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# ThresholdCells

VTKEx/Cxx/PolyData/ThresholdCells

Question

If you have a simple question about this example contact us at VTKExProject If your question is more complex and may require extended discussion, please use the VTK Discourse Forum

### Code

ThresholdCells.cxx

    #include <vtkIntArray.h>
    #include <vtkCellData.h>
    #include <vtkTriangle.h>
    #include <vtkCellArray.h>
    #include <vtkPoints.h>
    #include <vtkPolyData.h>
    #include <vtkUnstructuredGrid.h>
    #include <vtkPointData.h>
    #include <vtkSmartPointer.h>
    #include <vtkThreshold.h>

    int main(int, char *[])
    {
      vtkSmartPointer<vtkPoints> points =
        vtkSmartPointer<vtkPoints>::New();
      points->InsertNextPoint(0,0,0);
      points->InsertNextPoint(1,1,1);
      points->InsertNextPoint(2,2,2);
      points->InsertNextPoint(3,3,3);
      points->InsertNextPoint(4,4,4);

      // Create three triangles
      vtkSmartPointer<vtkTriangle> triangle0 =
        vtkSmartPointer<vtkTriangle>::New();
      triangle0->GetPointIds()->SetId ( 0, 0 );
      triangle0->GetPointIds()->SetId ( 1, 1 );
      triangle0->GetPointIds()->SetId ( 2, 2 );

      vtkSmartPointer<vtkTriangle> triangle1 =
        vtkSmartPointer<vtkTriangle>::New();
      triangle1->GetPointIds()->SetId ( 0, 1 );
      triangle1->GetPointIds()->SetId ( 1, 2 );
      triangle1->GetPointIds()->SetId ( 2, 3 );

      vtkSmartPointer<vtkTriangle> triangle2 =
        vtkSmartPointer<vtkTriangle>::New();
      triangle2->GetPointIds()->SetId ( 0, 2 );
      triangle2->GetPointIds()->SetId ( 1, 3 );
      triangle2->GetPointIds()->SetId ( 2, 4 );

      // Add the triangles to a cell array
      vtkSmartPointer<vtkCellArray> triangles =
        vtkSmartPointer<vtkCellArray>::New();
      triangles->InsertNextCell ( triangle0 );
      triangles->InsertNextCell ( triangle1 );
      triangles->InsertNextCell ( triangle2 );

      // Setup index array
      vtkSmartPointer<vtkIntArray> index =
        vtkSmartPointer<vtkIntArray>::New();
      index->SetNumberOfComponents(1);
      index->SetName("index");
      index->InsertNextValue(0);
      index->InsertNextValue(1);
      index->InsertNextValue(2);

      // Add points, cells and index array to a polydata
      vtkSmartPointer<vtkPolyData> polydata =
        vtkSmartPointer<vtkPolyData>::New();
      polydata->SetPoints(points);
      polydata->SetPolys(triangles);
      polydata->GetCellData()->AddArray(index);

      std::cout << "There are " << polydata->GetNumberOfCells()
                << " cells before thresholding." << std::endl;

      vtkSmartPointer<vtkThreshold> threshold =
        vtkSmartPointer<vtkThreshold>::New();
      threshold->SetInputData(polydata);
      threshold->ThresholdByLower(1);
      // doesn't work because the array is not added as SCALARS, i.e. via SetScalars
      // threshold->SetInputArrayToProcess(0, 0, 0, vtkDataObject::FIELD_ASSOCIATION_CELLS, vtkDataSetAttributes::SCALARS);
      // use like this:
      threshold->SetInputArrayToProcess(0, 0, 0, vtkDataObject::FIELD_ASSOCIATION_CELLS, "index");
      threshold->Update();

      vtkUnstructuredGrid* thresholdedPolydata = threshold->GetOutput();
      std::cout << "There are " << thresholdedPolydata->GetNumberOfCells()
                << " cells after thresholding." << std::endl;

      return EXIT_SUCCESS;
    }

### CMakeLists.txt

    cmake_minimum_required(VERSION 3.3 FATAL_ERROR)

    project(ThresholdCells)

    find_package(VTK COMPONENTS
      vtkCommonCore
      vtkCommonDataModel
      vtkFiltersCore QUIET)
    if (NOT VTK_FOUND)
      message("Skipping ThresholdCells: ${VTK_NOT_FOUND_MESSAGE}")
      return ()
    endif()
    message (STATUS "VTK_VERSION: ${VTK_VERSION}")
    if (VTK_VERSION VERSION_LESS "8.90.0")
      # old system
      include(${VTK_USE_FILE})
      add_executable(ThresholdCells MACOSX_BUNDLE ThresholdCells.cxx )
      target_link_libraries(ThresholdCells PRIVATE ${VTK_LIBRARIES})
    else ()
      # include all components
      add_executable(ThresholdCells MACOSX_BUNDLE ThresholdCells.cxx )
      target_link_libraries(ThresholdCells PRIVATE ${VTK_LIBRARIES})
      # vtk_module_autoinit is needed
      vtk_module_autoinit(
        TARGETS ThresholdCells
        MODULES ${VTK_LIBRARIES}
        )
    endif ()

### Download and Build ThresholdCells

Click here to download ThresholdCells and its CMakeLists.txt file. Once the tarball ThresholdCells.tar has been downloaded and extracted,

    cd ThresholdCells/build

If VTK is installed:

    cmake ..

If VTK is not installed but compiled on your system, you will need to specify the path to your VTK build:

    cmake -DVTK_DIR:PATH=/home/me/vtk_build ..

Build the project:

    make

and run it:

    ./ThresholdCells

WINDOWS USERS

Be sure to add the VTK bin directory to your path. This will resolve the VTK dll's at run time.