[vtkusers] Cannot use vtkConnectivty on GroupDataSet

[Cory Quammen]

Robert,

The GroupDatasets filter produces a vtkMultiBlockDataSet as output. A
vtkMultiBlockDataSet is a tree with leaves consisting of vtkDataObjects. It
is common for filters to operate on these data sets block-by-block, and
vtkConnectivity is no exception. It essentially runs on each leaf
vtkDataObject independently. As a consequence, different connected
components in different blocks may be assigned the same RegionId.
Unfortunately, this does not really make sense and is a bug. I have filed
an issue here: https://gitlab.kitware.com/vtk/vtk/issues/17329.

For you to make a test data set, however, you do not need to wait until the
bug is fixed. Within ParaView use the Append Geometry filter instead of
Group Datasets on your three input Spheres. This will produce a vtkPolyData
as output that you can then set as input to the vtkConnectivity filter and
get the output you expect.

HTH,
Cory

On Wed, May 23, 2018 at 5:24 AM Robert Sawko <robertsawko at gmail.com> wrote:

> Hi,
>
> I am trying with ParaView to create a test data set for my VTK application.
> I simply want three spheres with varying radius. The application operates
> on
> vtk contour plots that are produced by OpenFOAM. Using
> vtkConnectivityFilter
> it computes disjoint surfaces and then it iterates over each surface to
> obtain mass properties: volume and surface area.
>
> In order to create my data set I use sources in ParaView to create three
> disjoint spheres and I combine them into a single set by GroupDatasets
> filter. Then I use SaveData and save into .vtm file format. If I try to
> apply Connectivity on the group it find sonly one group even though spheres
> are clearly disjoint. I tried various extract filters, but I am clearly
> missing something.
>
> Connectivity does work on my "production" data, but just not my mock set
> constructed through PV sources and GroupDatasets. I would like to
> understand
> why that happens and if there's a correct way of producing my three sphere
> data set.
>
> Thanks.
>
>
>
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-- 
Cory Quammen
Staff R&D Engineer
Kitware, Inc.
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