Merge faces : bug, mistake or mistery?

Hi guys
i have a simple question, why if i use merge faces node with a brep in input my output is always a trimmed surface? (the node input and output are called brep)
i’ve tryed some way to have a brep in input and output but is impossible… also with very simple brep
it’s a bug?
same problem for rhino 5 and 6

Hello @francescoperego,

A trimmed surface IS a brep, albeit a special one. This means all trimmed surfaces are breps and pluging your output into a brep component should give no error.

Is there an error with your geometry, too?

Compare these two cases. If you have a surface with complex boundary, that will lead your merge faces into a Trimmed Surface.

Merge (13.6 KB)

no error in output and my geometry is good
but i can’t join a trimmed surfaces in order to unroll it without separationunroll

How are you trying to join that? It should be possible. Have you tried running your trimmed surfaces through a brep component before proceeding?

yes i’ve tryed several ways but in every case is not possible to join the trimmed surface. the only case that work is without merge faces.

Can you post a small example file with a part of your geometry that doesn’t join?

It could be a document tolerances issue (if the tolerance is set higher than default).

A couple tests to try for debugging:

Bake the surfaces and then join manually - does that work?

Extract the border curves of each surface and generate new planar surfaces in GH. Do the new surfaces join?

Here are two workarounds while i play around a bit more…
This is indeed kind of wierd and should just work out of the box, especially since in the file i posted, the second approach uses trimmed surfaces and it works :thinking: (20.1 KB)

Ok after some more testing here is the updated file with many workarounds: (27.8 KB)

Still i feel this is a bug, even when you bake the PolySurfaces, rhino fails to join them.

@pascal could you take a look at the gh file i uploaded? There is definitely something off with “MergeAllFaces”

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Hello - It looks to me like GH and RhinoCommon etc are not part of this at all, right? The two input breps, when merged in Rhino with MergeAllFaces, make edges that Rhino does not want to join.If you SplitEdge the longer edge at the end point of the other surface’s edge, Join works as expected - am I looking at the right thing?


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yes and thats what i find a bit problematic. The edges coming out of mergeallfaces could be better i guess, because this is a mergeable case.

use the original polysurfaces:

  • If you join them first and then run mergeallfaces, it works
  • If you dupborder -> planarsrf on both curves, join them, it works
  • you can also mergeallfaces and then boolean union, it works

Its at least inconsistent in a wierd way…

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yes i’ve tryed to bake and join and extract border and generate new planar surface.

thanks guys for your precious help