Geometric center of open polyline, line

[maahee]

(setq gct (osnap (vlax-curve-getStartPoint bm) "gcen"))

The above code line gives only closed polyline

How to find the geometric center of an open polyline and a single line

[DATVO]

1 hour ago, maahee said:

(setq gct (osnap (vlax-curve-getStartPoint bm) "gcen"))

The above code line gives only closed polyline

How to find the geometric center of an open polyline and a single line

 

Only the geometric center is available for closed areas, since open polylines have no area and therefore no true center. If it’s just a line, the "geometric center” is the midpoint. 

[Saxlle]

You can actually calculate it, here is the code (you can change it for your purposes):

 

(setq ptlist (mapcar 'cdr (vl-remove-if-not (function (lambda (x) (= (car x) 10))) (setq ent (entget (car (entsel "\nSelect the line:")))))) ;; point list
      sum_x 0
      sum_y 0
      vertices (cdr (assoc 90 ent)) ;; number of vertices for LWPOLYLINE
      )

(foreach pt ptlist
  (setq sum_x (+ sum_x (car pt))) ;; sum only the X vertices values
  )

(foreach pt ptlist
  (setq sum_y (+ sum_y (cadr pt))) ;; sum only the Y vertices values
  )

(setq sum_x (/ sum_x vertices) ;; dividing the total sum X by the number of vertices
      sum_y (/ sum_y vertices) ;; dividing the total sum Y by the number of vertices
      pt (list sum_x sum_y) ;; geometric center
      )

 

Below are the pictures with geometric center for open and closed polyline (picture 1 open, picture 2 closed).

 

Picture 1.

 

Picture 2.

 

50 minutes ago, DATVO said:

If it’s just a line, the "geometric center” is the midpoint. 

 

This is true.

 

Best regards.

[BIGAL]

Not quite the middle of say 2 points. The red is the average answer. The green is gcen. so 4 answers. If the shape is say 4 points then you could force a close get the Gcen then do a undo.

 

[lrm]

The geometric center (a.k.a. centroid) of an open polyline is deteremined by summing the length of each line segment times the coordinates of the midpoint of the respective segment. The result is then divided by the total length of the polyline. 

It is NOT the average of the point locations.

; Calculates the centroid for an open lwpolyline
; lrm 8/9/2025
(defun c:pl_centroid (/ ss pt_lst sum_x sum_y sum_d n i d mid_pt centroid)
  (setq	ss     (entget (car (Entsel "\nSelect Open Polyline")))
	pt_lst (mapcar
		 'cdr
		 (vl-remove-if-not '(lambda (x) (= (car x) 10)) ss)
	       )
  )
(setq sum_x 0
      sum_y 0
      sum_d 0.0
      n	    (length pt_lst)
      i	    0
)
(while (<= i (- n 2))
  (setq d (distance (nth i pt_lst) (nth (+ i 1) pt_lst)))
  (setq
    mid_pt (mapcar '/
		   (mapcar '+ (nth i pt_lst) (nth (+ i 1) pt_lst))
		   '(2 2 2)
	   )
  )
(setq sum_x (+ sum_x (* (car mid_pt) d))
      sum_y (+ sum_y (* (cadr mid_pt) d))
      sum_d (+ d sum_d)
      i	    (+ i 1)
)
)
(setq centroid (list
		 (/ sum_x sum_d)
		 (/ sum_y sum_d)
	       )
)
(princ "\n Centroid at: ")
(princ centroid)
(command "_point" "_non" centroid)
(princ)
)

 

 

[devitg]

@Mahee , please give it a try , will work both  for open and closed lwpolylines it get the Centroid from a temporary Region from a temporary closed polyline  if poly is open 

 

 

get-center.LSP 

get-center.LSP get poly center.dwg

Edited 2 hours ago by devitg
miss spelling