Transform gray image to create a pattern effect on while pixels given a mask

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I need to create a pattern on white pixels that basically turn whites into blacks or a given brightness (50 of 255 for example) given a mask, it need to follow some rules:

1) Have a margin, for example 5 pixels each side Giving this and lets asume only 2px margin wide:

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

Turn into:

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 1 1
1 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 1
1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 1 1
1 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 1
1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 1 1
1 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 1
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

Example: (Sorry for bad resolution, but is the best image example i could find)

I tried erode with a kernel but it erodes bounds only. I guess theres a easy way to do this...