Better calculation of angles

src/frameworks/UBGeometryUtils.cpp

@@ -255,8 +255,6 @@ QPolygonF UBGeometryUtils::arcToPolygon(const QLineF& startRadius, qreal spanAng
  */
 QPolygonF UBGeometryUtils::curveToPolygon(const QList<QPointF>& points, qreal startWidth, qreal endWidth)
 {
-    typedef QPair<QPointF, QPointF> pointPair;
-
     int n_points = points.size();
 
     if (n_points < 2)
@@ -431,7 +429,18 @@ void UBGeometryUtils::crashPointList(QVector<QPointF> &points)
 /**
  * @brief Return the angle between three points
  */
-qreal UBGeometryUtils::angle(const QPointF& a, const QPointF& b, const QPointF& c)
+qreal UBGeometryUtils::angle(const QPointF& p1, const QPointF& p2, const QPointF& p3)
 {
-    return (QLineF(a, b).angle() - QLineF(b, c).angle());
+    // Angle between three points, using the law of cosines:
+    // The angle at B equals arccos((a^2-b^2-c^2)/(2*a*c)), where a, b and c are the sides of the triangle
+    // opposite A, B and C, respectively
+
+    qreal a, b, c, beta;
+    a = qSqrt(qPow(p2.x() - p3.x(), 2) + qPow(p2.y() - p3.y(), 2));
+    b = qSqrt(qPow(p1.x() - p3.x(), 2) + qPow(p1.y() - p3.y(), 2));
+    c = qSqrt(qPow(p1.x() - p2.x(), 2) + qPow(p1.y() - p2.y(), 2));
+
+    beta = qAcos((a*a - b*b - c*c)/(2*a*c));
+
+    return 2*3.14159*beta;
 }

src/frameworks/UBGeometryUtils.h

@@ -55,7 +55,7 @@ class UBGeometryUtils
 
         static void crashPointList(QVector<QPointF> &points);
 
-        static qreal angle(const QPointF& a, const QPointF& b, const QPointF& c);
+        static qreal angle(const QPointF& p1, const QPointF& p2, const QPointF& p3);
 
         const static int centimeterGraduationHeight;
         const static int halfCentimeterGraduationHeight;