docs: def vignettingEq(xxx_todo_changeme, f=100, alpha=0, Xi=0, tau=0, cx=50, cy=50):
      '''
      Vignetting equation using the KARL-WEISS-MODEL
      see http://research.microsoft.com/en-us/people/stevelin/vignetting.pdf
  
      f - focal length
      alpha - coefficient in the geometric vignetting factor
      Xi - tilt factor of a planar scene
      tau - rotation angle of a planar scene
      cx - image center, x
      cy - image center, y
      '''
      (x, y) = xxx_todo_changeme
      dist = ((x - cx)**2 + (y - cy)**2)**0.5
  
      A = old_div(1.0, (1 + (old_div(dist, f))**2)**2)
      G = (1 - alpha * dist)
      T = np.cos(tau) * (1 + (old_div(np.tan(tau), f))
                         * (x * np.sin(Xi) - y * np.cos(Xi)))**3
  
      return A * G * T