[Paraview] multiple execution of Image Reader when running PV in parallel

[Jean Favre]

I am having trouble developping a custom reader (public inheritance of
vtkImageAlgorithm). When running PV on a single node, my reader executes
once only - as expected - and creates a vtkImageData. I can then use any
number of Filters downstream

When running PV on multiple nodes, my reader executes every time I start
a new filter. Can't figure out what triggers it.

Forget about my own source code. I went back to the basics and put a
DebugOn() statement in vtkImageReader. It exhibits exactly the same
behavior:

on a single node, execute once and only once as expected.
on multiple nodes, executes multiple times. Here is an example running
on 2 nodes,

First read of a 90^3 raw binary file (the extents are split in the Z
direction):

Reading extent: 0, 89, 0, 89, 0, 45
Reading extent: 0, 89, 0, 89, 43, 89

then I extracted a I=29 ortho-slice and the reader executed again

Reading extent: 29, 29, 0, 89, 0, 89
Reading extent: 29, 29, 0, 89, 0, 89

then I extracted an J=45 orthoslice and the reader executed again
Reading extent: 0, 89, 45, 45, 0, 89
Reading extent: 0, 89, 45, 45, 0, 89

then I executed an isosurface and the reader executed again

Reading extent: 0, 89, 0, 89, 0, 45
Reading extent: 0, 89, 0, 89, 43, 89

We have only found this multiple execution with ImageData. Other readers
(for example for StructuredGrid) do not execute multiple times.

is this a feature reserved to ImageData?

thanks for any tip on this issue.

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