# Offset Surface Problem

Speaker.3dm (1.6 MB)

I want to offset this surface. It doesn’t work properly. What can I do to fix this problem?

Hi ruesti - I see that it does not create solids- you’ll need to fix this up by hand - I’ll see if I can make you an example file.
If you use Corners=Sharp, the individual rings do create solids- I don’t know if that is what you need.

For Corners=Round, it looks like the easiest way, if you want to offset the individual ‘rings’ to be solids is probably as in the attached file - it’s a bit of work but not hard…

SpeakersCorner.3dm (134.8 KB)

If you want to do the whole thing at once, there there is more to do.

-Pascal

I want to create an inner shell of all the rings

Because the outer shell is made of triangular flat surfaces, the shell can be represented exactly by a mesh.

(Edit) Join the rings into a single polysurface.

Create a mesh from the surface using the Mesh command. Use “Simple Controls” with the slider all the way to the left - Fewer Polygons.

Offset the Mesh with OffsetMesh. Use the Solid option to create a solid if desired.

If a NURBS surface is needed use MeshToNURB.

(Edit) The distance between the initial and the “offset” surface may not be uniform when OffsetMesh is used. The distance between corresponding nodes should be the uniform and the offset amount specified in OffsetMesh.

Example attached: Speaker (1) DC1.3dm (2.9 MB)

Two causes for the problems:

1. Each facet of the shell is a triangle which is represented by a quad with a zero length side. The zero length sides cause problems in OffsetSrf. Triangular surfaces can also be represented by trimmed planar quads.

One way to replace the with zero length side quads with trimmed quads is to convert the surface to a mesh with Mesh, and then convert the mesh back to a NURBS polysurface with MeshToNurb. This polysurface will have trimmed quads.

The polysurface with trimmed quads can offset. However there will still be some problems.

1. The fundamental geometry of the shell appears to be such that when the individual surfaces are offset and extended, the resulting offset surfaces at some of the nodes of more than three surfaces do not meet at a single point. This complicates the offset surface and causes some problems.