[vtkusers] Merging selections

[David Doria]

If I do this:

  vtkSmartPointer<vtkSelectionNode> selectionNode1 =
      vtkSmartPointer<vtkSelectionNode>::New();
  selectionNode1->SetFieldType(vtkSelectionNode::POINT);
  selectionNode1->SetContentType(vtkSelectionNode::INDICES);
  selectionNode1->SetSelectionList(ids1);

    vtkSmartPointer<vtkSelectionNode> selectionNode2 =
      vtkSmartPointer<vtkSelectionNode>::New();
  selectionNode2->SetFieldType(vtkSelectionNode::POINT);
  selectionNode2->SetContentType(vtkSelectionNode::INDICES);
  selectionNode2->SetSelectionList(ids2);

  vtkSmartPointer<vtkSelection> selection =
      vtkSmartPointer<vtkSelection>::New();
  selection->AddNode(selectionNode1);
  selection->AddNode(selectionNode2);

followed by a vtkExtractSelection, shouldn't it extract everything
selected by both selections? (It doesn't work:
http://www.vtk.org/Wiki/VTK/Examples/MergeSelections)

What is the difference between doing the above and creating one
vtkSelection object, then calling Union() on a second vtkSelection?

Thanks,

David