Edit Ctrlpoint weight with a single slider (Voronoi-ish System)

Im new to Grasshopper. Im trying to setup a system, that can control the shape of a closed nurbs curve , by changing the weight of half the control points - or rather: every second control point - with a single slider.

Step 1: The system should be based on a number of “corner” points, that I draw manually in Rhino.
NOTE: These points are NOT in a plane, they will be very arbitrary from each other.

next steps should be defined in Grasshopper, and the aim is a system, where I can a) move the cornerpoints manually in Rhino and b) change the “corner” ctrl-pt weight with a single slider in Grasshopper.
I hope the attached image can explain what I am trying to do…

Step 2: These points represent the corners in a polygon.

step 3: How to set the midpoint of each side in the polygon ?

step 4: How to define a Closed Nurbs curve from the corner points AND the midpoints?

step 5: How to select ONLY the “corner” control-points and change their weight with a slider ( range 0.4 - 10) ?

You can see in the attached image, and the attached Rhino file, that “corner” control points value of 10 creates a “near-polygon” with very sharp corners

Step 6: - and a value of 0.4 creates a very smooth transition in the “corner” curvature

If you are curious what I am up to, please see the attached test3-pullsrfGH, where I am pulling a polygon onto a surface in Grasshopper. My problem is, that I actually want only the adjustable Nurbs curve on the surface.

The ultimate aim, is to be able to take any surface, and by use of adjustable points - and the control point weight slider, I can project a cellular grid onto the surface, and adjust the “sharpness” of the corners in the grid.

Its a bit like Voronoi, only that I control the design manually by nudging the corner points, not the centerpoints.

If you know of other Voronoi techniques like this, please share with me !

Thankyou very much ! This has very high priority for me !

Hans Henrik


Test3-pullsrfGH.3dm (3.1 MB)

Test3-pullsrf.gh (12.6 KB)

Do you know if the future curves will aways be pentagon-ish?

Do you know at least if it will always have odd number of corners?

These the first thing to consider if you are creating a re-usable definition.

Polygons in a cellular grid can be a mix of 3, 4, 5, 6 perhaps even 7 sided.

see examples from these sculptures, from my website:




  • at this point I am learning Grasshopper, about different ways to do each step.

I am an artist / sculptor, so It is ok for me if some steps I have to do manually in Rhino.

So maybe I am more trying to develop a certain workflow between Rhino design, and Grasshopper definitions, rather than a full parametric Grasshopper definition.

I have experimented with different setups in Grasshopper.
Please see attached set of Rhino / GH files, with comments and questions included in Rhino


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Howdy Hans,
I am trying to do something similar. have you tried taking the lines shortening them and then bridging them with nurbs?

I think this accomplishes the goal of the original post → controlling the shape of a polygon by adjusting the weights of its corner-points and mid-points.

CTRL POINT WEIGHT_re-02.gh (17.6 KB)


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