha = GetOpacityFactor();
+        frag_info.alpha =
+            GetOpacityFactor() * GetGeometry()->ComputeAlphaCoverage(entity);
 
         auto& host_buffer = renderer.GetTransientsBuffer();
         auto colors = CreateGradientColors(colors_, stops_);
@@ -136,7 +137,7 @@ bool SweepGradientContents::RenderTexture(const ContentContext& renderer,
       };
   return ColorSourceContents::DrawGeometry<VS>(
       renderer, entity, pass, pipeline_callback, frame_info,
-      [this, &renderer, &gradient_texture](RenderPass& pass) {
+      [this, &renderer, &gradient_texture, &entity](RenderPass& pass) {
         FS::FragInfo frag_info;
         frag_info.center = center_;
         frag_info.bias = bias_;
@@ -145,7 +146,8 @@ bool SweepGradientContents::RenderTexture(const ContentContext& renderer,
             gradient_texture->GetYCoordScale();
         frag_info.tile_mode = static_cast<Scalar>(tile_mode_);
         frag_info.decal_border_color = decal_border_color_;
-        frag_info.alpha = GetOpacityFactor();
+        frag_info.alpha =
+            GetOpacityFactor() * GetGeometry()->ComputeAlphaCoverage(entity);
         frag_info.half_texel =
             Vector2(0.5 / gradient_texture->GetSize().width,
                     0.5 / gradient_texture->GetSize().height);

impeller/entity/contents/tiled_texture_contents.cc

@@ -150,7 +150,7 @@ bool TiledTextureContents::Render(const ContentContext& renderer,
       };
   return ColorSourceContents::DrawGeometry<VS>(
       renderer, entity, pass, pipeline_callback, frame_info,
-      [this, &renderer, &is_external_texture](RenderPass& pass) {
+      [this, &renderer, &is_external_texture, &entity](RenderPass& pass) {
         auto& host_buffer = renderer.GetTransientsBuffer();
 
         pass.SetCommandLabel("TextureFill");
@@ -159,13 +159,15 @@ bool TiledTextureContents::Render(const ContentContext& renderer,
           FSExternal::FragInfo frag_info;
           frag_info.x_tile_mode = static_cast<Scalar>(x_tile_mode_);
           frag_info.y_tile_mode = static_cast<Scalar>(y_tile_mode_);
-          frag_info.alpha = GetOpacityFactor();
+          frag_info.alpha =
+              GetOpacityFactor() * GetGeometry()->ComputeAlphaCoverage(entity);
           FSExternal::BindFragInfo(pass, host_buffer.EmplaceUniform(frag_info));
         } else {
           FS::FragInfo frag_info;
           frag_info.x_tile_mode = static_cast<Scalar>(x_tile_mode_);
           frag_info.y_tile_mode = static_cast<Scalar>(y_tile_mode_);
-          frag_info.alpha = GetOpacityFactor();
+          frag_info.alpha =
+              GetOpacityFactor() * GetGeometry()->ComputeAlphaCoverage(entity);
           FS::BindFragInfo(pass, host_buffer.EmplaceUniform(frag_info));
         }
 

impeller/entity/geometry/geometry.h

@@ -107,6 +107,10 @@ class Geometry {
 
   virtual bool CanApplyMaskFilter() const;
 
+  virtual Scalar ComputeAlphaCoverage(const Entity& entitys) const {
+    return 1.0;
+  }
+
  protected:
   static GeometryResult ComputePositionGeometry(
       const ContentContext& renderer,

impeller/entity/geometry/geometry_unittests.cc

@@ -4,9 +4,11 @@
 
 #include <memory>
 #include "flutter/testing/testing.h"
+#include "gtest/gtest.h"
 #include "impeller/entity/contents/content_context.h"
 #include "impeller/entity/geometry/geometry.h"
 #include "impeller/entity/geometry/stroke_path_geometry.h"
+#include "impeller/geometry/constants.h"
 #include "impeller/geometry/geometry_asserts.h"
 #include "impeller/geometry/path_builder.h"
 #include "impeller/renderer/testing/mocks.h"
@@ -134,5 +136,21 @@ TEST(EntityGeometryTest, GeometryResultHasReasonableDefaults) {
   EXPECT_EQ(result.mode, GeometryResult::Mode::kNormal);
 }
 
+TEST(EntityGeometryTest, AlphaCoverageStrokePaths) {
+  Entity entity;
+  entity.SetTransform(Matrix::MakeScale(Vector2{3.0, 3.0}));
+  EXPECT_EQ(Geometry::MakeStrokePath({}, 0.5)->ComputeAlphaCoverage(entity), 1);
+  EXPECT_EQ(Geometry::MakeStrokePath({}, 0.1)->ComputeAlphaCoverage(entity), 1);
+  EXPECT_EQ(Geometry::MakeStrokePath({}, 0.05)->ComputeAlphaCoverage(entity),
+            1);
+  EXPECT_NEAR(Geometry::MakeStrokePath({}, 0.01)->ComputeAlphaCoverage(entity),
+              0.6, 0.1);
+  EXPECT_NEAR(
+      Geometry::MakeStrokePath({}, 0.0000005)->ComputeAlphaCoverage(entity),
+      1e-05, 0.001);
+  EXPECT_EQ(Geometry::MakeStrokePath({}, 0)->ComputeAlphaCoverage(entity), 1);
+  EXPECT_EQ(Geometry::MakeStrokePath({}, 40)->ComputeAlphaCoverage(entity), 1);
+}
+
 }  // namespace testing
 }  // namespace impeller

impeller/entity/geometry/stroke_path_geometry.cc

@@ -525,6 +525,18 @@ Join StrokePathGeometry::GetStrokeJoin() const {
   return stroke_join_;
 }
 
+Scalar StrokePathGeometry::ComputeAlphaCoverage(const Entity& entity) const {
+  Scalar scaled_stroke_width =
+      entity.GetTransform().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) {
+    return 1.0;
+  }
+  // This scalling is eyeballed from Skia.
+  return std::clamp(scaled_stroke_width * 20.0f, 0.f, 1.f);
+}
+
 GeometryResult StrokePathGeometry::GetPositionBuffer(
     const ContentContext& renderer,
     const Entity& entity,
@@ -568,8 +580,7 @@ GeometryResult StrokePathGeometry::GetPositionBuffer(
               .index_type = IndexType::kNone,
           },
       .transform = entity.GetShaderTransform(pass),
-      .mode = GeometryResult::Mode::kPreventOverdraw,
-  };
+      .mode = GeometryResult::Mode::kPreventOverdraw};
 }
 
 GeometryResult::Mode StrokePathGeometry::GetResultMode() const {

impeller/entity/geometry/stroke_path_geometry.h

@@ -28,6 +28,8 @@ class StrokePathGeometry final : public Geometry {
 
   Join GetStrokeJoin() const;
 
+  Scalar ComputeAlphaCoverage(const Entity& entity) const override;
+
  private:
   // |Geometry|
   GeometryResult GetPositionBuffer(const ContentContext& renderer,