Swirl: swirling vector field
  A(x,y) = w((x-rX)/rX, (y-rY)/rY)*rot(dir(x-rX, y-rY))
  w(u,v) = (1 - u^2)^2 * (1 - v^2)^2
  dir(u,v) = (u,v)/sqrt(u^2+v^2)
  rot(u,v) = (-v,u)
  rX =    31.500000
  rY =    31.500000

A range = [   -1.5000 _     1.5000]B range = [   -1.5000 _     1.5000]