Help creating curve from multiple edges on open polysurface

curvecomplexpoly2.3dm (1000.0 KB)

Hello everyone,

I am having difficulty defining a curve from the multiple (naked) edges of a trimmed polysurface. I would like to unify (reduce?) all the naked edges at the ‘opening’ into one continuous curve.

At the moment, the huge number of edges makes operations like Booleanunion, pipe and blend (as per the tail on in the penguin tutorial of the “getting started” pdf) very difficult… at least for me, but I’m a novice!

I’ve attached the .3dm file. Can anyone offer some advice?

Many thanks in advance,



Does DupBorder do what you need here?

Your polysurface has 464 triangular NURBS surfaces, each of which appears to be a planar rectangle trimmed to a triangle. Did you import a triangular mesh and use the convenient but dangerous MeshToNURB command? MeshToNURB is dangerous because anything other than a very simple mesh will result in a very complex polysurface.

Polysurfaces with such complexity are generally undesirable in Rhino and can lead to numerous difficulties. Also the faceted surface has an edge which is a polyline with kinks. My guess is the source of problems with BooleanUnion and blending originate with the complexity of the polysurface. Problems with Pipe may originate with the kinks in the polyline.

So what’s needed is to build a much simpler NURBS surface or polysurface from the mesh. Try the Patch command and experiment with it’s settings.

Many thanks to you both. I will work towards simplifying the mesh prior to attempting Rhino commands.

Hi John- if the goal is to make something smooth from the faceted object then simplifying the mesh is not going to be helpful - It’s hard to say what to do without knowing more about what you’re up to, but trying to treat a faceted thing like this, derived from a mesh the same as a smooth surface of the same general shape is likely to be a frustrating experience.


Hi Pascal. All I’m looking for is a way to blend a mesh, or a nurbs dervied from it, into an intersecting smooth solid, such as a sphere. If I create a boolean union between this mesh-like shape and a sphere the intersecting join is abrupt, not smooth like the penguin’s tail. I just wondered if there was a way to create a smoother finish?

If you want the intersection of two surfaces to be smooth then both intersecting surfaces must be smooth. That’s fundamental geometry.

So you need to replace the faceted shape with a smooth shape. Simply converting the mesh into a NURBS surface does not make it smooth. Do you need assistance in creating the smooth shape? If so how close does it need to be to the faceted surface?

Hi David,

Thanks for your reply.

Yes, creating a smooth surface derived from the faceted surface would be great, and I suppose I’d like it to be as close as possible. I did experiment with the Rhinoresurf plugin, but with no success (but, again, I’m a novice).

Is there a way to turn this faceted shape into a smooth surface?

Many thanks for your help.