code: def isChanged(self, min_changed_pixels, screen_state):
          
          r = self.clipRegionToScreen()
          current_state = r.getBitmap()
          diff = numpy.subtract(current_state, screen_state)
          return (numpy.count_nonzero(diff) >= min_changed_pixels)
docs: Returns true if at least ``min_changed_pixels`` are different between
          ``screen_state`` and the current state.