How to determine which side of a polygon edge is inside a polygon, and which is outside?

Question

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?

score 3 · Accepted Answer · answered Mar 03 '14 at 22:45

3

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.

answered Mar 03 '14 at 22:45

Beta

96,650
16
149
150

2

should have linked to this: http://stackoverflow.com/questions/1165647/how-to-determine-if-a-list-of-polygon-points-are-in-clockwise-order/1165943#1165943 – agentp Mar 04 '14 at 12:56

score 2 · Answer 2 · edited Jun 20 '20 at 09:12

2

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.

edited Jun 20 '20 at 09:12

Community

1
1

answered Mar 04 '14 at 12:51

agentp

6,849
2
19
37

How to determine which side of a polygon edge is inside a polygon, and which is outside?

2 Answers2

Linked