Surface will not translate as BREP to Revit

[scotttd]

This surface will not import through RIR as a Directshape BREP surface. When used it translates to a mesh. What is interesting, if I use the Rhino3DM importer into Revit then it does import as a BREP.

Steps to reproduce the behavior:

1. Used a BREP param and a GeometryasDirectshape component to input the BREP.
2. The result is a Mesh.

If I restart Revit, then use Insert CAD in Revit and import the 3DM file, it is a BREP

Why is it different?

Related To

• This surface is part of a much larger model, part of this thread Sample File: SubD-PatchV6.zip
• https://discourse.mcneel.com/t/wall-by-face-from-mass-generated-in-rhino/91227/10

[eirannejad]

Investigated this in Revit 2019 Confirmed the example patch is converted to directshape mesh

Native Revit imported, imports the surface (haven't verified if it makes any changes to the surface)

Native Revit import, builds a DB.HermiteFace from this geometry:

[scotttd]

Interesting. This is from part of a Rhino SUBD object.

The 9 point degree 3 patches what to be hermites. I wonder if all Subd Patches are Hermite?

Looks like the 7 point degree 3 patches along the edges will work with the standard BREP translation.

[eirannejad]

Investigated this a bit more. Revit API does not have Nurbs implementation so any surface other than the known topologies (Conical, Cylindrical, ...) is going to be created as a Hermite Surface

A simple rebuild on the Nurbs patch given in the example file above (to the exact matching # of control points) will cleanup the surface definition and Revit can actually understand the surface data and correctly create a HermiteSurface

We need to investigate and see what are the differences between these two surfaces:

SubD-PatchV6_edited.3dm.zip

Comparison Results

Confimed comparison results from McNeel mathematicians:

The surfaces have IDENTICAL NURBS structure (degree, counts, ... ). The ONLY differences are ones that should not matter.

• Locations of the control points
• and scaling of the knots

Use the list command to see the knots for one surface are 0,1,2,3,4,5,6 and for the other surface are 0, 1/8, 2/8, 3/8, 4/8, 5/8, 6/8, 7/8, 1

Seems like we need to focus on fixing the know scaling before passing the geometry to Revit.

[kike-garbo]

I tried normalizing the knots and doesn't work either.

Testing this I noticed that surfaces here SubD-PatchV6_edited.3dm.zip are like:

Rhino surface:

• Knots in U: 0,1,2,3,4,5,6
• Knots in V: 0,1,2,3,4,5,6

Revit Surface:

• Knots in U: 0, 1/8, 2/8, 3/8, 4/8, 6/8, 1 but not 5/8
• Knots in V: 0, 2/8, 4/8, 5/8, 6/8, 7/8, 1 but not 1/8 nor 3/8

[kike-garbo]

Those surfaces are a bit special, The technical issue is the two first partial derivatives are nearly parallel, and this produces numerical instability at the rounded corner of those surfaces. This is very rare in typical NURBS, but may happen in NURBS that come from a SubD.

[eirannejad]

Tested this with the most recent Rhino.Inside.Revit and issue still stands