code: def down(self, point):
          
          self._vdown = arcball_map_to_sphere(point, self._center, self._radius)
          self._qdown = self._qpre = self._qnow
          if self._constrain and self._axes is not None:
              self._axis = arcball_nearest_axis(self._vdown, self._axes)
              self._vdown = arcball_constrain_to_axis(self._vdown, self._axis)
          else:
              self._axis = None
docs: Set initial cursor window coordinates and pick constrain-axis.