[LISP程序]：怎样使多段线总是逆时针的

孤独客

怎样使多段线总是逆时针的

Underway

正时针，逆时针
http://www.4d-technologies.com/techcenter/geom/ge_whatpoly.lsp
info:
http://www.4d-technologies.com/techcenter/index.htm

http://www.faqs.org/faqs/CAD/autolisp-faq/part2/section-5.html
[23.6] How to REVERT a polyline direction?

2. 
   
3. ;; ! ***************************************************************************
   
4. ;; ! GE_WhatPoly
   
5. ;; ! ***************************************************************************
   
6. ;; ! Function : Find the direction of the polygon (if Cw or CCw)
   
7. ;; ! Argument : ptlist - list of points forming the polygon
   
8. ;; ! Returns  :
   
9. ;; !            1 - clockwise
   
10. ;; !           -1 - counterclockwise
    
11. ;; !            0 - Cannot be determined
    
12. ;; ! Updated  : September 19, 1998
    
13. ;; ! (C) 1999-2004, Four Dimension Technologies, Bangalore
    
14. ;; ! e-mail   : [email]rakesh.rao@4d-technologies.com[/email]
    
15. ;; ! Web      : [url]www.4d-technologies.com[/url]
    
16. ;; ! ****************************************************************************
    
17. 
    
18. (defun GE_WhatPoly (ptlist / nverts cnt area tmp pt1 pt2)
    
19. (setq
    
20.         cnt 0
    
21.         nverts (length ptlist)
    
22.         area 0.0
    
23. )
    
24. 
    
25. (while (< cnt (1- nverts))
    
26. (setq
    
27.         pt1 (nth cnt ptlist)
    
28.         pt2 (nth (1+ cnt) ptlist)
    
29.         area (+ area (* (cadr pt1) (car pt2)))
    
30.         cnt (1+ cnt)
    
31. )
    
32. )
    
33. 
    
34. (setq
    
35.         pt1 (nth (1- nverts) ptlist)
    
36.         pt2 (nth 0 ptlist)
    
37.         area (+ area (* (cadr pt1) (car pt2)))
    
38.         cnt 0
    
39.         tmp 0.0
    
40. )
    
41.    
    
42. (while (< cnt (1- nverts))
    
43. (setq
    
44.         pt1 (nth cnt ptlist)
    
45.         pt2 (nth (1+ cnt) ptlist)
    
46.         tmp (+ tmp (* (cadr pt2) (car pt1)))
    
47.         cnt (1+ cnt)
    
48. )
    
49. )
    
50. 
    
51. (setq
    
52.         pt1 (nth 0 ptlist)
    
53.         pt2 (nth (1- nverts) ptlist)
    
54.         tmp (+ tmp (* (cadr pt1) (car pt2)))
    
55.         area (* 0.5 (- area tmp))
    
56. )
    
57. 
    
58. (cond
    
59.         ((< area 0.0) (setq area -1) )
    
60.         ((> area 0.0) (setq area  1) )
    
61.         (T (setq area 0) )
    
62. )
    
63. area
    
64. )

<br /> ;; ! ***************************************************************************<br /> ;; ! GE_WhatPoly<br /> ;; ! ***************************************************************************<br /> ;; ! Function : Find the direction of the polygon (if Cw or CCw)<br /> ;; ! Argument : ptlist - list of points forming the polygon<br /> ;; ! Returns  :<br /> ;; !            1 - clockwise<br /> ;; !           -1 - counterclockwise<br /> ;; !            0 - Cannot be determined<br /> ;; ! Updated  : September 19, 1998<br /> ;; ! (C) 1999-2004, Four Dimension Technologies, Bangalore<br /> ;; ! e-mail   : [email]rakesh.rao@4d-technologies.com[/email]<br /> ;; ! Web      : [url]www.4d-technologies.com[/url]<br /> ;; ! ****************************************************************************<br /> <br /> (defun GE_WhatPoly (ptlist / nverts cnt area tmp pt1 pt2)<br /> (setq<br />         cnt 0<br />         nverts (length ptlist)<br />         area 0.0<br /> )<br /> <br /> (while (< cnt (1- nverts))<br /> (setq<br />         pt1 (nth cnt ptlist)<br />         pt2 (nth (1+ cnt) ptlist)<br />         area (+ area (* (cadr pt1) (car pt2)))<br />         cnt (1+ cnt)<br /> )<br /> )<br /> <br /> (setq<br />         pt1 (nth (1- nverts) ptlist)<br />         pt2 (nth 0 ptlist)<br />         area (+ area (* (cadr pt1) (car pt2)))<br />         cnt 0<br />         tmp 0.0<br /> )<br />     <br /> (while (< cnt (1- nverts))<br /> (setq<br />         pt1 (nth cnt ptlist)<br />         pt2 (nth (1+ cnt) ptlist)<br />         tmp (+ tmp (* (cadr pt2) (car pt1)))<br />         cnt (1+ cnt)<br /> )<br /> )<br /> <br /> (setq<br />         pt1 (nth 0 ptlist)<br />         pt2 (nth (1- nverts) ptlist)<br />         tmp (+ tmp (* (cadr pt1) (car pt2)))<br />         area (* 0.5 (- area tmp))<br /> )<br /> <br /> (cond<br />         ((< area 0.0) (setq area -1) )<br />         ((> area 0.0) (setq area  1) )<br />         (T (setq area 0) )<br /> )<br /> area<br /> )