mixed elements (including polyhedron) in vtu file

ParaView Support
1

I am currently trying to bridge my CFD solver with paraview and I am about to implement support for polyhedral meshes and want to explore how to write the corresponding output file.

I have used the legacy vtk file format before and it seems (?!) that polyhedral cells cannot be written in that file format (or put differently, I am not able to find any documentation on that). I was able to find some documentation on polyhedral cells using the vtu xml-based file format and got it eventually working for a single polyhedron, however, now when I want to write out a mesh (and ultimately a solution on it) consisting of vtk inbuild types (like tetrahedron, hexahedron etc.) + polhedral cells I am getting an error message that my faceoffset array is not long enough. While the error message is fairly verbose, I am not sure which should be the correct offset I should use. Here is an example output file consisiting of only a hexahedron and a polyhedron:

<VTKFile type="UnstructuredGrid" version="1.0" byte_order="LittleEndian" header_type="UInt64">
  <UnstructuredGrid>
    <Piece NumberOfPoints="14" NumberOfCells="2">
      <Points>
        <DataArray type="Float32" Name="Points" NumberOfComponents="3" format="ascii" RangeMin="0" RangeMax="2">
          0 0 0
          1 0 0
          1 1 0
          0 1 0
          0 0 1
          1 0 1
          1 1 1
          0 1 1
          1.5 0 0
          2 0 0.5
          2 0 1
          2 1 1
          1.5 1 0
          2 1 0.5
        </DataArray>
      </Points>
      <Cells>
        <DataArray type="Int64" Name="connectivity" format="ascii" RangeMin="0" RangeMax="13">
          0 1 2 3 4 5 6 7
          1 8 9 13 12 2 5 10 11 6
        </DataArray>
        <DataArray type="Int64" Name="offsets" format="ascii" RangeMin="8" RangeMax="18">
          8
          18
        </DataArray>
        <DataArray type="UInt8" Name="types" format="ascii" RangeMin="12" RangeMax="42">
          12
          42
        </DataArray>
        <DataArray type="Int64" Name="faces" format="ascii" RangeMin="1" RangeMax="13">
          7
          5  1  8  9  10 5
          5  2  12 13 11 6
          4  1  2  6  5
          4  9  10 11 13
          4  8  9  13 12
          4  1  8  12 2
          4  5  10 11 6
        </DataArray>
        <DataArray type="Int64" Name="faceoffsets" format="ascii" RangeMin="38" RangeMax="38">
          38
        </DataArray>
      </Cells>
    </Piece>
  </UnstructuredGrid>
</VTKFile>

Any help would be appreciated!

Edit: This is the exact error message I am getting in Paraview: “ERROR: In C:\bbd\ecd3383f\build\superbuild\paraview\src\VTK\IO\XML\vtkXMLUnstructuredDataReader.cxx, line 831
vtkXMLUnstructuredGridReader (0000018446A52840): Cannot read face offsets from Cells in piece 0 because the “faceoffsets” array is not long enough.”

1 Like
2

The problem is that the faceoffsets array has to be the same size as the number of cells. That is, you have to have an entry for every cell, even those that are not polyhedron-type. For those cells that do not use faces, put a -1 in that place in the array. So, your faceoffsets array should look like this:

        <DataArray type="Int64" Name="faceoffsets" format="ascii" RangeMin="-1" RangeMax="38">
          -1 38
        </DataArray>
2 Likes
3

Thanks Kenneth, that did the trick for me!