Topic: 3 points are clockwise or counter-clockwise?

[FELIX]

3 data points PT1, PT2 and PT3 are determining whether clockwise or counterclockwise?

[Kerry]

Have a play with this (from a site search) :-

http://www.theswamp.org/index.php?topic=13526.msg234360#msg234360
From gile

Code - Auto/Visual Lisp: [Select]

1. ;;; Clockwise-p Evaluates if p1 p2 et p3 are clockwise
2. ;; gile http://www.theswamp.org/index.php?topic=13526.msg234360#msg234360
3.  
4. (defun clockwise-p (p1 p2 p3)
5.   (< (sin (- (angle p1 p3) (angle p1 p2))) -1e-14)
6. )

[FELIX]

Very good, thank you so much!

[gile]

Hi,

Another way using the double signed area of the triangle:

Code - Auto/Visual Lisp: [Select]

1. (defun clockwise-p (p1 p2 p3)
2.   (minusp
3.     (- (* (- (car p2) (car p1)) (- (cadr p3) (cadr p1)))
4.        (* (- (car p3) (car p1)) (- (cadr p2) (cadr p1)))
5.     )
6.   )
7. )

[Lee Mac]

Another few:

http://lee-mac.com/clockwisep.html