reuse sqrt(1-z^2)

Sample/HardParticle/HemiEllipsoid.cpp

@@ -59,7 +59,8 @@ complex_t HemiEllipsoid::formfactor_at_bottom(C3 q) const
                    complex_t qxRz = q.x() * Rz;
                    complex_t qyWz = q.y() * Wz;
 
-                   complex_t gamma = std::sqrt(qxRz * qxRz + qyWz * qyWz);
+                   complex_t gamma = std::sqrt((q.x() * R)*(q.x() * R)
+                                               +  (q.y() * W) * (q.y() * W)) * std::sqrt(1.0 - z * z);
                    complex_t J1_gamma_div_gamma = Math::Bessel::J1c(gamma);
 
                    return Rz * Wz * J1_gamma_div_gamma * exp_I(q.z() * H * z);