Finding Polyline Intersection

Replies to This Discussion

Permalink Reply by Damien Alomar on November 9, 2009 at 7:53am

Well, since OnPolyLine isn't really a curve class, you won't be able to do much with it as an OnPolyLine unless you took care of the intersection yourself (which wouldn't necessarily be too complicated here). However, if you convert the polylines to a class that derives from OnCurve (such as OnPolyLineCurve) then you can use the IntersectCurve method that is exposed to OnCurve classes. Here's a quick run down...


'These constructors should work because OnPolyLine
'derives from ArrayOn3dPoint
Dim plCrv1 as New OnPolyLineCurve (polyLine1)
Dim plCrv2 as New OnPolyLineCurve (polyLiine2)

Dim intersectionEvents as New ArrayOnX_EVENT()

plCrv1.IntersectCurve (plCrv2, intersectionEvents, doc.AbsoluteTolerance)

'ToDo: Sort through intersection events...


One thing to note is that polylines created by GH are already PolyLineCurves, so you shouldn't have to "convert" them.

Permalink Reply by Ben Sitler on November 10, 2009 at 1:50pm

Thank you for your help..you are completely right on the lack of need to "convert" anything- I mistakenly was creating OnPolyLine instead of OnPolyLineCurve from my array of On3dPoint.

However, one further question... the IntersectCurve output doesn't seem to give me the intersection point, which is what I really need. Is there a way to extract that point?

Permalink Reply by Damien Alomar on November 10, 2009 at 2:50pm

That function outputs an array of OnX_Events, which will actually tell you more about the intersection than just a point. It carries the intersection type, which in almost all cases is going to be a point intersection, although there is a possibility of an overlap condition (one line being on top of another). It also carries the parameters of the intersections for both curves. There may be multiple intersections, so this information is stored as an array. Below is a very quick way to point from an intersection event. This doesn't really do any sorting or anything, which may be helpful in a more complex situation


'''continuing from previous example...
Dim intersectionPoints as New List (of On3dPoint)

If intersectionEvents.count > 0 Then
'there's at least one intersection
For i as integer = 0 to intersectionEvents.count-1
intersectionPoints.Add(plCrv1.PointAt(intersectionEvents(i).m_a(0))
Next
End If