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Counter-clockwise
Posted by taubz [send private reply] at October 03, 2001, 04:36:51 PM
Hiya... Does anyone know how to determine if an ordering of 2D points that form a polygon is in clockwise or counterclockwise order? Maybe the sum of the angles between neighboring segments is positive if clockwise?
Thanks. - taubz
Posted by Psion [send private reply] at October 03, 2001, 10:23:01 PM
A formula from an algorithms book:
z = (x_i - x_0)(y_c - y_0) - (y_i - y_0)(x_c - x_0) z > 0 iff p_i (with coords x_i and y_i) is clockwise from p_c with respect to p_0.... looks like a cross-product :-)
Posted by gian [send private reply] at October 04, 2001, 03:44:12 AM
* gian takes one look, goes insane and starts running into walls, head-first.
Posted by taubz [send private reply] at October 04, 2001, 12:31:28 PM
Duh... okay.... is that formula for finding the clockwise-nature (?) of a polygon or just for two points? I think that's just for two line segments joined at a point (if it isn't, then I don't follow the formula), which doesn't tell you anything about the polygon.
Thanks, but try again? I'll try gamedev.net, not that this is for a game. - taubz
Posted by Psion [send private reply] at October 04, 2001, 07:54:50 PM
I think you just pick one point as the start and compare the pairs of adjacent points afterward with that first point as the reference.
Posted by taubz [send private reply] at October 05, 2001, 11:28:18 AM
Well, either way, I found the algorithm on gavedev.net... it wasn't as simple as that formula. Thanks, tho.
- taubz
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