How to write a logic for extending the given region.

Iam able to write for regular polygons. Iam not sure wheather Iam correc or not.
I found all external lines parallel the sides of the given polygon at a
given units away from the polygon lines and found intersection of all those lines. That gave
me the coordinates of the Extended region.

Is above logic is correct? How to handle for the irregular polygons ?

-Thanks
Vamsi.