# non-manifold error when clipping polyhedra+hexahedron domain

**URL:** https://discourse.vtk.org/t/non-manifold-error-when-clipping-polyhedra-hexahedron-domain/15683
**Category:** Support
**Created:** [May 29, 2025, 11:55am UTC](https://discourse.vtk.org/t/non-manifold-error-when-clipping-polyhedra-hexahedron-domain/15683 "2025-05-29T11:55:56Z")
**Posts on this page:** 4
**Page:** 1

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### Author: ![pcristini](https://discourse.vtk.org/letter_avatar_proxy/v4/letter/p/c68b51/32.png) [@pcristini](https://discourse.vtk.org/u/pcristini)
#### Post date: [May 29, 2025, 11:55am UTC](https://discourse.vtk.org/t/non-manifold-error-when-clipping-polyhedra-hexahedron-domain/15683/1 "2025-05-29T11:55:57Z")

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I have a domain that I’m exporting from CFD results (exporter written by me). This domain has mixed cell types, mostly polyhedra & hexahedra.

When I clip the domain, without crinkle cut, I receive the following warning

```auto
Generic Warning: In vtkPolyhedron.cxx, line 1801
A cell with a non-manifold triangulation has been encountered. This cell cannot be contoured.

```

When I crinkle-clip the clip, the error is gone.

Can someone:  
a) what is causing the different behaviour when clipping with crinkle vs without  
b) explain how I can find the troublesome edges to check if it’s my export process causing this.

I’m not sure if the following is related, but when opening my files in paraview 5.11 the hexahedra cells look “broken” without crinkle clip.  
Without crinkle:

 ![image](https://discourse.vtk.org/uploads/default/original/2X/d/d2fbefea6a5cb1d861be4892b0929eef8f7cc008.png)

With crinkle:

 ![image](https://discourse.vtk.org/uploads/default/original/2X/5/5da970716861f0e53dcfab328d8816cf31d2d5b4.jpeg)

5.13 does not exhibit this issue, but I’m wondering if this is related to the non-manifold issue.

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### Author: ![Paulo\_Carvalho](https://discourse.vtk.org/user_avatar/discourse.vtk.org/paulo_carvalho/32/370_2.png) [@Paulo\_Carvalho](https://discourse.vtk.org/u/Paulo_Carvalho)
#### Post date: [May 29, 2025, 4:46pm UTC](https://discourse.vtk.org/t/non-manifold-error-when-clipping-polyhedra-hexahedron-domain/15683/2 "2025-05-29T16:46:16Z")

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Hello,

In a nutshell, a non-manifold mesh is one that cannot be unfolded into a 2D figure with all its normals pointing to the same direcition. A non-manifold geometry generally means some algorithms won’t work well. If it worked before maybe because of a more generalizable (slow) algorithm was used. Perhaps the new implementation is faster but it assumes a manifold well-behaved geometry. So, maybe you need to check for “non-manifoldness” and pre-process your mesh in order to make it manifold (check [VTK: vtkFeatureEdges Class Reference](https://vtk.org/doc/nightly/html/classvtkFeatureEdges.html)).

best,

PC

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### Author: ![dgobbi](https://discourse.vtk.org/user_avatar/discourse.vtk.org/dgobbi/32/18_2.png) [@dgobbi](https://discourse.vtk.org/u/dgobbi)
#### Post date: [May 29, 2025, 5:49pm UTC](https://discourse.vtk.org/t/non-manifold-error-when-clipping-polyhedra-hexahedron-domain/15683/3 "2025-05-29T17:49:11Z")

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Related topic: [Clipping breaks the hexahedron adjacent to the polyhedron](https://discourse.vtk.org/t/clipping-breaks-the-hexahedron-adjacent-to-the-polyhedron)

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### Author: ![pcristini](https://discourse.vtk.org/letter_avatar_proxy/v4/letter/p/c68b51/32.png) [@pcristini](https://discourse.vtk.org/u/pcristini)
#### Post date: [June 2, 2025, 7:47am UTC](https://discourse.vtk.org/t/non-manifold-error-when-clipping-polyhedra-hexahedron-domain/15683/4 "2025-06-02T07:47:25Z")

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Ah! I didn’t come across this while searching, so it was actually a bug and not something in my mesh.

Thank you!