Quick troubleshoot -- can't get this surface to offset

For some reason, I can’t get Offset Surface to work on this nurbs surface (attached). It’s one side of a ship’s hull, created in MaxSurf. Earlier versions of the same surface certainly worked.

So if I want to offset by say 10 inches, a surface will be generated, but not offset – it will just lie identical with the original surface. This happens if the offset is specified to the inner or outer side of the original surface. I’ve set to ‘Loose’ to make it more forgiving, but that doesn’t solve the problem.

Any ideas?

AD hull surface.3dm (896.5 KB)

Hi Ian - there will be a problem at this control point:

At that location, the surface U and V are exactly parallel- this is pretty common in boat hulls but causes problems when offsetting (including filleting, which uses offsets) is needed. The surface normal is calculated from the cross product of the U and V directions and at that point, these being colinear, the normal is undefined and offsetting fails or goes haywire. As a rule, it’s best to avoid having adjacent untrimmed edges of a surface be tangent.


Ok, thanks Pascal. I’ll see right now what I can do about that.

You were right of course, Pascal.

I had to nudge two control points out from being co-linear or co-planar in that bezier patch at the foot (the control point directly above the arrowed point in your image, and the one forward of your arrowed point), then it worked. Thanks.

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