Figure 3 Low-pass filter convolution masks

       0  1  0
1/6 *  1  2  1
       0  1  0

       1  1  1
1/9 *  1  1  1
       1  1  1

       1  1  1
1/10 * 1  2  1
       1  1  1

       1  2  1
1/16 * 2  4  2
       1  2  1