[Impeller] Fixes stroke path geometry that can draw outside of path if the path ends at sharp turn.

impeller/aiks/aiks_unittests.cc

@@ -1206,6 +1206,37 @@ TEST_P(AiksTest, CanRenderRoundedRectWithNonUniformRadii) {
   ASSERT_TRUE(OpenPlaygroundHere(canvas.EndRecordingAsPicture()));
 }
 
+TEST_P(AiksTest, CanRenderStrokePathThatEndsAtSharpTurn) {
+  Canvas canvas;
+
+  Paint paint;
+  paint.color = Color::Red();
+  paint.style = Paint::Style::kStroke;
+  paint.stroke_width = 200;
+
+  Rect rect = {100, 100, 200, 200};
+  PathBuilder builder;
+  builder.AddArc(rect, Degrees(0), Degrees(90), false);
+
+  canvas.DrawPath(builder.TakePath(), paint);
+  ASSERT_TRUE(OpenPlaygroundHere(canvas.EndRecordingAsPicture()));
+}
+
+TEST_P(AiksTest, CanRenderStrokePathWithCubicLine) {
+  Canvas canvas;
+
+  Paint paint;
+  paint.color = Color::Red();
+  paint.style = Paint::Style::kStroke;
+  paint.stroke_width = 20;
+
+  PathBuilder builder;
+  builder.AddCubicCurve({0, 200}, {50, 400}, {350, 0}, {400, 200});
+
+  canvas.DrawPath(builder.TakePath(), paint);
+  ASSERT_TRUE(OpenPlaygroundHere(canvas.EndRecordingAsPicture()));
+}
+
 TEST_P(AiksTest, CanRenderDifferencePaths) {
   Canvas canvas;
 
@@ -1952,6 +1983,22 @@ TEST_P(AiksTest, DrawRectStrokesRenderCorrectly) {
   ASSERT_TRUE(OpenPlaygroundHere(canvas.EndRecordingAsPicture()));
 }
 
+TEST_P(AiksTest, DrawRectStrokesWithBevelJoinRenderCorrectly) {
+  Canvas canvas;
+  Paint paint;
+  paint.color = Color::Red();
+  paint.style = Paint::Style::kStroke;
+  paint.stroke_width = 10;
+  paint.stroke_join = Join::kBevel;
+
+  canvas.Translate({100, 100});
+  canvas.DrawPath(
+      PathBuilder{}.AddRect(Rect::MakeSize(Size{100, 100})).TakePath(),
+      {paint});
+
+  ASSERT_TRUE(OpenPlaygroundHere(canvas.EndRecordingAsPicture()));
+}
+
 TEST_P(AiksTest, SaveLayerDrawsBehindSubsequentEntities) {
   // Compare with https://fiddle.skia.org/c/9e03de8567ffb49e7e83f53b64bcf636
   Canvas canvas;

impeller/core/formats.h

@@ -325,11 +325,33 @@ enum class IndexType {
   kNone,
 };
 
+/// Decides how backend draws pixels based on input vertices.
 enum class PrimitiveType {
+  /// Draws a triage for each separate set of three vertices.
+  ///
+  /// Vertices [A, B, C, D, E, F] will produce triages
+  /// [ABC, DEF].
   kTriangle,
+
+  /// Draws a triage for every adjacent three vertices.
+  ///
+  /// Vertices [A, B, C, D, E, F] will produce triages
+  /// [ABC, BCD, CDE, DEF].
   kTriangleStrip,
+
+  /// Draws a line for each separate set of two vertices.
+  ///
+  /// Vertices [A, B, C] will produce discontinued line
+  /// [AB, BC].
   kLine,
+
+  /// Draws a continuous line that connect every input vertices
+  ///
+  /// Vertices [A, B, C] will produce one continuous line
+  /// [ABC].
   kLineStrip,
+
+  /// Draws a point at each input vertex.
   kPoint,
   // Triangle fans are implementation dependent and need extra extensions
   // checks. Hence, they are not supported here.

impeller/entity/geometry/stroke_path_geometry.cc

@@ -253,6 +253,7 @@ StrokePathGeometry::CreateSolidStrokeVertices(
   for (size_t contour_i = 0; contour_i < polyline.contours.size();
        contour_i++) {
     auto contour = polyline.contours[contour_i];
+    size_t contour_component_i = 0;
     size_t contour_start_point_i, contour_end_point_i;
     std::tie(contour_start_point_i, contour_end_point_i) =
         polyline.GetContourPointBounds(contour_i);
@@ -308,27 +309,55 @@ StrokePathGeometry::CreateSolidStrokeVertices(
     // Generate contour geometry.
     for (size_t point_i = contour_start_point_i + 1;
          point_i < contour_end_point_i; point_i++) {
+      if ((contour_component_i + 1 >= contour.components.size()) &&
+          contour.components[contour_component_i + 1].component_start_index <=
+              point_i) {
+        // The point_i has entered the next component in this contour.
+        contour_component_i += 1;
+      }
       // Generate line rect.
       vtx.position = polyline.points[point_i - 1] + offset;
       vtx_builder.AppendVertex(vtx);
       vtx.position = polyline.points[point_i - 1] - offset;
       vtx_builder.AppendVertex(vtx);
-      vtx.position = polyline.points[point_i] + offset;
-      vtx_builder.AppendVertex(vtx);
-      vtx.position = polyline.points[point_i] - offset;
-      vtx_builder.AppendVertex(vtx);
 
-      if (point_i < contour_end_point_i - 1) {
-        compute_offset(point_i + 1);
+      auto is_end_of_contour = point_i == contour_end_point_i - 1;
+
+      if (!contour.components[contour_component_i].is_curve) {
+        // For line components, two additional points need to be appended prior
+        // to appending a join connecting the next component.
+        vtx.position = polyline.points[point_i] + offset;
+        vtx_builder.AppendVertex(vtx);
+        vtx.position = polyline.points[point_i] - offset;
+        vtx_builder.AppendVertex(vtx);
 
-        // Generate join from the current line to the next line.
-        join_proc(vtx_builder, polyline.points[point_i], previous_offset,
-                  offset, scaled_miter_limit, scale);
+        if (!is_end_of_contour) {
+          compute_offset(point_i + 1);
+          // Generate join from the current line to the next line.
+          join_proc(vtx_builder, polyline.points[point_i], previous_offset,
+                    offset, scaled_miter_limit, scale);
+        }
+      } else {
+        // For curve components, the polyline is detailed enough such that
+        // it can avoid worrying about joins altogether.
+        if (!is_end_of_contour) {
+          compute_offset(point_i + 1);
+        } else {
+          // If this is a curve and is the end of the contour, two end points
+          // need to be drawn with the contour end_direction.
+          auto end_offset =
+              Vector2(-contour.end_direction.y, contour.end_direction.x) *
+              stroke_width * 0.5;
+          vtx.position = polyline.points[contour_end_point_i - 1] + end_offset;
+          vtx_builder.AppendVertex(vtx);
+          vtx.position = polyline.points[contour_end_point_i - 1] - end_offset;
+          vtx_builder.AppendVertex(vtx);
+        }
       }
     }
 
     // Generate end cap or join.
-    if (!polyline.contours[contour_i].is_closed) {
+    if (!contour.is_closed) {
       auto cap_offset =
           Vector2(-contour.end_direction.y, contour.end_direction.x) *
           stroke_width * 0.5;  // Clockwise normal

impeller/geometry/path.cc

@@ -324,9 +324,10 @@ Path::Polyline Path::CreatePolyline(Scalar scale) const {
         return Vector2(0, -1);
       };
 
+  std::vector<PolylineContour::Component> components;
   std::optional<size_t> previous_path_component_index;
   auto end_contour = [&polyline, &previous_path_component_index,
-                      &get_path_component]() {
+                      &get_path_component, &components]() {
     // Whenever a contour has ended, extract the exact end direction from the
     // last component.
     if (polyline.contours.empty()) {
@@ -339,6 +340,8 @@ Path::Polyline Path::CreatePolyline(Scalar scale) const {
 
     auto& contour = polyline.contours.back();
     contour.end_direction = Vector2(0, 1);
+    contour.components = components;
+    components.clear();
 
     size_t previous_index = previous_path_component_index.value();
     while (!std::holds_alternative<std::monostate>(
@@ -363,14 +366,26 @@ Path::Polyline Path::CreatePolyline(Scalar scale) const {
     const auto& component = components_[component_i];
     switch (component.type) {
       case ComponentType::kLinear:
+        components.push_back({
+            .component_start_index = polyline.points.size(),
+            .is_curve = false,
+        });
         collect_points(linears_[component.index].CreatePolyline());
         previous_path_component_index = component_i;
         break;
       case ComponentType::kQuadratic:
+        components.push_back({
+            .component_start_index = polyline.points.size(),
+            .is_curve = true,
+        });
         collect_points(quads_[component.index].CreatePolyline(scale));
         previous_path_component_index = component_i;
         break;
       case ComponentType::kCubic:
+        components.push_back({
+            .component_start_index = polyline.points.size(),
+            .is_curve = true,
+        });
         collect_points(cubics_[component.index].CreatePolyline(scale));
         previous_path_component_index = component_i;
         break;
@@ -386,13 +401,14 @@ Path::Polyline Path::CreatePolyline(Scalar scale) const {
         const auto& contour = contours_[component.index];
         polyline.contours.push_back({.start_index = polyline.points.size(),
                                      .is_closed = contour.is_closed,
-                                     .start_direction = start_direction});
+                                     .start_direction = start_direction,
+                                     .components = components});
         previous_contour_point = std::nullopt;
         collect_points({contour.destination});
         break;
     }
-    end_contour();
   }
+  end_contour();
   return polyline;
 }
 

impeller/geometry/path.h

@@ -61,8 +61,17 @@ class Path {
   };
 
   struct PolylineContour {
+    struct Component {
+      size_t component_start_index;
+      /// Denotes whether this component is a curve.
+      ///
+      /// This is set to true when this component is generated from
+      /// QuadraticComponent or CubicPathComponent.
+      bool is_curve;
+    };
     /// Index that denotes the first point of this contour.
     size_t start_index;
+
     /// Denotes whether the last point of this contour is connected to the first
     /// point of this contour or not.
     bool is_closed;
@@ -71,6 +80,12 @@ class Path {
     Vector2 start_direction;
     /// The direction of the contour's end cap.
     Vector2 end_direction;
+
+    /// Distinct components in this contour.
+    ///
+    /// If this contour is generated from multiple path components, each
+    /// path component forms a component in this vector.
+    std::vector<Component> components;
   };
 
   /// One or more contours represented as a series of points and indices in

impeller/geometry/path_component.h

@@ -47,9 +47,13 @@ struct LinearPathComponent {
   std::optional<Vector2> GetEndDirection() const;
 };
 
+// A component that represets a Quadratic Bézier curve.
 struct QuadraticPathComponent {
+  // Start point.
   Point p1;
+  // Control point.
   Point cp;
+  // End point.
   Point p2;
 
   QuadraticPathComponent() {}
@@ -87,10 +91,15 @@ struct QuadraticPathComponent {
   std::optional<Vector2> GetEndDirection() const;
 };
 
+// A component that represets a Cubic Bézier curve.
 struct CubicPathComponent {
+  // Start point.
   Point p1;
+  // The first control point.
   Point cp1;
+  // The second control point.
   Point cp2;
+  // End point.
   Point p2;
 
   CubicPathComponent() {}