def _to_zyx(vec3, order):
    vec3 = np.asarray(vec3, dtype=float).ravel()
    if order == "xyz":
        return vec3[::-1].copy()      # (x, y, z) -> (z, y, x)
    if order == "zyx":
        return vec3.copy()
    raise ValueError(f"unknown geometry_order {order!r}")