CurveCurve intersection broken?

I've made a model that requires extracting the parameters from CurveCurve intersections between a number of curves (most are linear, some actually are polylines). Some of the parameters returned are really screwy (I've only noticed the errors on the linear curves), for example returning parameter of 71.2 when the calling getLength on the same exact curve returns 6.55. Some parameters being returns are negative, oddly enough. I've attached a screenshot and pasted a short example script.

Dim int As rhino.Geometry.Intersect.CurveIntersections = rhino.Geometry.Intersect.Intersection.CurveCurve(x(0), x(1), 0.001, 0.001)

print("Len Curve A " & CStr(x(0).getlength))

print("Len Curve B " & CStr(x(1).getlength))

print("ParamA " & CStr(int.Item(0).parameterA))

print("ParamB " & CStr(int.Item(0).parameterB))

It works fine if I extract the end points of the linear curves and replace the curve with new lines going from startpoint to endpoint and then calling Line.ToNurbsCurve. For now I will use this fix, but this seems like a serious bug (or I am way off base and don't understand how parameters work, which is certainly a possibility).

Anyone have an answer?

Cheers,

Ben

Attachments:

intersect.JPG, 177 KB

Replies to This Discussion

Permalink Reply by David Rutten on October 7, 2010 at 8:10am

Hi Ben,

curve parameters do not necessarily correspond with length. You need to use Curve.PointAt(t) in order to go from curve-parameter to 3d-point.

--
David Rutten
david@mcneel.com
Poprad, Slovakia

Permalink Reply by Ben Sitler on October 7, 2010 at 10:48am

Hmm,
Well I did try that, but to no avail. For the linear Curves: Int.Item(k).PointA universally gives the correct answer, but if I grabbed the ParameterA and then called CurveA.PointAt(ParamA) the resulting point was always wayyyyyyyyyyyyy off... and not necessarily even on CurveA if you extend it to infinity.

Another thing to note is that if I go over the lines that are giving me problems, and delete and replace them this seems to fix the issue. It is quite apparently something to do with the specific linear curves that I am working with, which is really odd as they are were all generated by New Line(PtA,PtB) in an earlier script.

Thanks for the fast reply.

Cheers,

Ben

Permalink Reply by David Rutten on October 7, 2010 at 12:25pm

You'll have to supply some curves that don't work. This (very simple) test fails to reproduce the problem:

Private Sub RunScript(ByVal x As Curve, ByVal y As Curve, ByRef A As Object)

  Dim ccx As Rhino.Geometry.Intersect.CurveIntersections = Rhino.Geometry.Intersect.Intersection.CurveCurve(x, y, 0.01, 0.01)

  If (ccx Is Nothing) Then

    Print("No intersections found")

    Return

  End If



  Print("{0} intesection(s) found", ccx.Count)



  Dim pts As New List(Of Point3d)

  For i As Integer = 0 To ccx.Count - 1

    pts.Add(x.PointAt(ccx(i).ParameterA))

  Next



  A = pts

End Sub