In fact, with such a path, the modification mapping can be defined as follows, any pair (x, y) on the path is expanded to (x + 1, y) and (x, y + 1), any pair (x, y) upon the path is shifted to (x, y + 1), and any pair (x, y) below the path is shifted to (x + 1, y). With this mapping, for any pair in the rectangle region except (0,0), there is only one mapping direction to it (i.e., there is only one pair which is mapped to it), which guarantees the reversibility. For example, Fig. 5 presents one expansion path and its corresponding modification mapping. Thereby, from all these paths in the rectangle region, we can select the optimal one that minimize the embedding distortion.