center line for two lines.

[David Bethel]

I would definitely recommend using a 3D point value for midpoint calculations:

 

(defun mid_pt (p1 p2 / xp yp zp)               ;Mid Point
   (setq xp (+ (nth 0 p1) (* 0.5 (- (nth 0 p2) (nth 0 p1)))))
   (setq yp (+ (nth 1 p1) (* 0.5 (- (nth 1 p2) (nth 1 p1)))))
   (setq zp (+ (nth 2 p1) (* 0.5 (- (nth 2 p2) (nth 2 p1)))))
   (list xp yp zp))

(defun mid_point (s e)
  (mapcar '(lambda (a b) (* (+ a b) 0.5)) s e))

(defun midpt (p1 p2)
 (cal "(p1+p2)/2"))

 

The missing z axis could lead to all kinds on oddities. -David