# Labelling the Outer Points of a Pentagram

• May 22nd 2013, 07:14 AM
Stockgoblin
Labelling the Outer Points of a Pentagram
Hi,

I'm trying to program an outer point labelling system for pentagrams and I'm having trouble with the logic.

I have 5 pairs of outer x & y coordinates
I have 5 pairs of inner x & y coordinates

I have decided that point "A" is the outer x & y coordinate pair that has an x value closest to the x value of the center of the inner pentagon. That's easy. The center just being the average of the 5 inner pairs.

I would like the outer points to be labelled clockwise, B, C, D & E.

What logic can I use to determine which point is which?

Any help is appreciated.

Thanks
• May 22nd 2013, 06:44 PM
chiro
Re: Labelling the Outer Points of a Pentagram
Hey Stockgoblin.

If you want to create a clock-wise ordering of points then you need to calculate the convex hull and reverse the order obtained by that algorithm (which creates an ordered list of points that is counter-clockwise in order).

For your problem, you have a 2D version which is a lot simpler than three or higher dimensions.