Figure 14 Cases that do and do not require a special opening routine

  Cases where you cannot erode center pixel

  0  200  0  200    0  200  200  200    0
  0  200  0    0  200    0    0  200    0
  0  200  0    0    0    0    0    0  200

Cases where you can erode the center pixel

200  200  200  200    0  0    0  200  0
  0  200    0  200  200  0  200  200  0
  0    0    0    0    0  0    0    0  0