[Paraview-developers] VTK xml formats with external geometry references

[Mark Olesen]

I realize this discussion pops up at regular intervals, but I haven't 
seen it for a while and I have some use cases that would really need it.

Seeing that there are multiblock and parallel VTK file formats such as 
.vtm, .pvtp, .pvtu serving as collections for externally defined data, I 
was wondering if it would be feasible to use a similar idea to hold the 
geometry separate from the cell/point data?
AFAIK this isn't really something that the normal (eg, vtp, vtu) formats 
support, but something that would really help me.

A fairly common situation is sampling to predefined surface that is 
reused for different times and for different fields. At the moment I 
have the choice of writing geometry+data for each sampled field at each 
time value. This can lead to fairly excessive disk usage.

To avoid this duplicate geometry, I would need a complete (non-trivial) 
rewrite of my sampling. But even after that I would still have geometry 
duplication if the geometry is not changing in time. I also lose the 
ability to easily remove unneeded fields a posteriori.

The desired meta-format might look like this:

<VTKFile type="XXXUnstructuredGrid"...>
   <XXXUnstructuredGrid>
     <Piece file="abc.vtu" NumberOfPoints="1000" NumberOfCells="10">
       <CellData Name="p" format="file" file="abc_p.dvtu"/>
       <CellData Name="T" format="file" file="abc_T.dvtu"/>
       <CellData Name="conc0" format="file" file="abc_scalar.dvtu"/>
       <CellData Name="conc1" format="file" file="abc_scalar.dvtu"/>
       <CellData Name="conc2" format="file" file="abc_scalar.dvtu"/>
       <CellData Name="conc3" format="file" file="abc_scalar.dvtu"/>
     </Piece>
   </XXXUnstructuredGrid>
</VTKFile>

This is somewhat like a pvtu file, but with the geometry and data being 
able to reside in separate files.

The "abc.vtu" base geometry is just a regular VTU file with or without 
an fields.

The "abc_T.dvtu" (dvtu for a data vtu) has data but no geometry. For 
sanity checks it should have the basic sizing information 
(NumberOfPoints, NumberOfCells). To be thoroughly redundant, I guess a 
geometry file could also be specifically named in the respective dvtu file.

Is there any thing within the current framework that could be easily 
leveraged to solve such a problem, and is such a way that it eventually 
be part of the VTK file-format collections. Is there actually a VTK 
format that already fits the bill and I just haven't found it?
I know that the EnSight format covers many of these points, but a pure 
VTK solution is what I'm looking for since it would be important to have 
parallel file format output as per PUnstructuredGrid etc.

Cheers,
/mark