Problem with vtkClipPolyData

[pahsieh at usgs.gov]

I am running into a problem with vtkClipPolyData when the
dataset contains cell scalars. The filter does not seem to
work correctly with GenerateClippedOutputOn, but
works fine with GenerateClippedOutputOff.

This problem is illustrated by the attached sample code.
A structured points data set is created with cell scalars,
and is then passed through a geometry filter to obtain
a poly dataset for clipping.

Clipping is done twice on the poly dataset, the first time
with GenerateClippedOutputOn, and the second time with
GenerateClippedOutputOff. The resulting actors are different.

This problem occurs in both vtk 2.3 and vtk 3.1 ("official"
release). The problem also occurs when the clipped dataset is
a structured grid that has been passed through a geometry filter,
or a poly data created from scratch.

Thanks for any help and best regards,
-- Paul

//======= sample code =======

#include "vtkRenderer.h"
#include "vtkRenderWindow.h"
#include "vtkRenderWindowInteractor.h"
#include "vtkStructuredPoints.h"
#include "vtkGeometryFilter.h"
#include "vtkPlane.h"
#include "vtkClipPolyData.h"
#include "vtkPolyDataMapper.h"
#include "vtkActor.h"

void main()
{
    // Create the rendering stuff
    vtkRenderer *ren = vtkRenderer::New();
    vtkRenderWindow *renWin = vtkRenderWindow::New();
    renWin->AddRenderer(ren);
    vtkRenderWindowInteractor *iren = vtkRenderWindowInteractor::New();
    iren->SetRenderWindow(renWin);

    // Create a structured points data set
    vtkStructuredPoints *sp = vtkStructuredPoints::New();
    sp->SetDimensions(11, 11, 1);
    sp->SetSpacing(1, 1, 1);
    sp->SetOrigin(0, 0, 0);

    // Create some cell scalar data.
    vtkScalars *scalars = vtkScalars::New();
    for (int i=0; i<100; i++)
    {
        scalars->InsertNextScalar(0.01 * i);
    }
    sp->GetCellData()->SetScalars(scalars);
    scalars->Delete();

    // Use the geometry filter to make a poly data set.
    vtkGeometryFilter *geom = vtkGeometryFilter::New();
    geom->SetInput(sp);

    // Define the clipping plane x = 2.5
    vtkPlane *plane = vtkPlane::New();
    plane->SetNormal(1, 0, 0);
    plane->SetOrigin(2.5, 0, 0);

    // Clip the data with GenerateClippedOutputOn
    vtkClipPolyData *clip1 = vtkClipPolyData::New();
    clip1->SetInput(geom->GetOutput());
    clip1->SetClipFunction(plane);
    clip1->GenerateClippedOutputOn();

    vtkPolyDataMapper *mapper1 = vtkPolyDataMapper::New();
    mapper1->SetInput(clip1->GetOutput());
    mapper1->SetScalarModeToUseCellData();

    vtkActor *actor1 = vtkActor::New();
    actor1->SetMapper(mapper1);

    // Clip the same date with GenerateClippedOutputOff
    vtkClipPolyData *clip2 = vtkClipPolyData::New();
    clip2->SetInput(geom->GetOutput());
    clip2->SetClipFunction(plane);
    clip2->GenerateClippedOutputOff();

    vtkPolyDataMapper *mapper2 = vtkPolyDataMapper::New();
    mapper2->SetInput(clip2->GetOutput());
    mapper2->SetScalarModeToUseCellData();

    vtkActor *actor2 = vtkActor::New();
    actor2->SetMapper(mapper2);
    actor2->AddPosition(12, 0, 0);


    // Display the 2 actors
    ren->AddActor(actor1);
    ren->AddActor(actor2);
    renWin->Render();
    iren->Start();

    // Clean up
    sp->Delete();
    geom->Delete();
    plane->Delete();
    clip1->Delete();
    mapper1->Delete();
    actor1->Delete();
    clip2->Delete();
    mapper2->Delete();
    actor2->Delete();
    ren->Delete();
    renWin->Delete();
    iren->Delete();
}

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