how to creat a line which is tangent from a given curve on certain t parameter

i have read vb part of the new tutorial.

i have some problems with the chapter points and vectors.

i tried to creat a line which is tangent from a given curve on certain t parameter.
i get stucked when i get an on3dpoint from the curve class-- curve.tangentat(t)
this appear to me is a on3dpoint. not sure though.

i used the one on tutorial : dim dir_vector as new on3dvector = p1-p0
the output said 'end statement expected‘

and i also wandered if i need something like rhino.pointadd in VB.net
what should i do?

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Permalink Reply by Dirk Anderson on July 8, 2009 at 5:09am

Try this code:

Sub RunScript(ByVal inCrv As OnCurve, ByVal t As Double, ByVal length As Double)
Dim tVec As New On3dVector
tVec = inCrv.TangentAt(t)
tVec = tVec * length

Dim ptStart As New On3dPoint(inCrv.PointAt(t))
Dim ptEnd As New On3dPoint
ptEnd = tVec + ptStart

Dim tLine As New OnLine(ptStart, ptEnd)
A = tLine
End Sub

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Permalink Reply by luming Wang on July 8, 2009 at 6:50am

cool.
thanks a lot.

i have got another question what is the difference bt. OnCurve and OnNurbsCurve?
and since OnCurve and OnLine is different, how to use the pointat in OnLine?

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Permalink Reply by Damien Alomar on July 8, 2009 at 7:06am

Check out my post @ the thread below. It explains a little bit about the different curve classes in the Rhino SDK.

http://grasshopper3d.ning.com/forum/topics/how-to-convert-online-to

Specifically, the PointAt method is going to be pretty much exactly the same for OnLine as it is in the OnCurve derived classes.

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Permalink Reply by Dirk Anderson on July 8, 2009 at 7:16am

Best to read the SDK help file on Classes.

OnCurve is the parent class for OnNurbsCurve (OnCurve is also within OnGeometry Class - Open Nurbs Geometry).
OnLine is a derivative of the Curve Class rather than OnGeometry Class, however, it can be cast to OnGeometry.

OnLine.PointAt(t) should work exactly the same.

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Permalink Reply by luming Wang on July 9, 2009 at 12:19am

thanks a lot.

just wanna to be more clear, so if i had a VB component output A is a OnNurbsCurve, can i use this output for another VB Component as an input x as OnCurve?