Use more numerically robust algorithm to compute QBezier::pointAt().

QBezier::pointAt() could potentially return values outside the bezier's
bounds, even when the bezier was a straight horizontal line. For
example, with y = 0.5, it would produce y=0.5 or y=0.49999999999999 for
different values of t, which when rounded cause jittering in a QML
PathView.

Task-number: QTBUG-17007
Task-number: QTBUG-18133
Cherry-pick-of: 8b66982ec7b4b5d2071931c288973dce73dc9875
Change-Id: I4ecac7b9085aaaaaaaaaaaaaaaaaaaaaa7d7b0bc
Reviewed-on: http://codereview.qt.nokia.com/2467
Reviewed-by: Samuel Rødal <samuel.rodal@nokia.com>

1 files changed, 18 insertions, 18 deletions

diff --git a/src/gui/painting/qbezier_p.h b/src/gui/painting/qbezier_p.h
index 399dd89140..f1f7eb155a 100644
--- a/src/gui/painting/qbezier_p.h
+++ b/src/gui/painting/qbezier_p.h
@@ -162,27 +162,27 @@ inline void QBezier::coefficients(qreal t, qreal &a, qreal &b, qreal &c, qreal &
 
 inline QPointF QBezier::pointAt(qreal t) const
 {
-#if 1
-    qreal a, b, c, d;
-    coefficients(t, a, b, c, d);
-    return QPointF(a*x1 + b*x2 + c*x3 + d*x4, a*y1 + b*y2 + c*y3 + d*y4);
-#else
     // numerically more stable:
+    qreal x, y;
+
     qreal m_t = 1. - t;
-    qreal a = x1*m_t + x2*t;
-    qreal b = x2*m_t + x3*t;
-    qreal c = x3*m_t + x4*t;
-    a = a*m_t + b*t;
-    b = b*m_t + c*t;
-    qreal x = a*m_t + b*t;
-    qreal a = y1*m_t + y2*t;
-    qreal b = y2*m_t + y3*t;
-    qreal c = y3*m_t + y4*t;
-    a = a*m_t + b*t;
-    b = b*m_t + c*t;
-    qreal y = a*m_t + b*t;
+    {
+        qreal a = x1*m_t + x2*t;
+        qreal b = x2*m_t + x3*t;
+        qreal c = x3*m_t + x4*t;
+        a = a*m_t + b*t;
+        b = b*m_t + c*t;
+        x = a*m_t + b*t;
+    }
+    {
+        qreal a = y1*m_t + y2*t;
+        qreal b = y2*m_t + y3*t;
+        qreal c = y3*m_t + y4*t;
+        a = a*m_t + b*t;
+        b = b*m_t + c*t;
+        y = a*m_t + b*t;
+    }
     return QPointF(x, y);
-#endif
 }
 
 inline QPointF QBezier::normalVector(qreal t) const