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Impeller: Fix for stroked circles with stroke half-width > radius, tessellation scaling - #191102
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`Tessellator::StrokedCircle` delegated to `GenerateStrokedCircle` whenever `half_width > 0`, but that generator zig-zags between an inner and an outer circle and asserts `half_width < radius`, since the inner radius goes negative once the stroke is at least as wide as the radius. That case should render as a filled circle whose radius reaches the outer edge of the stroke. We were hitting this when enabling construction lines during debugging. With `debugPaintSizeEnabled` set, `RenderSliver.debugPaint` draws a circle of radius 2.0 with a stroke width of 4.0, so toggling construction lines on in unopt builds triggered this `FML_DCHECK`: ``` [FATAL:flutter/impeller/tessellator/tessellator.cc(755)] Check failed: data.half_width > 0 && data.half_width < data.radii.width. ``` `GenerateStrokedArc` had the same issue but didn't assert: the inner radii went negative and the inner edge folded back through the centre of the oval, producing a self-intersecting strip. We now clamp to zero. Fixes: flutter#190597
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This pull request updates the Impeller Tessellator to correctly handle wide strokes for circles and arcs. When a stroked circle's stroke width is greater than or equal to its radius, it is now tessellated as a filled circle reaching the outer edge of the stroke. For stroked arcs, the inner radius is clamped to zero to prevent self-intersection when the stroke width exceeds the base radii. Corresponding unit tests have been added to verify these changes. There are no review comments, so I have no feedback to provide.
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Just nits and agreeing with your analysis of the broken code, but otherwise looks good.
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`Tessellator::StrokedCircle` computes its quadrant divisions based on the radius. The radius in this case should be the circle radius + the stroke half-width to get the outer radius of the resulting filled circle. The effect would be under-tessellation of the outer edge that gets worse with increasing half-width. This tosses a pair of brackets on to scale the whole thing.
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autosubmit label was removed for flutter/flutter/191102, because - The status or check suite Google testing has failed. Please fix the issues identified (or deflake) before re-applying this label. |
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Tessellator::StrokedCircledelegated toGenerateStrokedCirclewheneverhalf_width > 0, but that generator zig-zags between an inner and an outer circle and assertshalf_width < radius, since the inner radius goes negative once the stroke is at least as wide as the radius. That case should render as a filled circle whose radius reaches the outer edge of the stroke.We were hitting this when enabling construction lines during debugging. With
debugPaintSizeEnabledset,RenderSliver.debugPaintdraws a circle of radius 2.0 with a stroke width of 4.0, so toggling construction lines on in unopt builds triggered thisFML_DCHECK:GenerateStrokedArchad the same issue but didn't assert: the inner radii went negative and the inner edge folded back through the centre of the oval, producing a self-intersecting strip. We now clamp to the centre.Also fixed a small bug in stroked circle division count scaling.
Tessellator::StrokedCirclecomputes its quadrant divisions based on the radius. The radius in this case should be the circle radius + the stroke half-width to get the outer radius of the resulting filled circle.The effect would be under-tessellation of the outer edge that gets worse with increasing half-width. This tosses a pair of brackets on to scale the whole thing.
Fixes: #190597
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