Binary Erosion

Problem

Erode a binary mask with a full 3x3 structuring element and zero padding.

A pixel stays 1 only if all nine positions of its 3x3 window are 1; out-of-bounds positions count as 0, so border pixels are always eroded. The output has the same shape as the mask.

Hint

Treat any position outside the mask as a 0, which erodes the whole border.

Test cases

case 1

in: [[[1,1,1],[1,1,1],[1,1,1]]]

expected: [[0,0,0],[0,1,0],[0,0,0]]

case 2

in: [[[0,0,0],[0,1,0],[0,0,0]]]

expected: [[0,0,0],[0,0,0],[0,0,0]]

case 3

in: [[[1,1],[1,1]]]

expected: [[0,0],[0,0]]

case 4

in: [[[1,1,1,1],[1,1,1,1],[1,1,1,1]]]

expected: [[0,0,0,0],[0,1,1,0],[0,0,0,0]]

Starter codePython

def binary_erode(mask):
    # Your code here
    pass

4 test cases · run your code against them in the editor