[Paraview] Status of XDMF3 Reader in ParaView

[Andy Smith]

Currently in our CFD code we output XDMF and use the XDMF2 reader in
ParaView.
I would like to add the capability to output arbitrary polyhedra but it
appears support for this was only added in XDMF3.  Because of this I plan
to change the output to a format that is compatible with XDMF3 but face at
least two major issues that are preventing me from using XDMF3:

1. XDMF cannot handle functions properly (https://gitlab.kitware.com/
paraview/paregaview/issues/17516
<https://gitlab.kitware.com/paraview/paraview/issues/17516>).  I use
functions to join scalars to make vectors.

2. Inability to select certain blocks of a multiblock dataset before
reading data.  This feature allows the user to only load a subset of the
complete multiblock dataset and saves a large amount of time.  (see my
final comment in a post about a year ago
https://public.kitware.com/pipermail/paraview/2017-March/039605.html)

I can think of three options going forward:

1. Get functions and block selection during loading working in XDMF3 (best
option).
2. Back-port arbitrary polyhedra support to XDMF2 (looks doable, but not
appealing since eventually XDMF2 will go away).
3. Split arbitrary polyhedra into tetrahedra in the solver (probably most
complicated and least ideal since the visualization no longer completely
represents the actual flow field).

I've attached a simple multiblock dataset that works using the XDMF2 reader
and demonstrates both issues with the XDMF3 reader.  The function problem
is a known issue, but perhaps I'm doing something wrong in my file
formatting that prevents the selection of individual blocks during loading.

I'm open to any assessment of my proposed options and any suggestions on
what I can do to help.

Thanks,
Andy
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