'bbox in 0-based image coordinates'

def _update(self, x1, x2, y1, y2):

    if (self.valid() and (self._old_values == (x1, x2, y1, y2))):
        return
    self._update_grid(x1, y1, x2, y2)
    self._old_values = (x1, x2, y1, y2)
    self._force_update = False
