AutoLISP: Advanced Polyline Offset

Not knowing what to call this routine, I think that it is more of a combination of an advanced Offset command that automates the placement of the vertices at the (M2P) Mid-Between-2-Points.

If you need an offset that is between 2 polylines and the 2 polylines are not truly parallel to each other, the result of the polyline might not be the desired result.

OffsetPL2Mid 1

This process is automated by using this routine written by Alan Thompson and found at CADTutor:

(Please refer questions or requests to the forum where the lisp routine was originally posted)

OffsetPL2Mid 2

Note: that the output offers an option for the created polyline (LWPolyline or Polyline)

OffsetPL2Mid 3

As an added bonus, the routine handles polylines that are at different elevations (z values) and it even handles 3DPolylines pretty well.

OffsetPL2Mid 4

OffsetPL2Mid 5

Thanks Alan!!

Here’s how:

  • LBL <enter> to start
  • Select the 1st polyline
  • Select the 2nd polyline
  • Specify the output polyline (LWPoline or Polyline)
  • RE <enter> or REGEN <enter> to get rid of the temporary dashed red lines

(defun c:LBL (/ foo AT:GetSel _pnts _pline _lwpline _dist e1 e2)
  ;; Draw (LW)Polyline between two selected curves (at midpoint of vertices).
  ;; Alan J. Thompson, 09.29.10

  (defun foo (e)
    (and (wcmatch (cdr (assoc 0 (entget (car e)))) "LINE,*POLYLINE,SPLINE")
         (not (vlax-curve-isClosed (car e)))

  (defun AT:GetSel (meth msg fnc / ent)
    ;; meth - selection method (entsel, nentsel, nentselp)
    ;; msg - message to display (nil for default)
    ;; fnc - optional function to apply to selected object
    ;; Ex: (AT:GetSel entsel "\nSelect arc: " (lambda (x) (eq (cdr (assoc 0 (entget (car x)))) "ARC")))
    ;; Alan J. Thompson, 05.25.10
      (progn (setvar 'ERRNO 0)
             (setq ent (meth (cond (msg)
                                   ("\nSelect object: ")
             (cond ((eq (getvar 'ERRNO) 7) (princ "\nMissed, try again."))
                   ((eq (type (car ent)) 'ENAME)
                    (if (and fnc (not (fnc ent)))
                      (princ "\nInvalid object!")

  (defun _pnts (e / p l)
    (if e
      (cond ((wcmatch (cdr (assoc 0 (entget e))) "ARC,LINE,SPLINE")
             (list (vlax-curve-getStartPoint e) (vlax-curve-getEndPoint e))
            ((wcmatch (cdr (assoc 0 (entget e))) "*POLYLINE")
             (repeat (setq p (1+ (fix (vlax-curve-getEndParam e))))
               (setq l (cons (vlax-curve-getPointAtParam e (setq p (1- p))) l))

  (defun _pline (lst)
    (if (and (> (length lst) 1)
             (entmakex '((0 . "POLYLINE") (10 0. 0. 0.) (70 . 8)))
             (foreach x lst (entmakex (list '(0 . "VERTEX") (cons 10 x) '(70 . 32))))
      (cdr (assoc 330 (entget (entmakex '((0 . "SEQEND"))))))

  (defun _lwpline (lst)
    (if (> (length lst) 1)
      (entmakex (append
                  (list '(0 . "LWPOLYLINE")
                        '(100 . "AcDbEntity")
                        '(100 . "AcDbPolyline")
                        (cons 90 (length lst))
                        (cons 70 (* (getvar 'plinegen) 128))
                  (mapcar (function (lambda (p) (list 10 (car p) (cadr p)))) lst)

  (defun _dist (a b) (distance (list (car a) (cadr a)) (list (car b) (cadr b))))

      (setq e1 (_pnts (car (AT:GetSel entsel "\nSelect first open curve: " foo))))
      (setq e2 (_pnts (car (AT:GetSel entsel "\nSelect next open curve: " foo))))
      (not (initget 0 "Lwpolyline Polyline"))
      (setq *LBL:Opt* (cond ((getkword (strcat "\nSpecify line to draw: [Lwpolyline/Polyline] <"
                                               (cond (*LBL:Opt*)
                                                     ((setq *LBL:Opt* "Lwpolyline"))
                                               ">: "
     ((if (eq *LBL:Opt* "Lwpolyline")
       (vl-remove nil
                  (mapcar (function (lambda (a b)
                                      (if (and a b (not (grdraw (trans a 0 1) (trans b 0 1) 1 1)))
                                        (mapcar (function (lambda (a b) (/ (+ a b) 2.))) a b)
                          (if (< (_dist (car e1) (car e2))
                                 (_dist (car e1) (last e2))
                            (reverse e2)
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This blog serves as a knowledge base for myself so that I can reference tips & tricks that I have learned and also refer others to it as well. I hope that this blog helps you learn at least one tip to make your drafting experience better.
This entry was posted in AutoLISP, AutoLISP: Creating, AutoLISP: Modify, AutoLISP: Polylines. Bookmark the permalink.

4 Responses to AutoLISP: Advanced Polyline Offset

  1. Doug says:

    This is terrific. Thanks!

  2. Szigeti Pál says:

    Does it work on polylines with different number of points?

  3. laud8 says:

    You always make me learn a useful thing! Thanks Greg! :)

    • AutoCAD Tips says:

      Thanks for the compliment. I find your blog inspiring. Blending art and architecture and especially landscape architecture which I think is some of the hardest architecture to pull off and make it really stand out. You guys do a great job.

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