[Paraview] ImageAlgorithmm Readers, and Volume of Interest

[Mike Jackson]

I am trying to write a new reader for PV 3 based on the  
vtkImageAlgorithm class. I have some basics completed but my  
RequestData() method is running multiple times where I don't really  
think it should be. I had this problem last year with another reader  
but I don't think I ever figured out what I was doing wrong.

One of the sticking points I am having trouble getting my head  
wrapped around is how to extract a subvolume from the main volume.  
The size of the vtkImageData is like 10,000 x 10,000 x 300 which will  
cause memory issues. At this point I would just like to render a  
subvolume of data from the main volume. In our RequestInformation()  
method we are setting the WHOLE_EXTENT to the actual volume size (10k  
x 10k x 300).

int vtkH5RoboMetReader::RequestInformation(vtkInformation *vtkNotUsed 
(request),
vtkInformationVector **vtkNotUsed(inputVector),
               vtkInformationVector *outputVector)
{
...
   outInfo->Set( vtkStreamingDemandDrivenPipeline::WHOLE_EXTENT(),  
this->DataExtent, 6 );
...
}


and then in the RequestData() method we are only extracting volume of  
interest:

int vtkH5RoboMetReader::RequestData( vtkInformation* vtkNotUsed 
(request),
                     vtkInformationVector** vtkNotUsed(inputVector),
                     vtkInformationVector* outputVector )
{
vtkImageData *output =
    vtkImageData::SafeDownCast(outInfo->Get(vtkDataObject::DATA_OBJECT 
()));
int32 requestedExtent[6];
if(outInfo->Has(vtkStreamingDemandDrivenPipeline::UPDATE_EXTENT()))
{
  // Get the requested data extent.
   requestedExtent[0] = 0;
   requestedExtent[1] = 1291;
   requestedExtent[2] = 0;
   requestedExtent[3] = 967;
   requestedExtent[4] = 100;
   requestedExtent[5] = 101;
}

output->SetExtent(requestedExtent);
output->SetOrigin(static_cast<double>(requestedExtent[0]),
                  static_cast<double>(requestedExtent[2]),
                  static_cast<double>(requestedExtent[4]));
output->SetSpacing(this->DataSpacing);
output->SetScalarTypeToUnsignedChar();
output->SetNumberOfScalarComponents(1);
output->AllocateScalars();
//Load data from file

return 1;
}

When I run the test program that renders the subvolume I get the  
RequestData() method being called multiple times. Is there something  
obvious that I am just missing? I tried looking at some of the other  
readers in vtk/paraview but nothing is really jumping out at me.

Thanks for any help
--
Mike Jackson
imikejackson & gmail * com