[vtkusers] Creating parallel structured grid file (*.pvts) from multiple structured grid (*.vts) files

[André Malcher]

Hello,

I'm looking for resources/solutions to set up an parallel structured grid file (*.pvts).
I have an parallel Lattice-Boltzmann solver written in MPI in which each rank computes one part of a cubic domain.
I was able to output the *.vts files for each sub-domain, yet I am unable to compile those outputs to be shown in Paraview as a composition.
The problem seems to be related to the *.pvts file, but unfortunately it is very hard to find resources on the VTK xml formats in the internet.
Also, the "file-formats.pdf" provided at www.vtk.org lacks information about some details.

To describe my problem and for simplicity, I set up a cube with side length 8 (no. of elements) that is computed by 8 ranks,
i.e. I split the domain into eight cubes (of length 4 then) that are processed by each of the ranks.
My *.vts files have the following format (e.g. for rank 0):

<?xml version="1.0"?>
<VTKFile type="StructuredGrid" version="0.1" byte_order="LittleEndian">
   <StructuredGrid WholeExtent="0 3 0 3 0 3" >
   <Piece Extent="0 3 0 3 0 3" >
       <PointData Scalars="density" Vectors="velocity">
           <DataArray Name="velocity" type="Float32" NumberOfComponents="3" format="ascii">
           </DataArray>
           <DataArray Name="density" type="Float32">
<!-- Some values here -->
           </DataArray>
       </PointData>
       <CellData></CellData>
       <Points>
           <DataArray type="Float32" Name="coordinate" NumberOfComponents="3" format="ascii">
<!-- Some values here -->
           </DataArray>
       </Points>
    </Piece>
    </StructuredGrid>
</VTKFile>

These work fine and show the expected result when loaded into ParaView.
However, combining these files to one output fails. This is my *.pvts file that I create:

<?xml version="1.0"?>
<VTKFile type="PStructuredGrid" version="0.1" byte_order="LittleEndian">
   <PStructuredGrid WholeExtent="0 7 0 7 0 7 " GhostLevel="#">
       <PPointData Scalars="density" Vectors="velocity">
           <DataArray type="Float32" Name="velocity" NumberOfComponents="3" format="ascii" />
           <DataArray type="Float32" Name="density" />
       </PPointData>
		<PCellData></PCellData>
		<PPoints>
			<DataArray  type="Float32" Name="coordinate" NumberOfComponents="3" format="ascii" />
		</PPoints>
       <Piece Extent="0 3 0 3 0 3" Source="lbm_out_0.0.vts">
		</Piece>
       <Piece Extent="3 7 0 3 0 3" Source="lbm_out_1.0.vts">
		</Piece>
       <Piece Extent="0 3 3 7 0 3" Source="lbm_out_2.0.vts">
		</Piece>
       <Piece Extent="3 7 3 7 0 3" Source="lbm_out_3.0.vts">
		</Piece>
       <Piece Extent="0 3 0 3 3 7" Source="lbm_out_4.0.vts">
		</Piece>
       <Piece Extent="3 7 0 3 3 7" Source="lbm_out_5.0.vts">
		</Piece>
       <Piece Extent="0 3 3 7 3 7" Source="lbm_out_6.0.vts">
		</Piece>
       <Piece Extent="3 7 3 7 3 7" Source="lbm_out_7.0.vts">
		</Piece>
    </PStructuredGrid>
</VTKFile>

When I load this into ParaView and click apply it shows the right extents (0-7) but the density/velocity values as well as the bounds/ranges are totally wrong.
I attached a screenshot of the ParaView information window to give a better idea what I am talking about. By the way, since the data is useless,
ParaView also doesn't show any outline after clicking on "Apply". I would appreciate if you could provide me with some details about what I may be doing wrong.

Besides, I would like to know how the overlapping extent areas work. When they don't overlap, I get error messages and I have read about the fact that they are required to overlap.
But how does the extent include four elements when in (0..3) in the special case described above but five elements in (3..7)?

And last but not least, is it possible to load legacy *.vtk files as extent source?

Thank you very much in advance.
André