Problem with vtkExtractVOI and vtkExtractGrid

[pahsieh at usgs.gov]

The filters vtkExtractVOI and vtkExtractGrid appear to
lose cell data. This is illustrated by the attached
sample code. A structured points data set is created
with point and cell data (scalars). After filtering with
vtkExtractVOI, there are point scalars but no cell
scalars. The same thing happens for a structured grid
data set that is extracted by vtkExtractGrid (sample
code not attached).

As always, many thanks for you help and best regards,
Paul


//  ==== sample code ====
//  The output is:
//      The extracted grid has 27 point scalars.
//      The extracted grid has no cell scalars.


#include "vtkStructuredPoints.h"
#include "vtkExtractVOI.h"

void main()
{
    // Create a structured points data set
    vtkStructuredPoints *sp = vtkStructuredPoints::New();
    sp->SetDimensions(5, 5, 5);
    sp->SetSpacing(1, 1, 1);
    sp->SetOrigin(0, 0, 0);

    // Create cell scalar data.
    vtkScalars *scalars = vtkScalars::New();
    for (int i=0; i<64; i++)
    {
        scalars->InsertNextScalar(0.01 * i);
    }
    sp->GetCellData()->SetScalars(scalars);
    scalars->Delete();

    // Create point scalar data
    scalars = vtkScalars::New();
    for (i=0; i<125; i++)
    {
        scalars->InsertNextScalar(0.01 * i);
    }
    sp->GetPointData()->SetScalars(scalars);
    scalars->Delete();

    // Extract a portion of the grid
    vtkExtractVOI *voi = vtkExtractVOI::New();
    voi->SetInput(sp);
    voi->SetVOI(2, 4, 2, 4, 2, 4);
    voi->SetSampleRate(1, 1, 1);
    voi->Update();

    // Print the number of point scalars
    scalars = voi->GetOutput()->GetPointData()->GetScalars();
    if (scalars)
    {
        cout << "The extracted grid has " << scalars->GetNumberOfScalars()
             << " point scalars." << endl;
    }
    else
    {
        cout << "The extracted grid has no point scalars" << endl;
    }

    // Print the number of cell scalars
    scalars = voi->GetOutput()->GetCellData()->GetScalars();
    if (scalars)
    {
        cout << "The extracted grid has " << scalars->GetNumberOfScalars()
             << " cell scalars." << endl;
    }
    else
    {
        cout << "The extracted grid has no cell scalars." << endl;
    }

    // Clean up
    sp->Delete();
    voi->Delete();
}


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