x = cos(u) (a + sin(v)*cos(u/2) - sin(2*v) sin(u/2)/2)
   y = sin(u) (a + sin(v)*cos(u/2) - sin(2*v) sin(u/2)/2)
   z = sin(u/2) sin(v) + cos(u/2) sin(2*v)/2
ahol
   -pi <= u <= pi
és
   -pi <= v <= pi