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If it helps, try a simpler version. Just look at two neighbouring pixels in a row. First example: There is 10 and 10. Difference between them is 0, no difference, no edge. Second example: In the middle, there is 10 and 0. Difference is -10 because it drops from 10 to 0. There is a step of -10, that must be an edge. The 3x3 kernel just takes the source pixel ...


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