impeller: rrect path has dimples

[gaaclarke]

reproduction steps

See GradientOvalStrokeMaskBlurSigmaZero introduced in flutter/engine#55717

seen results

expected results

I'd expect smooth sides, notice the dimples at either ends of the shape.

version

https://github.com/flutter/engine/tree/772142e52853fcc3779f22bf5f3203010ee253ee

[jonahwilliams]

RRect?

[gaaclarke]

RRect?

Yup. https://github.com/flutter/engine/pull/55717/files#diff-de4beabd04cd7bef4aa21a5ee85881eb9df0c336b351bfef29bee69ad31b9a5cR97

[jonahwilliams]

Ahh, but stroked rrect so its going through the generic path tessellation code instead of the special rrect tessellation: https://github.com/flutter/engine/blob/main/impeller/display_list/canvas.cc#L676

[gaaclarke]

Here's the geometry where the dimples happen

[jonahwilliams]

Since this is a stroked path, I suspect the problem has to do with computations of the begin/end vectors

[jonahwilliams]

We've had similar bugs in the past

[gaaclarke]

The polyline from CreateTempPolyline is good in StrokePathGeometry::GetPositionBuffer:

[gaaclarke]

The source of this problem is in AddVerticesForLinearComponent. If you skip the loop if point_i == 11 || point_y == 33 the output will look almost the same but the dimples will be slightly larger. Here are the vertices that are getting added for the calls to AddVerticesForLinearComponent. The points for point_i==11 and point_i==33 look suspicious because points are repeated. Since this is a triangle strip I think it should be zigzagging back and forth around the polyline:

[ERROR:flutter/impeller/entity/geometry/stroke_path_geometry.cc(212)] 0 (15, 10)
(15, -10)
(185, 10)
(185, -10)

[ERROR:flutter/impeller/entity/geometry/stroke_path_geometry.cc(212)] 11 (210, 30)
(190, 30)
(210, 30)
(190, 30)

[ERROR:flutter/impeller/entity/geometry/stroke_path_geometry.cc(212)] 22 (185, 50)
(185, 70)
(15, 50)
(15, 70)

[ERROR:flutter/impeller/entity/geometry/stroke_path_geometry.cc(212)] 33 (-10, 30)
(10, 30)
(-10, 30)
(10, 30)

[gaaclarke]

I think it's because the 2 polylines points are really close:

- point_i
11
- polyline.GetPoint(point_i)
{...}
x =
200
y =
30.0000019
- polyline.GetPoint(point_i + 1)
{...}
x =
200
y =
29.9999981

[jonahwilliams]

Are we using a == where we need a ScalarNearlyEquals somewhere?