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[Impeller] Fix stroke curves. (flutter/engine#52978)

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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">
This commit is contained in:
Brandon DeRosier 2024-05-23 06:47:36 -07:00 committed by GitHub
parent 9a392e9e07
commit 5b84b2c85b
1 changed files with 10 additions and 2 deletions
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12
engine/src/flutter/impeller/entity/geometry/stroke_path_geometry.cc
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@ -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) {