Unable to join these surfaces


Can anyone shed some light on why these surfaces do not join as expected?
It must have something to do with them having singularities, yet I never ran into this type of problem and although that might be because I never had this situation in a model, yet it feels buggy they are not joining.

Select all and run _Join. It does join but not into a single polysurface.

UnableToJoin.3dm (97.7 KB)


If you use the JoinEdge command you will see to what extent there is a redolence issue.

Thanks, I’ll check on it.

Just checked, but the intolerance is way below the file tolerance.
Thus something is still wrong.


How were your triangular surfaces created? I did both 3pt and 4pt at your locations and they both joined without issue. I then did a curvature analysis on your original surfaces and they are not planar at the Apex

So only the shared base curve will join properly

Hi Danny,

They are straight lofts between the 2 sides. In theory they are flat, it’s just the mesh that messes up I guess.

I’m planning to use these surfaces for mapping to and from surface pairs in 3D and flat (on a layout)
Think of mapping a logo onto the 3D surfaces and then using a scripted flowalongsurface to map them onto the 2D layouts.

I do wonder now if I will get unwanted distortions at those singularities…l better test that now first.

EDIT: just tested and no noticeable distortions at the singularities.


There is something wrong with the boundary definitions.
When I select all the surfaces and run untrimall they join.

Hi Jim,

Thanks for looking into that. Seems like a bug in lofting straight…


if you select all the surfaces then _ChangeDegree, they will then join… even if you don’t actually change the degree (keep the U & V at 1… just run the command on the surfaces though)

i don’t have an explanation for this… just adding some clues to the mix :wink:

Hi Jeff,

Thanks, I guess at this point @Pascal should get this to the “bigger brains” at McNeel.


Yeah, something does not smell too good here. I’ll add this to the bug heap. PlanarSrfs from the borders of these do join correctly.

@Willem , Thanks for the example.