First, for the generated 2D-PEH, integrate it into one-quadrant since the four quadrants of the 2D-PEH are basically symmetric. Then, for a given b, divide its corresponding region into three regions as described in the end of the Section 3.2. As the two divided triangular regions are roughly symmetric along the diagonal, similarly, we further integrated them into one triangular region. And then, for given (n, m), determine the optimal expansion path by solving the optimization problem (11). We implement the above procedure several times for b ∈ {0, 1, 2, 3}, n = 7, 0 ≤ m ≤ 7 and T (ranged from 0 to its maximum value) to get the best modification mapping. Here, the best modification mapping means that the value of ED/EC is minimized, where ED and EC are the corresponding embedding distortion and capacity, respectively. For illustration, the obtained 2D-PEH modification mappings for the first layer of different images are presented in Fig. 9.