#http://www.mathcurve.com/surfaces/attraction/attraction.shtml
vertices = D1:100 D2:100
u = from 0 to (PI) D1
v = from (-0.5*PI) to (PI*0.5) D2
a = 10
t = a*sqrt(sin(u))
x = t*cos(u)*cos(v)
y = t*cos(u)*sin(v)
z = t*sin(u)
