Draw a line perpendicular to an object it started from

Hello.

I've searched on the forum and on the web for a way draw a line perpendicular to point A which is snapped to an object, but couldn't find a good solution that would work with every 2D object (lines, plines, splines, arcs, etc).

My current manual approach is to use offset for the object, it works fine but only on objects that can be offset.

Another method is to use polar tracking with "Relative to last segment", but it fails on curves and intersections.

So far the most promising solution I could find is to use polar command which detects the angle of a segment of a spline and then use polar command to draw line at 90 degree:

http://www.cadtutor.net/forum/showthread.php?5953-lines-perpendicular-to-spline&p=35506&viewfull=1#post35506

So my challenge now is to get these parts working:

1) when user selects point A, detect which object it was clicked at. This is necessary when the selected point is an intersection. For that I think we'll need capture cursor location for example with this:

(CAR (CDR (GRREAD nil 15)))

and then find the object near it.

Done. Post #2

2) find an angle of the object (segment) at point A

Done. Post #3

3) Determine if we need to use 90 degree or 270 degree angle for the line. I'm guessing this can be calculated from angle between point A and B (?)

Any help on these challenges would be greatly appreciated.

Thank you.

The 1) challenge is done using Lee-Mac's DynOffset

(DEFUN c:test (/ p1 cur ss EntLst)
   (SETQ p1  (GETPOINT "Select start point")
         cur (CAR (CDR (GRREAD nil 15))) ; cursor coordinates
         ss  (SSGET "_C" p1 p1) ;get all objects at selected point
         EntLst (vl-remove-if 'listp (mapcar 'cadr (ssnamex ss)))
         EntLst (VL-SORT ;sort objects by distance to cursor
                 EntLst
                 (FUNCTION
                     (LAMBDA (a b)
                         (< (DISTANCE
                                cur
                                (VLAX-CURVE-GETCLOSESTPOINTTO a cur)
                            )
                            (DISTANCE
                                cur
                                (VLAX-CURVE-GETCLOSESTPOINTTO b cur)
                            )
                         )
                     )
                 )
             )
   )
   (CDR (ASSOC 0 (ENTGET (car EntLst)))) ;closest to cursor object is the first item in the EntLst list
)

Solved 2) challenge:

(DEFUN c:test (/ p1 p3 ss EntLst cur ang)
   (SETQ p1     (GETPOINT "Select start point")
         cur    (CAR (CDR (GRREAD nil 15))) ; cursor coordinates
         ss     (SSGET "_C" p1 p1) ; get all objects at selected point
         EntLst (VL-REMOVE-IF 'LISTP (MAPCAR 'CADR (SSNAMEX ss)))
         EntLst (VL-SORT ; sort objects by distance to cursor
                    EntLst
                    (FUNCTION
                        (LAMBDA (a b)
                            (< (DISTANCE
                                   cur
                                   (VLAX-CURVE-GETCLOSESTPOINTTO a cur)
                               )
                               (DISTANCE
                                   cur
                                   (VLAX-CURVE-GETCLOSESTPOINTTO b cur)
                               )
                            )
                        )
                    )
                )
         ent    (CAR EntLst) ; closest to cursor
         p3     (VLAX-CURVE-GETPOINTATDIST ent (+ 0.0001 (VLAX-CURVE-GETDISTATPOINT ent p1))) ; coordinates on this entity 0.0001 away from p1
         ang    (ANGLE p1 p3) ; get angle between p1 and p3
   )
   (ENTMAKE (LIST '(0 . "LINE")
                  (CONS 10 (TRANS p1 1 0))
                  (CONS 11 (TRANS (POLAR p1 ang 1000) 1 0))
            )
   )

)

I've searched on the forum and on the web for a way draw a line perpendicular to point A which is snapped to an object, but couldn't find a good solution that would work with every 2D object (lines, plines, splines, arcs, etc).

Why just not :

(defun c:perpline ( / f p a o )

 (vl-load-com)

 (if (eq (getvar 'worlducs) 0)
   (progn
     (command "_.UCS" "_W")
     (setq f t)
   )
 )
 (while (or (null p) (not (and p (setq p (osnap p "_nea")))))
   (setq p (getpoint "\nPick point on curve - closest one will be accounted : "))
 )
 (setq a (angle '(0.0 0.0) (vlax-curve-getfirstderiv (setq e (car (nentselp p))) (vlax-curve-getparamatpoint e (vlax-curve-getclosestpointto e p)))))
 (command "_.UCS" "_3P" "_non" p "_non" (polar p a 1.0) "_non" (polar p (+ (* 0.5 pi) a) 1.0))
 (setq o (getvar 'orthomode))
 (setvar 'orthomode 1)
 (command "_.LINE" "_non" '(0.0 0.0 0.0) "\\" "")
 (setvar 'orthomode o)
 (command "_.UCS" "_P")
 (if f
   (command "_.UCS" "_P")
 )
 (princ)
)

That's actually pretty good, thank you.

Unfortunately it's not 100% working in my quick test.

For some reason on huge (dimension wise) entities it draws zero length line when I enter length manually:

test1: word view, pick first point on any object, then enter 10 as length.

result: line with 0 length

test2: zoom in to a part of an object, pick first point and enter 10 as length:

result: 10 inch line

[EDIT]

Found what's causing it: with too short line osnap would catch the starting coordinates as the end of line.

Drawing1.dwg

For 3) set SNAPANG to the value from 2) then put the cursor on the side you want the line and enter the distance.

Maybe this program can help?