I have an edge of a polygon (convex or concave). I want to find out if, going straight from start to end point of that edge, I have to turn right or left to get inside or outside of polygon. How can I check that?
Traverse the whole polygon in that direction. If you find that you went clockwise, then the interior is to the right; if you went counter-clockwise, it's to the left.
Another approach:
Project a perpendicular line and count how many times it crosses other edges.
odd -> interior
zero or even -> exterior
Equivalently If you happen to have a well optimized point-in-polygon routine available you can project a point some epsilon off the line and throw it into the inside test.
来源：https://stackoverflow.com/questions/22159120/how-to-determine-which-side-of-a-polygon-edge-is-inside-a-polygon-and-which-is