From 5b84b2c85b503bb67aa37c8c5322544ecae0e58d Mon Sep 17 00:00:00 2001
From: Brandon DeRosier <bdero@google.com>
Date: Thu, 23 May 2024 06:47:36 -0700
Subject: [PATCH] [Impeller] Fix stroke curves. (flutter/engine#52978)
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Resolves https://github.com/flutter/flutter/issues/147139.

Added an explanation inline for why we need to use different direction values depending on whether or not we're capping off the last component of the contour.

Before:
<img width="640" alt="Screenshot 2024-05-21 at 11 18 51 PM" src="https://github.com/flutter/engine/assets/919017/22d9fee7-834f-4cb4-8ed4-578a7c17f7a2">

<img width="121" alt="Screenshot 2024-05-21 at 11 19 35 PM" src="https://github.com/flutter/engine/assets/919017/419029b1-9a5f-4a30-bbb0-83dafc79f7df">

After:
<img width="640" alt="Screenshot 2024-05-21 at 11 17 25 PM" src="https://github.com/flutter/engine/assets/919017/629e30a5-d0c5-4b7a-a3e2-bf715aa4ba78">

<img width="115" alt="Screenshot 2024-05-21 at 11 16 38 PM" src="https://github.com/flutter/engine/assets/919017/12e51892-6009-47d0-a4e5-1f1b017c99e8">
---
 .../impeller/entity/geometry/stroke_path_geometry.cc | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/engine/src/flutter/impeller/entity/geometry/stroke_path_geometry.cc b/engine/src/flutter/impeller/entity/geometry/stroke_path_geometry.cc
index cc7a3da53ed..3ccf294cbb3 100644
--- a/engine/src/flutter/impeller/entity/geometry/stroke_path_geometry.cc
+++ b/engine/src/flutter/impeller/entity/geometry/stroke_path_geometry.cc
@@ -232,9 +232,17 @@ class StrokeGenerator {
       // For curve components, the polyline is detailed enough such that
       // it can avoid worrying about joins altogether.
       if (is_end_of_component) {
-        vtx.position = polyline.GetPoint(point_i + 1) + offset;
+        // Append two additional vertices to close off the component. If we're
+        // on the _last_ component of the contour then we need to use the
+        // contour's end direction.
+        // `ComputeOffset` returns the contour's end direction when attempting
+        // to grab offsets past `contour_end_point_i`, so just use `offset` when
+        // we're on the last component.
+        Point last_component_offset =
+            is_last_component ? offset : previous_offset;
+        vtx.position = polyline.GetPoint(point_i + 1) + last_component_offset;
         vtx_builder.AppendVertex(vtx.position);
-        vtx.position = polyline.GetPoint(point_i + 1) - offset;
+        vtx.position = polyline.GetPoint(point_i + 1) - last_component_offset;
         vtx_builder.AppendVertex(vtx.position);
         // Generate join from the current line to the next line.
         if (!is_last_component) {