vtkHDF: Add support for polyhedron cells

Development
15

Dear VTK-Team,

I am still enthusiastic about your prompt and positive reply. I expect that our (ParaView-) users would benefit a lot from a new unstructured grid format.
As in many finite volume codes we are storing data in the cell centers and the centers of the internal (those with a positive and negative number in my example) and the external(boundary) faces.
Thus the boundary data will need less storage than the data on the internal faces.
This could be reflecte by using something like face sections.
My idea would be something like :

/VTKHDF (Attribute Type UnstructuredGrid, Version )
   /NumberOfCells
   /NumberOfCellSides
   /NumberOfPoints
   /NumberOfFaces
   /NumberOfFaceVertices
   /NumberOfFaceSections
   /Connectivity
       /Faces
          /Vertices
          /Offsets
       /Cells
          /OneBasedFaceIDs
          /Offsets
       /FaceSection attribute  name = "family1" attribute boundaryCondition = "bc1"
      either /SectionRange  Start,End
      or     /FaceList  list of face indicators
       /FaceSection attribute name = "family2" 
      either /SectionRange  Start,End
      or     /FaceList  list of face indicators attribute boundaryCondition = "bc1"
   /Points
   /CellData
   /PointData
   /FaceData
   /FaceSectionData name="family1" 
   /FaceSectionData name="family2"

In this way, the all the data would be stored in a very compact way. Ideally, the /Cells /Faces and /FaceSection sections would be selectable similar to the “Extract Block” filter in ParaView.

If the 1 based faceIds are posing a problem, it would be possible to put the boundary faces in the beginning of the face list since their orientation would be always positive. A dummy face would do the trick as well.

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