Shorten Nurbs curve

[BillW]

I am trying to shorten a nurbs curve. There is an "ExtendNurbsCurve" function but no shorten option. I can move the start point of a nurbs curve with the following code

   curveH := CreateDuplicateObject(srcH,NIL);
   if (startoffset > 0) then begin
     curvelen := hlength(srcH);
     inPercentOfLength := startoffset / curvelen;
     if GetPointAndParameterOnNurbsCurveAtGivenLength(curveH,inPercentOfLength,px,py,pz,outParam,outIndex) then begin
       NurbsSetPt3D(curveH,0,outIndex,px,py,pz);
       resetobject(curveH);
     end;
   end;

The problem is with getting the curve alignment correct. I assume I have to set the correct knot values for curve vertex points 0 and 1 with  NurbsSetKnot(objH,curveH,0,0) and NurbsSetKnot(objH,curveH,0,1).  I can get the existing knot values with NurbsKnot(srcH,curveH,0,0) and NurbsKnot(objH,srcH,0,1) . The question is how will the knot values change - what is the relationship?

 

TIA

Bill Wood
 

[BillW]

Still trying to understand Nurbs particularly how knot values work. Still cant figure out the logic for knot values in all cases - see below (might help others understanding).

Please comment if the information is incorrect.

 

I have also noticed when I "Export to script" a nurbs curve with more than 1 piece (ie from NurbsCurveGetNumPieces and with different degree values), if imported (via the script) a nurbs curve is created for each piece. There doesn't seem to be a way of creating (via script) a multi piece nurbs curve as far as I can tell.

 

To solve my problem, I think I will have to convert the (mutli piece) nurbs curve to 3D polygons (one for each piece, grouped), create/modify the 3D polygons and convert back to nurbs.

I need the nurbs for internal workings of path based plugins.