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LibGfx: Approximate elliptical arcs with cubic beziers

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Unlike all other primitives elliptical arcs are non-trivial to
manipulate, it's tricky to correctly apply a Gfx::AffineTransform to
them. Prior to this change, Path::copy_transformed() was still
incorrectly applying transforms such as flips and skews to arcs.

This patch very closely approximates arcs with cubic beziers (I can not
visually spot any differences), which can then be easily and correctly
transformed in all cases.

Most of the maths here was taken from:
https://mortoray.com/rendering-an-svg-elliptical-arc-as-bezier-curves/
(which came from https://www.joecridge.me/content/pdf/bezier-arcs.pdf,
now a dead link).
This commit is contained in:
MacDue 2023-07-15 15:59:33 +01:00 • committed by Andreas Kling
parent 2a1bf63f9e
commit 1bc7b0320e
4 changed files with 54 additions and 101 deletions
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5
Userland/Libraries/LibGfx/Painter.cpp
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@ -2451,11 +2451,6 @@ void Painter::stroke_path(Path const& path, Color color, int thickness)
cursor = segment->point();
break;
}
case Segment::Type::EllipticalArcTo:
auto& arc = static_cast<EllipticalArcSegment const&>(*segment);
draw_elliptical_arc(cursor.to_type<int>(), segment->point().to_type<int>(), arc.center().to_type<int>(), arc.radii(), arc.x_axis_rotation(), arc.theta_1(), arc.theta_delta(), color, thickness);
cursor = segment->point();
break;
}
}
}