Recreate a smooth surface from SubD Outlines

I got a set of outlines from a subd model, I would like to recreate the geometry by using these outlines, I try loft and network surface and sweep but it appear that there is this hard edge from the model, the surface is not as smooth as the brep model that I convert from my subd model.

Any suggestion that I can use this set of outlines to recreate a surface that is as smooth as the brep convert from my subd model ?

SubD Outlines.3dm (217.6 KB)

I did not download your model, but from your description, I think there are some basic concepts of surface continuity.
In the picture above you are trying to create adjacent surfaces that are smoothly connected, at least G1 continuity in the red line.
These 2 surfaces are all untrimmed surfaces as a1 and b1 create (can be loft or sweep) surface 1, a1 and b1 are surface 1’s untrimmed edge, or we can call it NURBS surface edge.
The same goes with surface 2 and a2/b2.

In your case: a1 and a2 have a sharp corner, obviously, it has no G1 continuity, while b1 and b2 are continuous at least with G1 continuity.
So there is no way rhino is creating surface 1 and surface 2 and they have one side of edge curve with G1, other side of edge curve with G0, but they merge at G1. At least not by using a defaut way like loft or sweep.
The only way for rhino to achieve this with loft/sweep is to create a smooth change on the red line:
From A where continuity is G0, gently translate to B where continuity is G1.
This is why you can see the red line as some kind of “smooth becomes corner” or the other way “a corner fade into a smooth surface”

Thank you so much for your response.

Any suggestion on “From A where continuity is G0, gently translate to B where continuity is G1.”

Hi -


Could you clarify what problem you are trying to solve?
Do you only have the curves (as in the model that you uploaded) or do you also have the SubD object? If you have the SubD object, what is wrong with the ToNURBS version of that object?

Thanks for your response.

What I try to do is, behind this elements is the floor slab. I want to attach this elements to the slab. It is difficult for me to adjust the subd outlines with control points on to drag the points to match the slab outlines.

What I try before is extract the outlines of the subd model from grasshopper and then using ‘pull curve’ to pull the outlines to the surface extruded form the slab outlines. Then I will loft all the curve back to recreate the element, then I will have this element matching the slab outlines.

using this method for other part of the elements is good. It is just this corner got this sharp point and edge. When I loft it back there will be this hard edge.

Or if there is a way that can adjust this subd model, make it attach to the slab perfectly, not little gap in between.

From your rhino model, I don’t think there is any need to use SubD.
The shape is good to go with normal loft/sweep here.
I am not sure why you did this with SubD in the first place.

If you are interested in that particular area where you want G1 through all that red line.
It is not possible.
Just like 1+1 never equals 3. The reason behind it, some basic NURBS principles are explained in my previous reply.

Some suggestions based on construct level:
If some surface is hard to make in NURBS, then it is most likely hard to manufacture and construct.
Please gain some basic knowledge on how “undevelopable alum plates” are made in the factory.

Zhiwei is definitely a Chinese name, so let’s cut the crap here:




He wants to fit the SubD edge to some existed curve.

There is an old T-spline function likely named “fit TS to NURBS curve”.
But Rhino7 SubD does not have it.

It is still a problem, will have to wait for that function.

Thank you so much for your explanation and suggestion, it is really helpful, thanks.

Hello - it seems like a surface point arrangement like this

ought to do it.

SubD Outlines_Maybe.3dm (325.7 KB)