Figure 4 The result of eroding the rectangle in Figure 1 using a threshold of 2

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