NURBS CURVE

Replies to This Discussion

Permalink Reply by Hannes Löschke on February 3, 2012 at 2:49pm

Have a look >here/a> for info about NURBS control points and knots.

Weights are optional and define how closely the curve follows the specific control points. If you use weigths, you need to specify one weigth for each CP (I think). If you leave the weigths, the curve is buildt the standard way rhino does.

Permalink Reply by M NICK on February 3, 2012 at 3:03pm

Thanks for your help Hannes, but I want to change the weights, I tried to assign a weight for each cp but it didn't work!

Attachments:

• unnamed.gh, 3 KB

Permalink Reply by David Rutten on February 3, 2012 at 3:15pm

That's because you're feeding the Knot vector component the actual control-points rather than an integer describing how many there are. You need to insert a [List Length] component between your point parameter and your knot component.

--

David Rutten

david@mcneel.com

Poprad, Slovakia

Attachments:

• unnamed.gh, 2 KB

Permalink Reply by Hannes Löschke on February 3, 2012 at 3:22pm

Your def doesn't apply weigths... so I cannot see your problem.

Just to show how changing the weigths, I made this quick def.

Attachments:

• NURBS_weight.gh, 4 KB

Permalink Reply by Ninja on February 27, 2014 at 8:06am

 I saw that you altered one value of the 4 values in the weight list, can we alter all 4 of them and how?

Thanks

Permalink Reply by David Rutten on February 3, 2012 at 3:05pm

Hi Nick,  

the number of knots equals the degree + controlpointcount - 1. A degree 3 curve with 7 control points should then result in 3 + 7 - 1 = 9 knot values. You'll also find that for non-periodic curves (open curves and closed curves that have a kink at the seam) that the first few and last few knot values are identical. In fact, you need a few identical values at the start and end to ensure the curve reaches all the way to the first and last control-point. This is called "clamping". The number of repeated (or "multiplicity" or "valence") knots must be equal to the degree. So with the above example of a degree 3 curve with 7 control-points, a valid knot vector would be:

0;0;0;1;2;3;4;4;4

9 values, with the first three and last three being identical. You are never allowed to repeat the same knot value more often than the degree, and you are never allowed to decrease the values either.

Weights are single numbers attached to individual control-points. Weights are most often used to create Conic Section curves (arcs, ellipses, parabolas, hyperbolas) and these are always degree=2. You can also assign weights to freeform curves if you want of course. When you choose to do so, you must supply an equal number of weights and control-points. Furthermore, weights are relative only to each other. If you assign a weight value of 1.0 to all control-points, you get the exact same curve as you assign a weight of 4.9 to all control-points.

The difference between "Curve control points" in NurbCrv and "Control point count" in the Knot component is that one actually represents a list of points, whereas the other is merely an integer representing how long that list is.

In order to create a valid knot vector, all I need to know is the degree, the number of control-points involved and whether or not the curve is supposed to be periodic or not (periodic curves are a special case of nurbs curve, best to stay away until you're comfortable with the regular nurbs curve type). It doesn't matter where the control-points are, I only care about how many there are.

--

David Rutten

david@mcneel.com

Poprad, Slovakia

Permalink Reply by M NICK on February 6, 2012 at 6:52pm

Thanks David for the great explanation....

I developed the attached GH definition to find the curves on the NURBS Surface, but at the end of the definition, the brep comp. gives me 12 edges instead of 10! I know it is because that it counts all 4 edges of sub surfaces. 

Is there anyone who can help me to find the 10 curves?

Thanks,,

Nick

Attachments:

• unnamed 3.3dm, 34 KB
• unnamed 3.gh, 6 KB

Permalink Reply by M NICK on February 6, 2012 at 7:37pm

I came up with the new definition, Is there any better way to find the curve on the surface? Also, this definition gives me the continuous curve in which I need to find the divided curves (Curves between the vertices separately)!!!

I would be appreciated if anyone could give me some info.!!!

N

Attachments:

• 4.gh, 5 KB

Permalink Reply by Isabella Butts on February 25, 2020 at 11:51am

Oh thanks, I also needed this information