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IDProjectCategoryView StatusLast Update
0002219OpenFOAMPatchpublic2017-09-03 19:41
ReporterwyldckatAssigned Towyldckat 
Status closedResolutionsuspended 
Product Versiondev 
Fixed in Version 
Summary0002219: Proposition for deprecating the representation of tetWedge with a collapsed VTK_WEDGE when exporting to VTK
DescriptionWhile diagnosing what has been discussed in issue #2213, namely the consolidation of VTK exporting capabilities, I finally noticed and tested what I didn't find in the fix for issue #2099: how to decompose "tetWedge" cells.

The detail I had missed for #2099 was that there is a preparation section of the code that calculates how many cells are needed. Once tetWedge was scheduled for decomposition, it worked just fine, similarly to the decomposition done by the internal reader in ParaView/VTK.

The reason why I'm proposing this is because it feels like this hack'ish representation is no longer needed, given that VTK_POLYHEDRON rendering is now commonplace in most ParaView/VTK versions for the past few years. I'm not certain of this, but given this exists since at least OpenFOAM 1.1, I deduced that the reason why there was a need to have "tetWedge" represented in ParaView was for allowing proper (or at least a fair) visualization of these kinds of cells, which can easily show up in meshes for OpenFOAM to use.

Therefore, attached is the package "deprecated_dedicated_tetWedge_VTK_export_v1.tar.gz" which provides the following updated files for OpenFOAM-dev:


The change made in these files is the same: completely removed "tetWedge" from the list of cells that were rendered with dedicated VTK cell-shapes (VTK_WEDGE in this case), given that the renderer would show a triangle inside the mesh that shouldn't be shown, as documented in issue #2099.

The benefit of using this is that the decomposed representation of a "tetWedge" is a bit more consistent, given it does not have a collapsed represented face. I'll attach an image in a few minutes to show what I mean.

The downside is that the cell count for the decomposed mesh increases a bit. Using the last big mesh from issue #2099 as a rough sample, it gave the following cell counts:

  - 380840 cells : Polyhedral (original) mesh
  - 1590393 cells : Decomposed mesh 4.x
  - 1593669 cells : Decomposed mesh (also decomposing tetWedge)

Nearly 3300 additional cells were needed for rendering the original "tetWedge" cells, which isn't much of an increase, when compared to the overall cell count.
TagsNo tags attached.


related to 0002099 resolvedhenry snap stage generates internal triangles 
related to 0002213 resolvedhenry "vtkTopo.C" in foamToVTK library is missing the same fix as provided in #1633 



2016-08-26 00:20


deprecated_dedicated_tetWedge_VTK_export_v1.tar.gz (4,849 bytes)


2016-08-26 00:44



2016-08-26 00:44



2016-08-26 00:58

updater   ~0006769

This is odd... I know I saw yesterday the decomposed tetWedge had a better render than the polyhedron render... but it looks like whatever I saw was either a fluke or I messed up the decomposed cell selection...

Anyway, attached at the following the images "decomposed_tetWedge_render_*.png" (using ParaView 5.0.1), which show two slightly different perspectives for two representations of the decomposed tetWedge:

  - On the left is a representation for when the 4 cells (2 pyramids and 2 tets) as a single mesh entity; which is pretty much the same exact render as given with the single polyhedral cell or the VTK_WEDGE with a collapsed edge.

  - On the right is the representation of two groups of cells: 3 cells are separated from the 4th cell, because the 4th cell is the one that is used for rendering the collapsed face of the tetWedge.

     - This may give a little bit better representation of the original cell, but this 4th cell is actually a warped prism, which is why the blue edge goes into the grey surface on this representation.

Therefore, the only real benefit of decomposing a tetWedge, instead of using VTK_WEDGE with a collapsed edge, is that it will also fix the issue of the triangles showing up inside the mesh when the mesh is decomposed.


2016-08-26 08:17

manager   ~0006771

Should this approach be applied to all cell which have collapsed edges? Or should the mesher guarantee that collapsed edges are removed from the mesh? During mesh-motion collapsed edges could be generated and removal would require topology change which otherwise might not be needed.

How small an edge should be collapsed? What is the minimum edge-length that VTK can handle and render the corresponding face correctly?


2016-08-26 09:07

manager   ~0006772

On the point about maintaining support for cell-decomposition rather than replacing it with VTK_POLYHEDRON is that the current situation is partly historical and partly that VTK_POLYHEDRON is (was?) rather restrictive requiring the cells to be convex which cannot be guaranteed, even with foamyHexMesh with edge collapse. Do you know if VTK_POLYHEDRON has been generalized yet or is it still restricted to convex shapes?


2016-08-26 19:13

updater   ~0006774

> Should this approach be applied to all cell which have collapsed edges?

The only reason (that I've seen so far) as why to not use "VTK_WEDGE" to represent a "tetWedge", is mostly just to avoid future reports of "there's some weird triangles inside my mesh" and also to reduce maintenance for this particular cell render, as done so far.
The only specific problem with edge collapsing is related to how the 5 vertex "tetWedge" is fitted into the 6 vertex "VTK_WEDGE". The limitation is on the ParaView/VTK rendering side, not on OpenFOAM's side.

The perfect solution would be to have a dedicated cell shape that could be used in ParaView/VTK to represent these "tetWedge" cells, so that they would be properly rendered in ParaView/VTK. That would probably also sort out some other rendering limitations in there as well. Unfortunately, that's beyond my current capabilities/time-frame.

I did some research into what ParaView/VTK is currently able to handle and the only/closest related detail to the "tetWedge" that I could find was a diagnostic algorithm that was extracted from a software named VERDICT, which in turn refers to this cell shape as "knife": - at least it's what it looks like from the description.

> Do you know if VTK_POLYHEDRON has been generalized yet or is it still restricted to convex shapes?

Quoting from the VTK code documentation:

  The class does not require the polyhedron to be convex. However, the polygonal faces must be planar. Non-planar polygonal faces will definitely cause problems, especially in severely warped situations.

This is consistent with what I've seen so far:

  - tetWedge does have one face that is not planar and therefore it's not rendered properly, no matter which strategy is used... or at least I haven't found one yet.

  - I saw some complex polyhedrons on the meshes from issue #2099 which were not rendered properly, also due to non-planar faces.

> On the point about maintaining support for cell-decomposition [...]

Sorry, I formed my sentence(s) on this topic incorrectly. What I meant to say is that given we have VTK_POLYHEDRON as an option, it can be used when detailed diagnosing of the mesh is needed, instead of relying on the VTK_WEDGE render for the tetWedge.

Support for cell-decomposition should be kept in and support maintained for it, at the very least as a fail-safe measure, because several of the post-processing algorithms in ParaView/VTK still have problems handling polyhedron cells, many of which lead to crashes. Progress has been made in recent versions, but last time I checked, it's still not bullet-proof.


2016-08-26 20:25

manager   ~0006775

> The only specific problem with edge collapsing is related to how the 5 vertex "tetWedge" is fitted into the 6 vertex "VTK_WEDGE". The limitation is on the ParaView/VTK rendering side, not on OpenFOAM's side.

Understood, but collapsed edges may occur for any cell-shape presumably generating the same problem; how should it be handled in general?

> However, the polygonal faces must be planar

There is no way we can guarantee planar faces without tet-decomposition.


2016-08-27 00:59

updater   ~0006779

> Understood, but collapsed edges may occur for any cell-shape presumably generating the same problem; how should it be handled in general?

OK, now I finally managed to understand the line of thought on what you wrote. And I can't believe I missed that the "wedge" is represented as a "VTK_HEXAHEDRON" with a collapsed edge... I looked at the code so many times the past few days and I always overlooked that detail. That was why I wasn't being able to see from that perspective.

I ran the tutorial case "mesh/foamyHexMesh/flange" and I believe I finally saw what you've written about. On the surface, after using "collapseEdges", there are several holes in the surface of the mesh, specifically due to these "wedge" cells. (Oh... wow... I remembered to look inside the mesh... and there are a ton of floating triangles inside it...)

From what I can see, there is at least one vertex that is incorrectly assigned as well, when doing "wedge" -> "VTK_HEXAHEDRON", as what happened for #1633, but a similar fix would not solve this issue either, namely the triangles inside the mesh.

I still have to test this, but "VTK_POLYHEDRON" should be able to take over this particular cell representation, although the collapsed face might look as incorrect as the one on "tetWedge", i.e. it flattens part of the face.
Hopefully the decomposition of the "wedge" cell will preserve the overall representation... but I still need to test that as well.

As for "when should edges be collapsed": I did a test with "combustion/engineFoam/kivaTest", by setting the parameters in "constant/engineGeometry" to result in the piston going farther into the top of the chamber; as long as the cells didn't overlapped each other, ParaView/VTK was able to render the nearly flat cells without artefacts. The positions of the vertexes were roughly 5e-6 m apart from each other (if I remember correctly). It did not have render problems, because the vertexes were defined as independent positions; the problems usually occur after cleaning up the mesh, i.e. when collapsing small edges, as you've mentioned.

> There is no way we can guarantee planar faces without tet-decomposition.

I still need to figure out what should be the correct representation for the non-planar faces. ParaView/VTK does have a few situations where it renders a face with omitted edges on its surface, since it's bending said surface... but still... it doesn't look right either way.


2016-08-29 21:35

updater   ~0006783

I've changed this from status "assigned" to "new", until I or anyone else can look better into this. I'll look into this in the upcoming weekend.


2016-10-31 14:38

updater   ~0007081

I haven't yet found time and inspiration to look better into this, but I do bring interesting news on the topic of concave cells and their rendering in ParaView.

The following email thread: - demonstrates that the current OpenGL2 implementation that ParaView is using in 5.2 is giving fair results when compared with OpenGL, as shown in the first email of the thread:

This means that OpenGL2 has helped to get better rendering results for the more complex shapes, although I don't know yet what are its limitations on this.


2016-11-25 15:23

reporter   ~0007329

Uploaded a 1 cell testcase which displays funny. Guess this is related to the discussion.

c0_0.vtk (536 bytes)


2017-09-03 19:41

updater   ~0008675

@MattijsJ: Many thanks!

@Henry: I'm suspending this issue for now. This can be revisited after the various VTK IO capabilities are consolidated.

Issue History

Date Modified Username Field Change
2016-08-26 00:20 wyldckat New Issue
2016-08-26 00:20 wyldckat Status new => assigned
2016-08-26 00:20 wyldckat Assigned To => henry
2016-08-26 00:20 wyldckat File Added: deprecated_dedicated_tetWedge_VTK_export_v1.tar.gz
2016-08-26 00:44 wyldckat File Added: decomposed_tetWedge_render_01.png
2016-08-26 00:44 wyldckat File Added: decomposed_tetWedge_render_02.png
2016-08-26 00:58 wyldckat Note Added: 0006769
2016-08-26 00:58 wyldckat Relationship added related to 0002099
2016-08-26 00:59 wyldckat Relationship added related to 0002213
2016-08-26 08:17 henry Note Added: 0006771
2016-08-26 09:07 henry Note Added: 0006772
2016-08-26 19:13 wyldckat Note Added: 0006774
2016-08-26 20:25 henry Note Added: 0006775
2016-08-27 00:59 wyldckat Note Added: 0006779
2016-08-29 21:34 wyldckat Assigned To henry =>
2016-08-29 21:35 wyldckat Note Added: 0006783
2016-08-29 21:35 wyldckat Status assigned => new
2016-10-31 14:38 wyldckat Note Added: 0007081
2016-11-25 15:23 MattijsJ File Added: c0_0.vtk
2016-11-25 15:23 MattijsJ Note Added: 0007329
2017-09-03 19:41 wyldckat Note Added: 0008675
2017-09-03 19:41 wyldckat Assigned To => wyldckat
2017-09-03 19:41 wyldckat Status new => closed
2017-09-03 19:41 wyldckat Resolution open => suspended