code: def place(self, center, radius):
          
          self._radius = float(radius)
          self._center[0] = center[0]
          self._center[1] = center[1]
docs: Place Arcball, e.g. when window size changes.
  
          center : sequence[2]
              Window coordinates of trackball center.
          radius : float
              Radius of trackball in window coordinates.