[vtk-developers] vtkWedge face node ordering and vtkUnstructuredGridGeometryFilter

[John Platt]

Hi,

I think this is also a problem for vtkDataSetSurfaceFilter when mixing 
quadratic hex's & wedges. If the actor opacity is reduced, the internal 
faces between the hex's & wedges are visible.

This can be fixed by turning the wedge inside-out i.e. swapping points 1 
& 2 and 4 & 5. The FE system I work with allows both right and left 
handed elements so the topology of each element must be checked and 
inverted if necessary.

I agree that the order of the nodes in the face arrays should be changed 
to be consistent with the other cells. I suspect this would largely go 
unnoticed given the time this inconsistency has been present and the 
number of reports on the behaviour.

Thanks,

John.



On 01/04/2015 16:42, Berk Geveci wrote:
> Hi Andreas,
>
> Changing the ordering of vtkWedge would be a very large undertaking
> given that there is years of documentation, file formats (in VTK and
> 3rd party), code etc. that depend on a certain ordering. One less
> intrusive way would be to change any vtkWedge methods that returns
> faces to return them in a different order. I suspect this option is
> doable but I'd like to hear from the community. The least intrusive
> options is to fix the few filters that have this issue but I feel like
> it is a band-aid.
>
> Best,
> -berk
>
> On Wed, Apr 1, 2015 at 9:51 AM, Andreas Buykx <A.Buykx at tnodiana.com> wrote:
>> Hi,
>>
>>
>>
>> To detect if coinciding faces of neighbouring 3D cells are connected,
>> vtkUnstructuredGridGeometryFilter walks the points of these faces in
>> opposite directions and expects the point ids to match. This fails for
>> vtkWedge attached to e.g. vtkTetra and vtkHexahedron as demonstrated by the
>> python script I attach. The root cause is that the vtkWedge::GetFaceArray
>> and vtkWedge::GetFace return the face points in an ordering that makes the
>> face normal point into the 3D cell instead of out of the  3D cell, like all
>> other 3D cells do. The two other algorithms that generate surface cells
>> (vtkDataSetSurfaceFilter and vtkGeometryFilter) don’t suffer from this
>> ordering because the former checks point ordering but explicitly allows both
>> loop directions, and the latter only takes cell connectivity into account
>> without checking point-ordering at all.
>>
>>
>>
>> I don’t know what the reason is to have a different face-point ordering for
>> the vtkWedge, but one of the other consequences is that the outer surface
>> will have inconsistent cell normals as can easily be seen when loading the
>> xml files produced by the tests in paraview, and showing the cell normals.
>> So I would prefer to change the face-point ordering in vtkWedge. If that is
>> not possible for some reason then I propose to make/update the case-blocks
>> in the surface filters cell-type switches to re-order the face-points for
>> the wedge faces. As long as this is not dealt with our FEA models will show
>> edges where they should not be so I will have to solve this issue one way or
>> another, and I’d like to contribute it to VTK as well.
>>
>>
>>
>> Shall I make a mantis issue for this? Who can help me with the git/gerrit
>> stuff?
>>
>>
>>
>> Thanks a lot,
>>
>> Andreas Buykx
>>
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