Use kPreventOverdraw for arcs with overlapping stroke caps #179312

b-luk · 2025-12-02T00:27:39Z

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flar

Some food for thought. I don't know if this can be moved somewhere where we compute the Arc::Iterator or not. It would give you helpful delta vectors to compute where the ends are.

engine/src/flutter/impeller/entity/geometry/arc_geometry.cc

flar

It all looks good, but I think I might have uncovered some related issues in my evaluation of the unit tests that bears investigation while we are solving the overlap issues.

flar · 2025-12-08T18:47:34Z

engine/src/flutter/impeller/entity/geometry/arc_geometry.cc

-    return ComputePositionGeometry(renderer, generator, entity, pass);
+    auto result = ComputePositionGeometry(renderer, generator, entity, pass);
+    if (CapsOverlap()) {
+      result.mode = GeometryResult::Mode::kPreventOverdraw;


Perhaps in a follow-on PR, we should consider having the generator specify the overlap type rather than patching it after the fact.

Acknowledged. I'll follow up with this.

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

…this for round caps too

flar · 2025-12-09T19:09:09Z

engine/src/flutter/impeller/entity/geometry/arc_geometry.cc

+      // inside the inner edge of the start cap. For a visualization of when
+      // this occurs, see
+      // https://github.com/flutter/flutter/issues/178746#issuecomment-3554526727
+      // Note that testing for overlap is completely non-dependent on the arc's


independent of

flar · 2025-12-09T19:24:56Z

engine/src/flutter/impeller/entity/geometry/arc_geometry.cc

+      // start angle is 0.
+      auto radius = arc_.GetOvalSize().width / 2.0f;
+      auto start_point = Point(radius, 0);
+      auto sweep_angle = Radians(arc_.GetSweep()).radians;


nit: sweep_radians since you've resolved it to radians and not just an angle

flar

LGTM, but we should have a golden that also verifies the output but I didn't see any golden changes from existing tests. Maybe a variety of angles and widths with each end cap. I don't think you need to test the non-overlaps (maybe 1 for reference?), just make sure a variety of overlaps don't overdraw. It might require 3 separate tests? Or if you can pack in a reasonably visible set of tests into one image that would be cool too. But we should have had golden changes for this and the fact that we didn't indicates a hole in our testing.

Some minor nits in the comments that would be nice to adopt.

flar · 2025-12-09T19:25:46Z

engine/src/flutter/impeller/entity/geometry/arc_geometry.cc

+      auto start_point = Point(radius, 0);
+      auto sweep_angle = Radians(arc_.GetSweep()).radians;
+      auto end_point = Point(cos(sweep_angle), sin(sweep_angle)) * radius;
+      return (end_point - start_point).GetLength() < stroke_width_;


start_point.GetDistanceSquared(end_point) < stroke_width * stroke_width to avoid the sqrt

flar · 2025-12-09T19:27:25Z

engine/src/flutter/impeller/entity/geometry/arc_geometry.cc

+      // coordinate of the ending cap, rather than needing to calculate overlap
+      // based on both x and y positions of both caps.
+      auto radius = arc_.GetOvalSize().width / 2.0f;
+      auto half_width = stroke_width_ / 2.0f;


* 0.5f might be faster than / 2.0f depending on compiler opts.

engine/src/flutter/impeller/entity/geometry/arc_geometry.cc

b-luk · 2025-12-09T20:57:16Z

LGTM, but we should have a golden that also verifies the output but I didn't see any golden changes from existing tests. Maybe a variety of angles and widths with each end cap. I don't think you need to test the non-overlaps (maybe 1 for reference?), just make sure a variety of overlaps don't overdraw. It might require 3 separate tests? Or if you can pack in a reasonably visible set of tests into one image that would be cool too. But we should have had golden changes for this and the fact that we didn't indicates a hole in our testing.

Some minor nits in the comments that would be nice to adopt.

Added a golden test each for square and round caps. They're modified versions of the existing RenderArcFarm tests.

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After:

flar

LGTM

flar · 2025-12-09T21:06:24Z

I'll want to take a final look at the goldens for the playground test, but I'm guessing it will work right.

flar · 2025-12-09T21:43:55Z

Goldens LGTM

auto-submit · 2025-12-09T22:00:28Z

autosubmit label was removed for flutter/flutter/179312, because - The status or check suite Linux mac_clang_tidy has failed. Please fix the issues identified (or deflake) before re-applying this label.

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Use kPreventOverdraw for arcs with overlapping stroke caps

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b-luk marked this pull request as ready for review December 2, 2025 00:40

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[Impeller] Stroked arcs with very wide stroke widths do not render stroke caps #179369

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Test a lot more sweep angles in unit tests

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Merge branch 'master' into overlappingcapspreventoverdraw

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b-luk added 2 commits December 8, 2025 12:17

Add comment about treating arcs as if their start angle is 0. And do …

0753e2b

…this for round caps too

Add for-loop to test different start angles

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small changes to address comments

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Remove redundant line

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Use int instead of Scalar/float for loop counter

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Use kPreventOverdraw for arcs with overlapping stroke caps #179312

Use kPreventOverdraw for arcs with overlapping stroke caps #179312

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b-luk commented Dec 2, 2025

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