code: def exponential(x, y, xscale, yscale):
      
      if xscale==0.0 or yscale==0.0:
          return x*0.0
  
      with float_error_ignore():
          x_w = np.divide(x,xscale)
          y_h = np.divide(y,yscale)
          return np.exp(-np.sqrt(x_w*x_w+y_h*y_h))
docs: Two-dimensional oriented exponential decay pattern.