求助： 如何判断一个环的顺时针与逆时针？？

zxfzxf

求助： 如何判断一个环的顺时针与逆时针？？

威名

(setq ast (cdr (assoc 50 (entget aab))))
  (setq sst (cdr (assoc 51 (entget aab))))
  (setq r (cdr (assoc 40 (entget aab))))
  (setq zhongxing (cdr (assoc 10 (entget aab))))
  (setq t0 (- sst ast))
       (cond ((< t0 0)
             (setq s1 (* (/ (+ 6.28 t0) 2) (* r r)))
      (setq s2 (/ (* (sin (+ 6.28 t0)) (* r r)) 2))
       ))
       (cond ((> t0 0)
             (setq s1 (* (/ t0 2) (* r r)))
      (setq s2 (/ (* (sin t0) (* r r)) 2))
              ))

zxfzxf

请楼上的大侠能否详细的解释？？？

HuaiYu

;; ! ***************************************************************************
;; ! GE_WhatPoly
;; ! ***************************************************************************
;; ! Function : Find the direction of the polygon (if Cw or CCw)
;; ! Argument : ptlist - list of points forming the polygon
;; ! Returns  :
;; !            1 - clockwise
;; !           -1 - counterclockwise
;; !            0 - Cannot be determined
;; ! Updated  : September 19, 1998
;; ! (C) 1999-2004, Four Dimension Technologies, Bangalore
;; ! e-mail   : rakesh.rao@4d-technologies.com
;; ! Web      : www.4d-technologies.com
;; ! ****************************************************************************

(defun GE_WhatPoly (ptlist / nverts cnt area tmp pt1 pt2)
  (setq cnt    0
 nverts (length ptlist)
 area   0.0
  ) ;_ Endsetq
  (while (< cnt (1- nverts))
    (setq pt1  (nth cnt ptlist)
   pt2  (nth (1+ cnt) ptlist)
   area (+ area (* (cadr pt1) (car pt2)))
   cnt  (1+ cnt)
    ) ;_ Endsetq
  ) ;_ Endwhile
  (setq pt1  (nth (1- nverts) ptlist)
 pt2  (nth 0 ptlist)
 area (+ area (* (cadr pt1) (car pt2)))
 cnt  0
 tmp  0.0
  ) ;_ Endsetq
  (while (< cnt (1- nverts))
    (setq pt1 (nth cnt ptlist)
   pt2 (nth (1+ cnt) ptlist)
   tmp (+ tmp (* (cadr pt2) (car pt1)))
   cnt (1+ cnt)
    ) ;_ Endsetq
  ) ;_ Endwhile
  (setq pt1  (nth 0 ptlist)
 pt2  (nth (1- nverts) ptlist)
 tmp  (+ tmp (* (cadr pt1) (car pt2)))
 area (* 0.5 (- area tmp))
  ) ;_ Endsetq
  (cond ((< area 0.0) (setq area -1))
 ((> area 0.0) (setq area 1))
 (t (setq area 0))
  ) ;_ Endcond
  area
) ;_ Enddefun

HuaiYu

或者你去根据内角去判断