Determine ordering of polygon 3D

Question

I have a list of vertices 3D, creating a polygon. I need to triangulate it and i use the cutting ears algorithm. My polygons are non-convex, so i need to find the concave angles in the polygon. I also dont know wich orientation are my polygons ( clockwise or counter clocwise)

Could anyone help me how to determine if is clockwise or counter clockwise

Thank you

clockwise or counter clockwise depends on the side you're looking the polygon from, it's not absolute in 3d, I think — fortran
Are you using a library (like OpenGL)? If so, you may be able to use something like GLUtesselator. This will scale easily even to non-convex polygons if you ever need it. — Tarydon

Beta Beta · Accepted Answer · 2009-07-30T14:57:04

It's probably simplest to project the polygon into a convenient plane (say z=0) and then use a method like reinier's.

1) Set all z=0. This won't change the answer.

2) Use a method like reinier's or this one to determine whether the polygon is clockwise or counterclockwise, and therefore whether the cross products "should be" +z or -z.

3) Any vertex with a cross product that points in the "wrong" direction is concave.

Determine ordering of polygon 3D

2 Answers