By changing the pattern and the step used the its rotation, it is possible to generate a large number of thinning operators. Depending of the pattern and step used, the operator can be homotopic (preserve the topology of the original image) or not.
Below are the original image and the result of application of four skeleton by thinning operators numbered from 1 to 4:
1)
2)
3)
4)
The patterns and rotation step used in the above skeletons are:
111 Pattern = .1. step 45 degres: 000
111 Pattern = .1. step 45 degres: 000
0.. Pattern = 011 step 90 degrees: 0..
1.. Pattern = .10 step 90 degres: 000