Are there any algorithms for calculation of nonconvex boundary of a set of

points.

Yes, I understand that it is non unambigous, but nevertheless

for the set:

x1 xxxxxxxxxxxxxxxxxx

x2xxxxxxxxxxxxxxxxxxx

x3xxxxxxxxxxxx

x4xxxxxxxxxxx

x5xxxxxxxxxxxxx

x6xxxxxxxxxxxxx

I wish to get x1-x2-x3-x4-x5-x6 or x1-x2-x3-x5-x6 or x1-x5-x6

but not x1-x6.