How can I implement mutual information?

 I have been trying to implement Mutual Information equation, but the code seems that its not working fine. Can anyone help me?                    

                          fixed_current = uint8(fixed_current);
                          moving_current = uint8(moving_current);
                          v_x_y = zeros (256);
                          for i = 1:256
                              for j = 1:256
                                  coord_r = fixed_current(i,j)+1;
                                  coord_c = moving_current(i,j)+1;

                                  v_x_y (coord_r,coord_c) = v_x_y (coord_r,coord_c) + 1;
                              end
                          end

                          p_x_y = v_x_y./ (256*256);
                          indNoZero = v_x_y ~= 0;
                          p_x_y_n0 = p_x_y(indNoZero);

                          % Define the entropy
                          H_x = entropy(fixed_current);
                          H_y = entropy(moving_current);
                          H_x_y = -sum(sum(p_x_y_n0.*log2(p_x_y_n0)));

                          % Define the MI
                          res = (H_x + H_y - H_x_y)/(max(H_x,H_y));

[Nirav Sharda]

Are you getting any error messages or is the code giving incorrect results? Also, I was able to find some file exchange examples on this link .