7x7 area with lines 0 10 0 10 0 10 0 0 0 0 0 0 0 0 10 0 10 0 10 0 10 0 0 0 0 0 0 0 0 10 0 10 0 10 0 0 0 0 0 0 0 0 10 0 10 0 10 0 10 result of convolution with 7x7 mask = 40 9x9 area with lines 0 0 0 0 0 0 0 0 0 10 0 10 0 10 0 10 0 10 0 0 0 0 0 0 0 0 0 0 10 0 10 0 10 0 10 0 0 0 0 0 0 0 0 0 0 10 0 10 0 10 0 10 0 10 0 0 0 0 0 0 0 0 0 0 10 0 10 0 10 0 10 0 0 0 0 0 0 0 0 0 0 result of convolution with 9x9 mask = -20