For example, we can get the eastern boundary
by accessing the cell to the right.
Likewise, we can get the southern boundary by accessing
the cell below the current one.
If we continue to do this for the entire silhouette map, we can represent all of the discontinuities in the texture properly.
Notice now how the edge discontinuities we represent in this silhouette map linearly approximate the two ellipses we saw in our original texture.