[Paraview] Algorithm causing reader RequestData when running

[Paul Edwards]

Hi,

I am writing an algorithm to operate on a vtkMultiBlockDataSet where I
would like to extract blocks depending on field data that is set.
Internally in my algorithm I am using the vtkExtractBlock filter and
when this is updated it causes the RequestData for my reader to be
executed again.  This doesn't happen when applying ExtractBlock in
ParaView.  Below I have included an example RequestData that causes
the data to be reloaded (note: this only happens when running in
parallel):

8<-----------------------------------------------------------------
int vtkTestMulti::RequestData(
        vtkInformation* vtkNotUsed(request),
        vtkInformationVector** inputVector,
        vtkInformationVector* outputVector)
{

    vtkInformation* inInfo = inputVector[0]->GetInformationObject(0);
    vtkMultiBlockDataSet* input = vtkMultiBlockDataSet::SafeDownCast(
            inInfo->Get(vtkDataObject::DATA_OBJECT()));

    vtkInformation* outInfo = outputVector->GetInformationObject(0);
    vtkMultiBlockDataSet* output = vtkMultiBlockDataSet::SafeDownCast(
            outInfo->Get(vtkDataObject::DATA_OBJECT()));

    VTK_CREATE(vtkExtractBlock, extractBlockFilter);
    extractBlockFilter->SetInput(input);
    extractBlockFilter->AddIndex(1);
    extractBlockFilter->Update();
    output->ShallowCopy(extractBlockFilter->GetOutput());

    return 1;
}
----------------------------------------------------------------->8

Can anyone see what I am doing wrong or provide any pointers for where to look?

Thanks in advance,
Paul