Surface modelling problem

Hi everyone,

I’m new about rhino and i want to model a surface which i draw the shape in the attached jpg file. The knot vectors are U={0,0,0,0.5,1,1,1} (degree of the knot vector U is p=2) and in the other direction the knot vector V={0,0,0,1,1,1} (degree of the knot vector V is q=2). Therefore the total number of control points are n x m = 4 x 3 = 12 and it is seen that i want to model a two element surface.

Could you examine the attached file and give an idea about how it could be done in the Rhino.

Best regards,
Çetin CUMA

No file attached. Just image.

Not possible according to the image - the count in the rows/columns of control points have to match. You maybe want to look into how NURBS surface geometry works. So you would probably need 5 x 3. However, a single NURBS surface normally doesn’t like to have kinks - although it’s legal - at best you will have a “compound” surface with an internal kink joint caused by the sharp corner at the upper left.


Much more concise than I could explain.


I’m sorry, i should have explained the model a bit more. There are two control points joint in the upper left corner and i want to define 2 elements in the U direction and one element in the V direction.

Can you explain how to define compound surfaces? Thank you.

Still doesn’t work. Perhaps you should re-do your diagram with lines different colors connecting the U points and the V points. A single surface MUST have the same number of points along U everywhere and along V everywhere. You can “collapse” a row of U or V points into one location to make a singularity (as in the poles of a NURBS sphere), but technically even though they show as one point, the full number of points are still there, stacked at that spot.

Compound is perhaps not the correct term, what I meant is that you will have a “kink” or fully multiple knotline at that spot. Legal, as I said, but many Rhino operations may fail with this surface.


I modified the control points and got the correct geometry but i cant obtain the knot vectors U={0,0,0,0.5,1,1,1} and V={0,0,0,1,1,1} which i need for analysis. Can you check if my geometry or python script is wrong.


Model.3dm (25.6 KB) (746 Bytes)

Hi Çetin - try Reparameterize. Set the domain 0 > 1 in each direction .


Thank you very much Pascal, i managed to set the knot vector domains accordingly but i still have issues with the extraction of data.

Hi Çetin - sorry - what do you mean here - what are you trying to do?


I am trying to extract control point and knot vector data for an isogeometric analysis software in Mathematica.

In what form do you need the ‘extraction’? Do you have an example format?


I am trying to get data for the Mathematica script in the attachment. I am sorry for the file is in rar format i couldnt upload a file with .nb extension.


MainNotebook46.rar (20.3 KB)