Meshtrim, MeshSplit, MeshBooleanUnion etc. not working correctly; bug

[Charles]

@piac
The model is created only with Rhino 8 tools.
2026-07-01_MeshSplit.3dm (8.5 MB)

The mesh comes from _ExtractRenderMesh.

I want to trim/split or whatever the small borders, in the end I want a closed, solid mesh.

Nothing works correctly, Meshtrim, MeshSplit, MeshBooleanUnion etc..
For me it looks like a simple task…
And things like that should really work.

I already tried to rebuild the mesh, made a mesh from the polysrf etc.
And also tried PermitSplittingWithEachOther.

MeshTrim incomplete result

Hi all the easiest thing to do here is to use an option --which is admittedly hard to find-- and will work in most of these each-with-each cases. Run _MeshSplit Select BOTH meshes and press _Enter Tap the newly appeared option “PermitSplittingWithEachOther”. All this option does, is to clear selection. This way you can perform the next step Select both meshes again This works immediately on the first file. Thanks, Giulio giulio@mcneel.com

Can you point out a reliable method to get the task done?

[theoutside (Kyle Houchens)]

so a bit of work to do here as for some reason this mesh was misbehaving.

1. copy and move the mesh in a bit
2. patch single face at the corners to make bridges from the original mesh to the copy.
3. fillmeshhole for each hole left by the above process
4. shrinkwrap the part at .01 units
5. mesh boolean split with the polysurface

result-

2026-07-01_MeshSplit_kfix.3dm (13.5 MB)

[Charles]

Oh well this is complicated and tedious.
For such a simple geometry.

I have found another method to get it done.
And as yours, it’s not error free, and requires repairs.
Repairing a fresh geometry, created in Rhino - embarrasing.
Enough ranting now.

I think the weak point here is the MeshSplit operations.
That’s why I use the term “bug”.

My proposal is to investigate and fix the issues.

[theoutside (Kyle Houchens)]

Agreed this is not ideal- @piac perhaps can take a peek and expand on the issues here?

[Charles]

@theoutside

@piac perhaps can take a peek

Tried again with the meshes in (9.0.26188.13293, 2026-07-07).
I could

It feels like nothing works as it should.
The same findings as @osuire.
That was over two years ago, and I haven’t seen anything improve since then.
I’d remove all the faulty functions to avoid disappointment.

Intersect with meshes and general mesh rant

I still don’t understand what’s so special about meshes that it requires its own intersect (and trim and split) tool(s). I think it’s more a case of programmers making it easy on themselves. Moreover, I think it really sucks that the concept of “IntersectTwoSets” isn’t available for meshes since intersecting a bunch of meshes is typically time consuming and produces LOTS of geometry. Without the ability to choose what should intersect what, intersections with meshes are a real chore. [24042…

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-Extremely poor shelling (completely useless ; I need to use MeshMixer every time for this)

I also never got a usable result, except in very simple, artificial test cases.

-Confusing “MeshRepair” dialog box

If it were a dialog…
MeshRepair lives in a panel for whatever reason, to me it makes no sense.
And it requires to re-select the mesh to repair for different ‘repair’ options.
Most of them don’t work as expected.

-Shrinkwrap produces meshes that are considered “Bad meshes” ; in particular, with self-intersections.

Garbage to gold…?