Handle all corner cases for stroke geometry, add bevel join & cap/join enums (flutter/engine#35)

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

@@ -293,6 +293,30 @@ const Color& SolidStrokeContents::GetColor() const {
   return color_;
 }
 
+static void CreateCap(
+    VertexBufferBuilder<SolidStrokeVertexShader::PerVertexData>& vtx_builder,
+    const Point& position,
+    const Point& normal) {}
+
+static void CreateJoin(
+    VertexBufferBuilder<SolidStrokeVertexShader::PerVertexData>& vtx_builder,
+    const Point& position,
+    const Point& start_normal,
+    const Point& end_normal) {
+  SolidStrokeVertexShader::PerVertexData vtx;
+  vtx.vertex_position = position;
+  vtx.pen_down = 1.0;
+  vtx.vertex_normal = {};
+  vtx_builder.AppendVertex(vtx);
+
+  // A simple bevel join to start with.
+  Scalar dir = start_normal.Cross(end_normal) > 0 ? -1 : 1;
+  vtx.vertex_normal = start_normal * dir;
+  vtx_builder.AppendVertex(vtx);
+  vtx.vertex_normal = end_normal * dir;
+  vtx_builder.AppendVertex(vtx);
+}
+
 static VertexBuffer CreateSolidStrokeVertices(const Path& path,
                                               HostBuffer& buffer) {
   using VS = SolidStrokeVertexShader;
@@ -300,44 +324,90 @@ static VertexBuffer CreateSolidStrokeVertices(const Path& path,
   VertexBufferBuilder<VS::PerVertexData> vtx_builder;
   auto polyline = path.CreatePolyline();
 
-  for (size_t i = 0, polyline_size = polyline.points.size(); i < polyline_size;
-       i++) {
-    const auto is_last_point = i == polyline_size - 1;
+  size_t point_i = 0;
+  if (polyline.points.size() < 2) {
+    return {};  // Nothing to render.
+  }
 
-    const auto& p1 = polyline.points[i];
-    const auto& p2 =
-        is_last_point ? polyline.points[i - 1] : polyline.points[i + 1];
+  VS::PerVertexData vtx;
 
-    const auto diff = p2 - p1;
+  // Cursor state.
+  Point direction;
+  Point normal;
+  Point previous_normal;  // Used for computing joins.
+  auto compute_normals = [&](size_t point_i) {
+    previous_normal = normal;
+    direction =
+        (polyline.points[point_i] - polyline.points[point_i - 1]).Normalize();
+    normal = {-direction.y, direction.x};
+  };
+  compute_normals(1);
 
-    const Scalar direction = is_last_point ? -1.0 : 1.0;
+  // Break state.
+  auto breaks_it = polyline.breaks.begin();
+  size_t break_end =
+      breaks_it != polyline.breaks.end() ? *breaks_it : polyline.points.size();
 
-    const auto normal =
-        Point{-diff.y * direction, diff.x * direction}.Normalize();
+  while (point_i < polyline.points.size()) {
+    if (point_i > 0) {
+      compute_normals(point_i);
 
-    VS::PerVertexData vtx;
-    vtx.vertex_position = p1;
-    auto pen_down =
-        polyline.breaks.find(i) == polyline.breaks.end() ? 1.0 : 0.0;
-
-    vtx.vertex_normal = normal;
-    vtx.pen_down = pen_down;
-    vtx_builder.AppendVertex(vtx);
-
-    vtx.vertex_normal = -normal;
-    vtx.pen_down = pen_down;
-    vtx_builder.AppendVertex(vtx);
-
-    // Put the pen down again for the next contour.
-    if (!pen_down) {
-      vtx.vertex_normal = normal;
-      vtx.pen_down = 1.0;
+      // This branch only executes when we've just finished drawing a contour
+      // and are switching to a new one.
+      // We're drawing a triangle strip, so we need to "pick up the pen" by
+      // appending transparent vertices between the end of the previous contour
+      // and the beginning of the new contour.
+      vtx.vertex_position = polyline.points[point_i - 1];
+      vtx.vertex_normal = {};
+      vtx.pen_down = 0.0;
       vtx_builder.AppendVertex(vtx);
-
-      vtx.vertex_normal = -normal;
-      vtx.pen_down = 1.0;
+      vtx.vertex_position = polyline.points[point_i];
       vtx_builder.AppendVertex(vtx);
     }
+
+    // Generate start cap.
+    CreateCap(vtx_builder, polyline.points[point_i], -direction);
+
+    // Generate contour geometry.
+    size_t contour_point_i = 0;
+    while (point_i < break_end) {
+      if (contour_point_i > 0) {
+        if (contour_point_i > 1) {
+          // Generate join from the previous line to the current line.
+          CreateJoin(vtx_builder, polyline.points[point_i - 1], previous_normal,
+                     normal);
+        } else {
+          compute_normals(point_i);
+        }
+
+        // Generate line rect.
+        vtx.vertex_position = polyline.points[point_i - 1];
+        vtx.pen_down = 1.0;
+        vtx.vertex_normal = normal;
+        vtx_builder.AppendVertex(vtx);
+        vtx.vertex_normal = -normal;
+        vtx_builder.AppendVertex(vtx);
+        vtx.vertex_position = polyline.points[point_i];
+        vtx.vertex_normal = normal;
+        vtx_builder.AppendVertex(vtx);
+        vtx.vertex_normal = -normal;
+        vtx_builder.AppendVertex(vtx);
+
+        compute_normals(point_i + 1);
+      }
+
+      ++contour_point_i;
+      ++point_i;
+    }
+
+    // Generate end cap.
+    CreateCap(vtx_builder, polyline.points[point_i - 1], -direction);
+
+    if (break_end < polyline.points.size()) {
+      ++breaks_it;
+      break_end = breaks_it != polyline.breaks.end() ? *breaks_it
+                                                     : polyline.points.size();
+    }
   }
 
   return vtx_builder.CreateVertexBuffer(buffer);
@@ -384,6 +454,30 @@ Scalar SolidStrokeContents::GetStrokeSize() const {
   return stroke_size_;
 }
 
+void SolidStrokeContents::SetStrokeMiter(Scalar miter) {
+  miter_ = miter;
+}
+
+Scalar SolidStrokeContents::GetStrokeMiter(Scalar miter) {
+  return miter_;
+}
+
+void SolidStrokeContents::SetStrokeCap(Cap cap) {
+  cap_ = cap;
+}
+
+SolidStrokeContents::Cap SolidStrokeContents::GetStrokeCap() {
+  return cap_;
+}
+
+void SolidStrokeContents::SetStrokeJoin(Join join) {
+  join_ = join;
+}
+
+SolidStrokeContents::Join SolidStrokeContents::GetStrokeJoin() {
+  return join_;
+}
+
 /*******************************************************************************
  ******* ClipContents
  ******************************************************************************/

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

@@ -113,6 +113,20 @@ class TextureContents final : public Contents {
 
 class SolidStrokeContents final : public Contents {
  public:
+  enum class Cap {
+    kButt,
+    kRound,
+    kSquare,
+    kLast,
+  };
+
+  enum class Join {
+    kMiter,
+    kRound,
+    kBevel,
+    kLast,
+  };
+
   SolidStrokeContents();
 
   ~SolidStrokeContents() override;
@@ -125,6 +139,18 @@ class SolidStrokeContents final : public Contents {
 
   Scalar GetStrokeSize() const;
 
+  void SetStrokeMiter(Scalar miter);
+
+  Scalar GetStrokeMiter(Scalar miter);
+
+  void SetStrokeCap(Cap cap);
+
+  Cap GetStrokeCap();
+
+  void SetStrokeJoin(Join join);
+
+  Join GetStrokeJoin();
+
   // |Contents|
   bool Render(const ContentContext& renderer,
               const Entity& entity,
@@ -133,6 +159,9 @@ class SolidStrokeContents final : public Contents {
  private:
   Color color_;
   Scalar stroke_size_ = 0.0;
+  Scalar miter_ = 0.0;
+  Cap cap_ = Cap::kButt;
+  Join join_ = Join::kMiter;
 
   FML_DISALLOW_COPY_AND_ASSIGN(SolidStrokeContents);
 };

engine/src/flutter/impeller/entity/entity_unittests.cc

@@ -7,6 +7,7 @@
 #include "impeller/entity/entity_playground.h"
 #include "impeller/geometry/path_builder.h"
 #include "impeller/playground/playground.h"
+#include "impeller/playground/widgets.h"
 
 namespace impeller {
 namespace testing {
@@ -45,25 +46,36 @@ TEST_F(EntityTest, ThreeStrokesInOnePath) {
 }
 
 TEST_F(EntityTest, TriangleInsideASquare) {
-  Path path = PathBuilder{}
-                  .MoveTo({10, 10})
-                  .LineTo({210, 10})
-                  .LineTo({210, 210})
-                  .LineTo({10, 210})
-                  .Close()
-                  .MoveTo({50, 50})
-                  .LineTo({100, 50})
-                  .LineTo({50, 150})
-                  .Close()
-                  .TakePath();
+  auto callback = [&](ContentContext& context, RenderPass& pass) {
+    Point a = IMPELLER_PLAYGROUND_POINT(Point(10, 10), 20, Color::White());
+    Point b = IMPELLER_PLAYGROUND_POINT(Point(210, 10), 20, Color::White());
+    Point c = IMPELLER_PLAYGROUND_POINT(Point(210, 210), 20, Color::White());
+    Point d = IMPELLER_PLAYGROUND_POINT(Point(10, 210), 20, Color::White());
+    Point e = IMPELLER_PLAYGROUND_POINT(Point(50, 50), 20, Color::White());
+    Point f = IMPELLER_PLAYGROUND_POINT(Point(100, 50), 20, Color::White());
+    Point g = IMPELLER_PLAYGROUND_POINT(Point(50, 150), 20, Color::White());
+    Path path = PathBuilder{}
+                    .MoveTo(a)
+                    .LineTo(b)
+                    .LineTo(c)
+                    .LineTo(d)
+                    .Close()
+                    .MoveTo(e)
+                    .LineTo(f)
+                    .LineTo(g)
+                    .Close()
+                    .TakePath();
 
-  Entity entity;
-  entity.SetPath(path);
-  auto contents = std::make_unique<SolidStrokeContents>();
-  contents->SetColor(Color::Red());
-  contents->SetStrokeSize(5.0);
-  entity.SetContents(std::move(contents));
-  ASSERT_TRUE(OpenPlaygroundHere(entity));
+    Entity entity;
+    entity.SetPath(path);
+    auto contents = std::make_unique<SolidStrokeContents>();
+    contents->SetColor(Color::Red());
+    contents->SetStrokeSize(20.0);
+    entity.SetContents(std::move(contents));
+
+    return entity.Render(context, pass);
+  };
+  ASSERT_TRUE(OpenPlaygroundHere(callback));
 }
 
 TEST_F(EntityTest, CubicCurveTest) {