Figure 6 Four 3x3 masks

vertical mask        horizontal mask

   0  1  0              0  0  0
   0  1  0              1  1  1
   0  1  0              0  0  0
horizontal mask and vertical mask

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