Talk:Zhang-Suen thinning algorithm

Could someone help explain why, in the example image the following transformation occurs (in the periods separating the R and C, and after the C):

.....    .....
.###. -> .....
.#?#.    ..#..
.....    .....

Surely the the cell labelled '?' will be culled at step 1:

- It is black with 8 neighbours
- B = 5 (2 <= 5 <= 6)
- A = 1
- At least one of P2 P4 P6 is white (P6 is white)
- At least one of P4 P6 P8 is white (P6 is white)

Why isn't it whitened at step 1?

--Tim-brown (talk) 17:33, 15 October 2013 (UTC)