Fix solid stroke contour switching + round cap smoothing, and add transparent path overdraw example (flutter/engine#112)

engine/src/flutter/impeller/aiks/aiks_unittests.cc

@@ -482,5 +482,43 @@ TEST_F(AiksTest, TransformMultipliesCorrectly) {
   // clang-format on
 }
 
+TEST_F(AiksTest, PathsShouldHaveUniformAlpha) {
+  // Compare with https://fiddle.skia.org/c/027392122bec8ac2b5d5de00a4b9bbe2
+  Canvas canvas;
+  Paint paint;
+
+  paint.color = Color::White();
+  canvas.DrawPaint(paint);
+
+  paint.color = Color::Black().WithAlpha(0.5);
+  paint.style = Paint::Style::kStroke;
+  paint.stroke_width = 10;
+
+  Path path = PathBuilder{}
+                  .MoveTo({20, 20})
+                  .QuadraticCurveTo({60, 20}, {60, 60})
+                  .Close()
+                  .MoveTo({60, 20})
+                  .QuadraticCurveTo({60, 60}, {20, 60})
+                  .TakePath();
+
+  canvas.Scale({3, 3});
+  for (auto join :
+       {SolidStrokeContents::Join::kBevel, SolidStrokeContents::Join::kRound,
+        SolidStrokeContents::Join::kMiter}) {
+    paint.stroke_join = join;
+    for (auto cap :
+         {SolidStrokeContents::Cap::kButt, SolidStrokeContents::Cap::kSquare,
+          SolidStrokeContents::Cap::kRound}) {
+      paint.stroke_cap = cap;
+      canvas.DrawPath(path, paint);
+      canvas.Translate({80, 0});
+    }
+    canvas.Translate({-240, 60});
+  }
+
+  ASSERT_TRUE(OpenPlaygroundHere(canvas.EndRecordingAsPicture()));
+}
+
 }  // namespace testing
 }  // namespace impeller

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

@@ -79,7 +79,11 @@ static VertexBuffer CreateSolidStrokeVertices(
       vtx.vertex_normal = {};
       vtx.pen_down = 0.0;
       vtx_builder.AppendVertex(vtx);
+
       vtx.vertex_position = polyline.points[contour_start_point_i];
+      // Append two transparent vertices at the beginning of the new contour
+      // because it's a triangle strip.
+      vtx_builder.AppendVertex(vtx);
       vtx_builder.AppendVertex(vtx);
     }
 
@@ -152,9 +156,13 @@ bool SolidStrokeContents::Render(const ContentContext& renderer,
   cmd.pipeline =
       renderer.GetSolidStrokePipeline(OptionsFromPassAndEntity(pass, entity));
   cmd.stencil_reference = entity.GetStencilDepth();
+
+  auto smoothing = SmoothingApproximation(
+      5.0 / (stroke_size_ * entity.GetTransformation().GetMaxBasisLength()),
+      0.0, 0.0);
   cmd.BindVertices(CreateSolidStrokeVertices(
       entity.GetPath(), pass.GetTransientsBuffer(), cap_proc_, join_proc_,
-      miter_limit_, arc_smoothing_approximation_));
+      miter_limit_, smoothing));
   VS::BindFrameInfo(cmd, pass.GetTransientsBuffer().EmplaceUniform(frame_info));
   VS::BindStrokeInfo(cmd,
                      pass.GetTransientsBuffer().EmplaceUniform(stroke_info));
@@ -166,7 +174,6 @@ bool SolidStrokeContents::Render(const ContentContext& renderer,
 
 void SolidStrokeContents::SetStrokeSize(Scalar size) {
   stroke_size_ = size;
-  arc_smoothing_approximation_ = SmoothingApproximation(5.0 / size, 0.0, 0.0);
 }
 
 Scalar SolidStrokeContents::GetStrokeSize() const {

engine/src/flutter/impeller/entity/contents/solid_stroke_contents.h

@@ -74,8 +74,6 @@ class SolidStrokeContents final : public Contents {
               RenderPass& pass) const override;
 
  private:
-  SmoothingApproximation arc_smoothing_approximation_;
-
   Color color_;
   Scalar stroke_size_ = 0.0;
   Scalar miter_limit_ = 4.0;