HorizontalCylinder::formfactor: integral now running over angle

Sample/HardParticle/HorizontalCylinder.cpp

@@ -57,14 +57,14 @@ complex_t HorizontalCylinder::formfactor(C3 q) const
         radial_part = M_TWOPI * R * R * Math::Bessel::J1c(qR) * exp_I(q.z() * R);
 
     else
-        radial_part =
-            2.
-            * ComplexIntegrator().integrate(
-                [=](double z) {
-                    double y = sqrt(R * R - z * z);
-                    return y * Math::sinc(q.y() * y) * exp_I(q.z() * (z - m_slice_bottom));
-                },
-                m_slice_bottom, m_slice_top);
+        // integration variable substituted as z = R * sin(t)
+        radial_part = 2. * pow(R, 2) * exp_I(q.z() * (-m_slice_bottom))
+                      * ComplexIntegrator().integrate(
+                          [=](double t) {
+                              return pow(cos(t), 2) * Math::sinc(q.y() * R * cos(t))
+                                     * exp_I(q.z() * R * sin(t));
+                          },
+                          asin(m_slice_bottom / R), asin(m_slice_top / R));
 
     return radial_part * axial_part;
 }