Obtaining knot values of control points

Rhino Developer
1

Dear Sir / Madame,

I’m new to Rhinoceros and interested in finite element analysis.

I need to obtain the knot values of the control points in my surface. Is there a way to get the knot values of a specific control point?, Is there a code which could give my selected control point’s corresponding knot values in u and v directions are, respectively as xi=1/3 , eta= 1/5

For example;

knotvector=rs.SurfaceKnots(surface) returns the knot values of the selected surface.

Could I write knotvalue=rs.ControlPointKnots(surface) to obtain the knotvalues of the control points in the surface ?

Best Regards,
Ahmet

2

Hi Ahmet,

A control point does not have a knot value. As the parameters of a surface are evaluated across the U- and V-domains, the knot values are used to determine how to interpolate the control point locations to a smooth surface.

What you can do, is get the edit points (the same as EditPtOn command in Rhino). These points are on the curve or surface and they do have knot values. These can be obtained with the so-called Greville parameters.

See for example for surfaces:

https://developer.rhino3d.com/api/RhinoCommon/html/M_Rhino_Geometry_Collections_NurbsSurfacePointList_GetGrevillePoint.htm

and

https://developer.rhino3d.com/api/RhinoCommon/search.html?SearchText=greville

3

The edit/Greville points have parameter values as does any point on a NURBS curve. In certain situations (such asequally spaced knots) some, but not all, the Greville points coincide with knots and have the same parameter values as knots. In other situations (such as unequally spaced knots) the edit/Greville points do not coincide with knots and the parameter values of the edit/Greville points will differ from the knot values.

1 Like
4

Thank you Mr. Cockey for your response.

5

Thank you Mr. Van Rijswijk for your answer.