[Impeller] Fix stroke cap drawing not correct. (flutter#39481)

* ++

* ++

Author: luckysmg

impeller/display_list/display_list_unittests.cc

@@ -121,13 +121,34 @@ TEST_P(DisplayListTest, CanDrawArc) {
     static float stroke_width = 10;
     static bool use_center = true;
 
+    static int selected_cap = 0;
+    const char* cap_names[] = {"Butt", "Round", "Square"};
+    flutter::DlStrokeCap cap;
+
     ImGui::Begin("Controls", nullptr, ImGuiWindowFlags_AlwaysAutoResize);
     ImGui::SliderFloat("Start angle", &start_angle, -360, 360);
     ImGui::SliderFloat("Sweep angle", &sweep_angle, -360, 360);
     ImGui::SliderFloat("Stroke width", &stroke_width, 0, 100);
+    ImGui::Combo("Cap", &selected_cap, cap_names,
+                 sizeof(cap_names) / sizeof(char*));
     ImGui::Checkbox("Use center", &use_center);
     ImGui::End();
 
+    switch (selected_cap) {
+      case 0:
+        cap = flutter::DlStrokeCap::kButt;
+        break;
+      case 1:
+        cap = flutter::DlStrokeCap::kRound;
+        break;
+      case 2:
+        cap = flutter::DlStrokeCap::kSquare;
+        break;
+      default:
+        cap = flutter::DlStrokeCap::kButt;
+        break;
+    }
+
     auto [p1, p2] = IMPELLER_PLAYGROUND_LINE(
         Point(200, 200), Point(400, 400), 20, Color::White(), Color::White());
 
@@ -136,7 +157,7 @@ TEST_P(DisplayListTest, CanDrawArc) {
     Vector2 scale = GetContentScale();
     builder.scale(scale.x, scale.y);
     builder.setStyle(flutter::DlDrawStyle::kStroke);
-    builder.setStrokeCap(flutter::DlStrokeCap::kButt);
+    builder.setStrokeCap(cap);
     builder.setStrokeJoin(flutter::DlStrokeJoin::kMiter);
     builder.setStrokeMiter(10);
     auto rect = SkRect::MakeLTRB(p1.x, p1.y, p2.x, p2.y);

impeller/entity/geometry.cc

@@ -302,6 +302,7 @@ VertexBuffer StrokePathGeometry::CreateSolidStrokeVertices(
     HostBuffer& buffer,
     Scalar stroke_width,
     Scalar scaled_miter_limit,
+    Cap cap,
     const StrokePathGeometry::JoinProc& join_proc,
     const StrokePathGeometry::CapProc& cap_proc,
     Scalar tolerance) {
@@ -370,9 +371,15 @@ VertexBuffer StrokePathGeometry::CreateSolidStrokeVertices(
 
     // Generate start cap.
     if (!polyline.contours[contour_i].is_closed) {
-      auto cap_offset =
-          Vector2(contour.start_direction.y, -contour.start_direction.x) *
-          stroke_width * 0.5;
+      Vector2 direction;
+      if (cap == Cap::kButt) {
+        direction =
+            Vector2(contour.start_direction.y, -contour.start_direction.x);
+      } else {
+        direction =
+            Vector2(-contour.start_direction.y, contour.start_direction.x);
+      }
+      auto cap_offset = direction * stroke_width * 0.5;
       cap_proc(vtx_builder, polyline.points[contour_start_point_i], cap_offset,
                tolerance);
     }
@@ -439,7 +446,8 @@ GeometryResult StrokePathGeometry::GetPositionBuffer(
   auto& host_buffer = pass.GetTransientsBuffer();
   auto vertex_buffer = CreateSolidStrokeVertices(
       path_, host_buffer, stroke_width, miter_limit_ * stroke_width_ * 0.5,
-      GetJoinProc(stroke_join_), GetCapProc(stroke_cap_), tolerance);
+      stroke_cap_, GetJoinProc(stroke_join_), GetCapProc(stroke_cap_),
+      tolerance);
 
   return GeometryResult{
       .type = PrimitiveType::kTriangleStrip,

impeller/entity/geometry.h

@@ -164,6 +164,7 @@ class StrokePathGeometry : public Geometry {
                                                 HostBuffer& buffer,
                                                 Scalar stroke_width,
                                                 Scalar scaled_miter_limit,
+                                                Cap cap,
                                                 const JoinProc& join_proc,
                                                 const CapProc& cap_proc,
                                                 Scalar tolerance);