Get angle for ear clipping

Question

I am trying to implement a ear clipping algorithm into a program of mine but I am having issues. While that I can get it to work in a lot of situations, I haven't found a good way to check for reflex angles.

I've been looking up ways - every method I've tried to date seems to have angle it won't work for. When I try to find more information, most people's tutorials/work just tell me to "find the reflex angle and test for ear" then describe how to test for ear but not how to get the reflex angle.

Can anyone tell me how to get the proper angle inside the triangle for a concave polygon, or point me in the right direction? Could be an understanding issue with me. Thanks.

Answer 1

Figured out my problem was one of how I was conceiving the issue. I was saying that if the point was outside the polygon it could still be in the polygon without adding in my head the fact I removed the last vertex. Been busting my brains trying to implement ear clipping for a few days and got it wrong at this point - the solution was the basic "check if the center point of the triangle was outside the polygon and mark it as reflex".