- step1 changes all black points to white if they are north border points that are 8_simple but neither 8_isolated nor 8_endpoint.
- step2 changes all black points to white if they are south border points that are 8_simple but neither 8_isolated nor 8_endpoint
- step3 changes all black points to white if they are east border points that are 8_simple but neither 8_isolated nor 8_endpoint
- step4 changes all black points to white if they are west border points that are 8_simple but neither 8_isolated nor 8_endpoint
Notice that all changes in each step are done in an temporary array and then the result of the changes is copied in the first array then the next step is executed in parallel on the result of the previous step so steps 1 to 4 are done in sequence until no pixels are changed from black to white
Finally when the skeleton is obtained, the array will be copied in a file which is called "skelet.pbm".
User can see the result by using "XV"
These are a few examples of executing the program :
The original pattern 
The skeleton of
the pattern
The original pattern 
The skeleton of
the pattern
The original pattern 
The skeleton of
the pattern
The original pattern 
The skeleton of
the pattern
The original pattern 
The skeleton of
the pattern
The eight connected algorithm is less noise sensitive than the four connected algorithm and it reserves thes eight connectivity of the skeleton of the pattern.