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Fix edge cases in arcTo 

The implementation is mathematically sound, but due to floating point
arithmetic the code would sometimes call acos with values very slightly
out of the valid [-1..1] interval. Using hypot and atan2 instead gives
the correct result.

Fixes: QTBUG-49049
Change-Id: I0185514a63dc2c99cb67e1f9951aac0ac32b5fd8
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
(cherry picked from commit c1938ddbb4)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
This commit is contained in:
Delf Neumärker 2021-12-09 10:28:15 +01:00 committed by Qt Cherry-pick Bot
parent d9da7d87b9
commit 0d0499dfc9
2 changed files with 13 additions and 18 deletions
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29
src/quick/items/context2d/qquickcontext2d.cpp
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@ -3884,16 +3884,16 @@ void QQuickContext2D::bezierCurveTo(qreal cp1x, qreal cp1y,
m_path.cubicTo(QPointF(cp1x, cp1y), QPointF(cp2x, cp2y), pt);
}
void QQuickContext2D::addArcTo(const QPointF& p1, const QPointF& p2, float radius)
void QQuickContext2D::addArcTo(const QPointF& p1, const QPointF& p2, qreal radius)
{
QPointF p0(m_path.currentPosition());
QPointF p1p0((p0.x() - p1.x()), (p0.y() - p1.y()));
QPointF p1p2((p2.x() - p1.x()), (p2.y() - p1.y()));
float p1p0_length = std::sqrt(p1p0.x() * p1p0.x() + p1p0.y() * p1p0.y());
float p1p2_length = std::sqrt(p1p2.x() * p1p2.x() + p1p2.y() * p1p2.y());
qreal p1p0_length = std::hypot(p1p0.x(), p1p0.y());
qreal p1p2_length = std::hypot(p1p2.x(), p1p2.y());
double cos_phi = (p1p0.x() * p1p2.x() + p1p0.y() * p1p2.y()) / (p1p0_length * p1p2_length);
qreal cos_phi = QPointF::dotProduct(p1p0, p1p2) / (p1p0_length * p1p2_length);
// The points p0, p1, and p2 are on the same straight line (HTML5, 4.8.11.1.8)
// We could have used areCollinear() here, but since we're reusing
@ -3903,16 +3903,16 @@ void QQuickContext2D::addArcTo(const QPointF& p1, const QPointF& p2, float radiu
return;
}
float tangent = radius / std::tan(std::acos(cos_phi) / 2);
float factor_p1p0 = tangent / p1p0_length;
qreal tangent = radius / std::tan(std::acos(cos_phi) / 2);
qreal factor_p1p0 = tangent / p1p0_length;
QPointF t_p1p0((p1.x() + factor_p1p0 * p1p0.x()), (p1.y() + factor_p1p0 * p1p0.y()));
QPointF orth_p1p0(p1p0.y(), -p1p0.x());
float orth_p1p0_length = std::sqrt(orth_p1p0.x() * orth_p1p0.x() + orth_p1p0.y() * orth_p1p0.y());
float factor_ra = radius / orth_p1p0_length;
qreal orth_p1p0_length = std::hypot(orth_p1p0.x(), orth_p1p0.y());
qreal factor_ra = radius / orth_p1p0_length;
// angle between orth_p1p0 and p1p2 to get the right vector orthographic to p1p0
double cos_alpha = (orth_p1p0.x() * p1p2.x() + orth_p1p0.y() * p1p2.y()) / (orth_p1p0_length * p1p2_length);
qreal cos_alpha = QPointF::dotProduct(orth_p1p0, p1p2) / (orth_p1p0_length * p1p2_length);
if (cos_alpha < 0.f)
orth_p1p0 = QPointF(-orth_p1p0.x(), -orth_p1p0.y());
@ -3920,20 +3920,15 @@ void QQuickContext2D::addArcTo(const QPointF& p1, const QPointF& p2, float radiu
// calculate angles for addArc
orth_p1p0 = QPointF(-orth_p1p0.x(), -orth_p1p0.y());
float sa = std::acos(orth_p1p0.x() / orth_p1p0_length);
if (orth_p1p0.y() < 0.f)
sa = 2 * M_PI - sa;
qreal sa = std::atan2(orth_p1p0.y(), orth_p1p0.x());
// anticlockwise logic
bool anticlockwise = false;
float factor_p1p2 = tangent / p1p2_length;
qreal factor_p1p2 = tangent / p1p2_length;
QPointF t_p1p2((p1.x() + factor_p1p2 * p1p2.x()), (p1.y() + factor_p1p2 * p1p2.y()));
QPointF orth_p1p2((t_p1p2.x() - p.x()), (t_p1p2.y() - p.y()));
float orth_p1p2_length = std::sqrt(orth_p1p2.x() * orth_p1p2.x() + orth_p1p2.y() * orth_p1p2.y());
float ea = std::acos(orth_p1p2.x() / orth_p1p2_length);
if (orth_p1p2.y() < 0)
ea = 2 * M_PI - ea;
qreal ea = std::atan2(orth_p1p2.y(), orth_p1p2.x());
if ((sa > ea) && ((sa - ea) < M_PI))
anticlockwise = true;
if ((sa < ea) && ((ea - sa) > M_PI))

2
src/quick/items/context2d/qquickcontext2d_p.h
View File

@ -240,7 +240,7 @@ public:
void arc(qreal x, qreal y, qreal radius,
qreal startAngle, qreal endAngle,
bool anticlockwise);
void addArcTo(const QPointF& p1, const QPointF& p2, float radius);
void addArcTo(const QPointF& p1, const QPointF& p2, qreal radius);
bool isPointInPath(qreal x, qreal y) const;