Interior angle of polygon

I have a closed polygon made up of n points. Could be convex, concave or self intersecting.

Given 3 consecutive points (ie two adjacent lines) I want to draw an arc between the two lines such that the arc is on the interior of the polygon.

Just like an angle indicator in geometry.

Starting from line 1, I can either draw the arc cw or ccw to get to line 2. One way will draw the interior angle, the other way will draw the exterior angle, but how do I tell which?

Hope the decsription is clear, I am going round in circles trying to find a reasonable way to do this. Maybe there is a simple solution I just can't see.

This post appears to be relevant to your question: http://stackoverflow.com/questions/5...igger-than-180