[vtkusers] Vtk xml file format questions.

[Greg Schussman]

Hi.

I've been googling around and reading up on the vtk xml file format, but
not having much luck understanding a few things in particular.

First, it seems that there are parts within the file, but there isn't
support for grouping those parts into groups so that one can easily toggle
visibility of various groups.  Or (hopefully), have I missed something?

Second, there seems to be a strong notion of keeping the data with the
mesh.  But for my purposes, there is too much data to fit in the same file
as the mesh.  Splitting the big file into multiple smaller files, but
having a redundant copy of the same mesh in each file would be overly
space-consuming.   I've seen older discussion of this (for timesteps, I
think), writing a mesh to one file, and data as vtkDataObjects to another,
and then using vtkMergeDataObjectFilter to combine the two.  But that
looked like it was using the legacy file format.  Is it possible to do this
with the vtk xml file format?  Would the data be written to a .vtu file
where the points and cells are just plain missing?  Or would that be an
invalid file from the reader's perspective?  (by the way, is there a dtd
avaliable for the vtk xml format?  I wasn't able to find one).

Third, is the vtk xml format friendly to only reading parts of a large
file, while still performing well?  That is, if the mesh and all associated
data were shoveled into one big file (tens to hundreds of gigabytes, with
one mesh, and possibly hundreds of attributes), could only the mesh and
specific attributes be extracted quickly and easily?

Fourth, for compressed data in the XML file (raw, appended, if I understand
right), does the overall file size compare favorably with a purely ASCII
file that has been gzipped?

Lastly, am I barking up the wrong tree?  The vtk xml format, as I
understand it, looks pretty nice.  But for the kinds of things I'm asking
about here, would some other format be better suited?  Or some other
approach to using this format?

Thanks.

Greg
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