HOW TO: self referencing vla-intersectWith.

[SOliver]

Alright folks,

 

I've made this post for two reason: Firstly I spent a fair bit of time looking through google on this subject with practically no pointers on how to achieve the above - so I thought "I'm sure the forum will find this interesting". Secondly: If this is wrong I'm sure someone will spot the error before I do. If this is old news then let the thread die - hate the code not the coder lol

 

So I've been scraping the internet looking for a efficient (not create-a-region) based approach to determining if a lwPolyline intersects with it's self. Anyway after a bit of lethargy-fuelled critical thinking I discovered the following.

(defun c:sample( / )
 (setq ply(vlax-ename->vla-object(car (entsel "\nSelect polyline"))))
 (setq uBound(vlax-safeArray-get-u-bound(vlax-variant-value(vla-get-coordinates ply)) 1))
 (setq intUBound(vlax-safeArray-get-u-bound(vlax-variant-value(vla-intersectWith ply ply acExtendNone)) 1))
 (princ "\nCoord len: ")(princ uBound)
 (princ "\nInter UB: ")(princ intUBound)
 (if(> intUBound uBound)
   (princ "\nIntersects with self")
    (princ "\nDoes not intersect with self")
 )
 (princ)
)

From what I can tell every point along a polyline is considered to be an intersection when self referencing an object with the vla-intersectWith method.

 

Since each point is considered an intersect a non self-intersecting lwPolyline will have the same safeArray upper bound as that of the results from vla-intersectWith - if they differ then the lwPolyline intersects itself.

 

EDIT:

 

In hindsight I probably shouldn't have called this a "how to" given that the information may be redundant || not validated by a group of peers.

 

 

Enjoy, use or disprove at your own leisure.

[Lee Mac]

Nice one SOliver

 

You might want to considering approaching it this way to avoid the Variants - but they both ultimately use the same method:

 

(defun SelfIntersecting ( poly )
 (< (length (vlax-get poly 'Coordinates)) (length (vlax-invoke poly 'IntersectWith poly 0)))
)

 

Lee

[SOliver]

Nice one SOliver

 

You might want to considering approaching it this way to avoid the Variants - but they both ultimately use the same method:

 

(defun SelfIntersecting ( poly )
 (< (length (vlax-get poly 'Coordinates)) (length (vlax-invoke poly 'IntersectWith poly 0)))
)

 

Lee

 

Thanks Lee.

 

I never realised until I read your code that vlax-get and vlax-invoke provide a standard list in the return. Would it be fair to say that that when using vlax-* methods (pertaining to safeArrays) the return will be in standard list format as opposed to vla-* methods which true to nature will return a safeArray of sorts?

[Lee Mac]

The vlax-get/invoke are undocumented and will return the data in native AutoLISP formats - however, IIRC there are a few methods/properties for which they fail.