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
Set initial cursor window coordinates and pick constrain-axis.
dataset/ETHPy150Open omangin/multimodal/multimodal/lib/transformations.py/Arcball.down