Problem with a closed brep from three curves


I’m trying to generate a closed BREP from 3 curves but to no avail. Any idea ? Is this related to the “periodic” nature of the curves ?

generating a volume from (13.9 KB)

thanks in advance

If I was a curve and looking like this, I would also refuse to become a Brep!
Try to model it manually and see what steps are required. Usually a shape, like this
need to be composed out of multiple sections. Usually modelling the less curved areas first, then
blending in between. Grasshopper on top of this is a terrible tool for surface modelling,
because it lacks many fundamental tools of Rhino or any other CAD.
And surfacing is not always about modelling the curves first. Especially, modelling a blend or periodic shape as curves first, is a terrible idea. Because blending/matching curves is not equal
to blending/matching surfaces. There is a lot more to it. On top of this, making any sort of surface representation is not what the aim should be. A simple, clean and concise representation should be the aim, always.

Hi Tom,

thanks for the answer. I have been trying to model the shape manually but to no avail. At least the “patch” component works in GH. I cannot have it work in Rhino. In my opinion, both curves are not that complicated. Both are just drawn on the surface of the cylinder (inner and outer surface of the cylinder).

My need is to represent this volume in 3D. What other aim could it be ?

Foremost, “patching” is a surface technique intended to quickly close small holes. It shouldn’t be used to create (clean) surface models. This is, simply because, it hammers a surface into the shape without considering the flow of the rest of your shape. An outline doesn’t give you all the required information about how curvature should flow on a shape.

It doesn’t matter how simple/complicated the curves are. A curve can be simple, but still be twisted or “unclean”. Of course, the simpler the curve, the less likely. A single-span curve, of degree 2, cannot twist at all. But what’s more important is, that an outline only give you hint on how the curvature flows at the edges of the surfaces. You can create a clean untwisted outline of a surface, but still once you create a surface out of the curves, you get a twisted surface.

Another fundamental concept of surface modelling is matching and blending surfaces. Until this day, Grasshopper doesn’t give you these fundamental tools.

So there is simply no way to model shapes like this the right way using Grasshopper.

In order to avoid the “patch” component, I ended breaking both input curves into 2 curves and lofting them. That works but I am still puzzled about the failure of the initial tactic.

generating a volume from (18.0 KB)

Because it’s pure garbage you produce here. Patching is flexible, but not as much if you model like this. Sure you get a closed Brep by lofting this, but unless this is all you need to do, then there is no point in creating a mess like this.