You'd better be careful how you pass perimeters to your subs or you'll wind up convexed and get the answers oblong.

(Aside from the bad puns, I've got nothing to add other than that PDL might have some support for this. If you're looking for algorithmic help see if you can't lay a hand on Computer Graphics, C Version (ISBN 0135309247); just be forewarned that the algorithm's probably going to be more geared towards rendering the merged result which may not be what you need.)