How to convert the Open Brep to a Surface?

Hi all,

When I loft 6 curves to create a surface, it creates an 'Open Brep' rather than a Surface.

How to convert the Open Brep to a Surface?

Replies to This Discussion

Permalink Reply by David Rutten on November 22, 2010 at 1:55pm

Hi Chun,

this might not be possible. What does the open brep look like?

--
David Rutten
david@mcneel.com
Seattle, WA

Permalink Reply by Chun Qing Li on November 23, 2010 at 5:43am

Hi david,

Please find the attached image for your information.

Permalink Reply by David Rutten on November 23, 2010 at 10:52am

Sorry, I meant the actual Brep geometry. If you bake the arcs and Loft in Rhino, does it also create a polysurface?

--
David Rutten
david@mcneel.com
Seattle, WA

Permalink Reply by Chun Qing Li on November 23, 2010 at 2:59pm

Yes it does.

Or bake the loft surface to Rhino, and reference it back to Grasshopper (but this is not convenient).

Any other solutions?

Many thanks...

Permalink Reply by David Rutten on November 23, 2010 at 5:19pm

Well, if the Rhino Loft command creates a polysurface then there's very little I can do. I know this is not a helpful reply. I can ask around for further information, but it will have to wait until after Thanksgiving.

--
David Rutten
david@mcneel.com
Seattle, WA

Permalink Reply by Chun Qing Li on November 24, 2010 at 10:15am

Ok David, thank you very much.

Permalink Reply by Brian Harms on October 13, 2011 at 6:04pm

Hey guys, this thread is probably pretty dead by now but I've always had good luck with rebuilding the curves/arcs before lofting to get untrimmed surfaces instead of open breps.

Works in both grasshopper and rhino. You loose out a little in terms of maintaining the exact original geometry, but with a couple hundred control points you get pretty darn close results.

hope this helps,

Brian

Permalink Reply by marina cisneros on February 10, 2012 at 9:26am

By no i´m sure is totally dead, but anway, the way i do it is by exploding the Brep. That gives me surfaces, and then i operate on the surfaces.

Permalink Reply by Wong Wai Hang on April 26, 2016 at 9:01pm

Hi,

I have the same issue happened when I loft the curves.

Sometime I got untrimmed surfaces, other will be open breps.

I think something to do with the control points in the drawn curves.

Less control points will result in untrimmed surface, otherwise they will be open breps.

When I got an open brep, I will rebuilt the surface under the grasshopper loft command.

Of course, how close to the original surface will depend on rebuilt setting.

That the way I do.

Permalink Reply by David Rutten on April 27, 2016 at 12:43am

2. A kink (discontinuity within the curve) in at least one of its curves.

And sometimes kinks are hard to spot, they need not be visually 'kinky'. I think any multiple knot in a nurbs curve with valence=degree causes the lofter to think it needs to make a polysurface.

Permalink Reply by Wong Wai Hang on April 27, 2016 at 1:11am

Hi David,

What do you mean "Valence = degree"?

Thanks

Permalink Reply by David Rutten on April 27, 2016 at 5:53am

To elaborate, the higher the degree the smoother the curve. Degree=1 curves are (poly)lines, degree=2 curves can be used to mimic conic sections such as circles, parabola, and hyperbola, degree=3 is the default for Rhino, degree=5 gives you smoother curves, but each control point matters less, degree=11 is the highest degree Rhino allows. Apart from 2, degrees tend to be odd rather than even, though Rhino supports both.

In addition to the degree and the control points, Nurbs curves have knots. A single knot is just a number, and the list of all knots is called the knot-vector of a curve. The number of knots depends on both the degree and the control-point count, and the spacing of the knots affects the shape of the curve a little bit. If there are <degree> knots with the same value, then the curve is somewhat discontinuous at that location which could manifest itself as a kink or as a clamped end-point. However it is not possible by just looking at the shape of a curve to say where stacked knots might be, but you can use the Rhino _List command to inspect all details of a Nurbs curve.