From 678bbc27da1644e664ec55cbc8406621661a92de Mon Sep 17 00:00:00 2001
From: Brandon DeRosier <bdero@google.com>
Date: Tue, 5 Mar 2024 16:24:58 -0800
Subject: [PATCH] [Impeller] Fix path winding when bridging from contours with
 an odd number of points.

---
 impeller/aiks/aiks_path_unittests.cc          | 48 +++++++++++++------
 impeller/tessellator/tessellator.cc           |  8 ++--
 impeller/tessellator/tessellator_unittests.cc |  8 ++--
 3 files changed, 42 insertions(+), 22 deletions(-)

diff --git a/impeller/aiks/aiks_path_unittests.cc b/impeller/aiks/aiks_path_unittests.cc
index a1f85dee47484..85d617856c533 100644
--- a/impeller/aiks/aiks_path_unittests.cc
+++ b/impeller/aiks/aiks_path_unittests.cc
@@ -412,20 +412,40 @@ TEST_P(AiksTest, CanRenderOverlappingMultiContourPath) {
 
   const Scalar kTriangleHeight = 100;
   canvas.Translate(Vector2(200, 200));
-  // Form a path similar to the Material drop slider value indicator.
-  auto path =
-      PathBuilder{}
-          .MoveTo({0, kTriangleHeight})
-          .LineTo({-kTriangleHeight / 2.0f, 0})
-          .LineTo({kTriangleHeight / 2.0f, 0})
-          .Close()
-          .AddRoundedRect(
-              Rect::MakeXYWH(-kTriangleHeight / 2.0f, -kTriangleHeight / 2.0f,
-                             kTriangleHeight, kTriangleHeight),
-              radii)
-          .TakePath();
-
-  canvas.DrawPath(path, paint);
+  // Form a path similar to the Material drop slider value indicator. Both
+  // shapes should render identically side-by-side.
+  {
+    auto path =
+        PathBuilder{}
+            .MoveTo({0, kTriangleHeight})
+            .LineTo({-kTriangleHeight / 2.0f, 0})
+            .LineTo({kTriangleHeight / 2.0f, 0})
+            .Close()
+            .AddRoundedRect(
+                Rect::MakeXYWH(-kTriangleHeight / 2.0f, -kTriangleHeight / 2.0f,
+                               kTriangleHeight, kTriangleHeight),
+                radii)
+            .TakePath();
+
+    canvas.DrawPath(path, paint);
+  }
+  canvas.Translate(Vector2(100, 0));
+  {
+    auto path =
+        PathBuilder{}
+            .MoveTo({0, kTriangleHeight})
+            .LineTo({-kTriangleHeight / 2.0f, 0})
+            .LineTo({0, -10})
+            .LineTo({kTriangleHeight / 2.0f, 0})
+            .Close()
+            .AddRoundedRect(
+                Rect::MakeXYWH(-kTriangleHeight / 2.0f, -kTriangleHeight / 2.0f,
+                               kTriangleHeight, kTriangleHeight),
+                radii)
+            .TakePath();
+
+    canvas.DrawPath(path, paint);
+  }
 
   ASSERT_TRUE(OpenPlaygroundHere(canvas.EndRecordingAsPicture()));
 }
diff --git a/impeller/tessellator/tessellator.cc b/impeller/tessellator/tessellator.cc
index e771c6dff9e84..40a1a542ab761 100644
--- a/impeller/tessellator/tessellator.cc
+++ b/impeller/tessellator/tessellator.cc
@@ -212,15 +212,15 @@ std::vector<Point> Tessellator::TessellateConvex(const Path& path,
 
     size_t a = start + 1;
     size_t b = end - 1;
-    while (a < b) {
+    while (a <= b) {
+      // If the contour has an odd number of points, two identical points will
+      // be appended when a == b. This ensures the triangle winding order will
+      // remain the same after bridging to the next contour.
       output.emplace_back(polyline.GetPoint(a));
       output.emplace_back(polyline.GetPoint(b));
       a++;
       b--;
     }
-    if (a == b) {
-      output.emplace_back(polyline.GetPoint(a));
-    }
   }
   return output;
 }
diff --git a/impeller/tessellator/tessellator_unittests.cc b/impeller/tessellator/tessellator_unittests.cc
index 7bbe1e314633c..d01a3ba33b2c3 100644
--- a/impeller/tessellator/tessellator_unittests.cc
+++ b/impeller/tessellator/tessellator_unittests.cc
@@ -112,7 +112,7 @@ TEST(TessellatorTest, TessellateConvex) {
         PathBuilder{}.AddRect(Rect::MakeLTRB(0, 0, 10, 10)).TakePath(), 1.0);
 
     std::vector<Point> expected = {
-        {0, 0}, {10, 0}, {0, 10}, {10, 10},  //
+        {0, 0}, {10, 0}, {0, 10}, {10, 10}, {10, 10},  //
     };
     EXPECT_EQ(pts, expected);
   }
@@ -125,9 +125,9 @@ TEST(TessellatorTest, TessellateConvex) {
                                       .TakePath(),
                                   1.0);
 
-    std::vector<Point> expected = {{0, 0},   {10, 0},  {0, 10},  {10, 10},
-                                   {10, 10}, {20, 20}, {20, 20}, {20, 20},
-                                   {30, 20}, {20, 30}, {30, 30}};
+    std::vector<Point> expected = {
+        {0, 0},   {10, 0},  {0, 10},  {10, 10}, {10, 10}, {10, 10}, {20, 20},
+        {20, 20}, {20, 20}, {30, 20}, {20, 30}, {30, 30}, {30, 30}};
     EXPECT_EQ(pts, expected);
   }
 }