Figure 2

Area 1:

      1  2  3
      4  5  6
      7  8  9

     Output of difference edge detector is:
     max of {
         | 1 - 9 |     | 7 - 3 |

         | 4 - 6 |     | 2 - 8 |
     } = 8


Area 2:

     10  10  10
     10  10  10
     10  10  1

     Output of difference edge detector is:
     max of {
         | 10 - 1  |   | 10 - 10 |

         | 10 - 10 |   | 10 - 10 |
     } = 9


Area 3:

     10  5  3
     10  5  3
     10  5  3

     Output of difference edge detector is:
     max of {
         | 10 - 3 |  | 10 - 3 |

         | 10 - 3 |  | 5 - 5 |
     } = 7