Bretzel5 Implicit Surface

Bretzel5 001
Bretzel5 001
bretzel5
bretzel5
bretzel5 st
bretzel5 st
the  Bretzel 5 surface the function f(x,y,z) is given by:
  f(x,y,z) :=  (x^2 + (y^2)/4 - 1) * ( (x^2)/4 + y^2 - 1) - ff
  (default: ff = 0.075)