def invert_depth(x):
    assert ((len(x) == 3) and (x[2] != 0))
    return (np.array([x[0], x[1], 1]) / x[2])
def invert_depth(x):
    assert ((len(x) == 3) and (x[2] != 0))
    return (np.array([x[0], x[1], 1]) / x[2])