DXF data extraction -- Find circle center of circle segment that is part of a polyline

[ibSteveB]

I draw polylines that have circle segments for smoothing inside corners and such.

 

I want to extract the polyline data from the DXF.  I can find the polyline in the DXF and extract the start and end points of the various "VERTEX" items.  The circle segments have an identifier " 42 " that clues me in that the current segment is a circle segment.  But I am only finding the start point and end point for the circle segment.

 

How can I find the center point for each circle segment listed in the polyline data?

 

Attached are a simple polyline and CSV I extracted from the DXF.

CircleSegPolyLine.CSV CircleSegment-PolyLine.dxf

[Steven P]

The entity definition - for example, Polyline, tries to be as efficient as it can be - uses the least dotted pair identifiers as possible... and so a smaller file size, stuff like that.

 

That means for polylines, code '42' identifies the bulge of a segment, 0 being flat (no curve) and is a result of the arcs centre point, radius and angle.

 

Afra Lisp gives a better description than I can here: https://www.afralisp.net/archive/lisp/Bulges1.htm

Lee Mac gives some LISPs here also: https://lee-mac.com/bulgeconversion.html

[GLAVCVS]

Hi

Try this

(defun dameCentroRadio
	 (pt1 pt2 asoc42 / dst alfa radio h angPerpn centro)
    (setq dst	   (distance pt1 pt2)
	  alfa	   (* 4 (atan asoc42))	; Ángulo del arco
	  radio	   (/ dst (* 2 (sin (/ alfa 2)))) ; Radio del arco
	  h	   (sqrt (- (* radio radio) (* (/ dst 2) (/ dst 2))))
					; Distancia del centro al punto medio				
	  angPerpn (+ (angle pt1 pt2)
		      (* (/ pi 2)
			 (if (> asoc42 0)
			   1
			   -1
			 )
		      )
		   )			; Ángulo perpendicular				
	  centro
		   (list
		     (+ (/ (+ (car pt1) (car pt2)) 2) (* h (cos angPerpn)))
		     (+ (/ (+ (cadr pt1) (cadr pt2)) 2) (* h (sin angPerpn)))
		   )
    )
    (list centro radio)	
) 

 

[GLAVCVS]

2 hours ago, GLAVCVS said:

Hi

Try this

(defun dameCentroRadio
	 (pt1 pt2 asoc42 / dst alfa radio h angPerpn centro)
    (setq dst	   (distance pt1 pt2)
	  alfa	   (* 4 (atan asoc42))	; Ángulo del arco
	  radio	   (/ dst (* 2 (sin (/ alfa 2)))) ; Radio del arco
	  h	   (sqrt (- (* radio radio) (* (/ dst 2) (/ dst 2))))
					; Distancia del centro al punto medio				
	  angPerpn (+ (angle pt1 pt2)
		      (* (/ pi 2)
			 (if (> asoc42 0)
			   1
			   -1
			 )
		      )
		   )			; Ángulo perpendicular				
	  centro
		   (list
		     (+ (/ (+ (car pt1) (car pt2)) 2) (* h (cos angPerpn)))
		     (+ (/ (+ (cadr pt1) (cadr pt2)) 2) (* h (sin angPerpn)))
		   )
    )
    (list centro radio)	
) 

 

This function will return a list with the center and radius.
Once you have both parameters, you can get all the points you want along the arc using:
 

(polar centro [sequence of possible angles between pt1 and pt2] radius)

 

Edited March 17 by GLAVCVS

[GLAVCVS]

Of course:
-pt1 is the start point of the arc
-pt2 is the end point of the arc
-asoc42 is the value returned by (cdr (assoc 42 ...))' on that segment