# DeleteCells

VTKExamples/Cxx/PolyData/DeleteCells

### Description

This example demonstrates how to delete a cell from a vtkPolyData. This is a three-step process: first, build upward-links from points to cells; second, mark cells for deletion; third, delete the marked cells. The first step is necessary to create a data structure suitable for efficient delete operations. Creating this data structure is computationally expensive, so it is only carried out before operations that need it, e.g. those that require changes to the topology.

### Code

DeleteCells.cxx

    #include <vtkSmartPointer.h>

    #include <vtkActor.h>
    #include <vtkCellArray.h>
    #include <vtkLine.h>
    #include <vtkPoints.h>
    #include <vtkPolyData.h>
    #include <vtkPolyDataMapper.h>
    #include <vtkProperty.h>
    #include <vtkRenderer.h>
    #include <vtkRenderWindow.h>
    #include <vtkRenderWindowInteractor.h>

    #include <vtkNamedColors.h>

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

      vtkSmartPointer<vtkLine> line0 =
        vtkSmartPointer<vtkLine>::New();
      line0->GetPointIds()->SetId(0, 0);
      line0->GetPointIds()->SetId(1, 1);

      vtkSmartPointer<vtkLine> line1 =
        vtkSmartPointer<vtkLine>::New();
      line1->GetPointIds()->SetId(0, 1);
      line1->GetPointIds()->SetId(1, 2);

      vtkSmartPointer<vtkLine> line2 =
        vtkSmartPointer<vtkLine>::New();
      line2->GetPointIds()->SetId(0, 2);
      line2->GetPointIds()->SetId(1, 3);

      vtkSmartPointer<vtkCellArray> lines =
        vtkSmartPointer<vtkCellArray>::New();
      lines->InsertNextCell(line0);
      lines->InsertNextCell(line1);
      lines->InsertNextCell(line2);

      vtkSmartPointer<vtkPolyData> polydata =
        vtkSmartPointer<vtkPolyData>::New();
      polydata->SetPoints(points);
      polydata->SetLines(lines);

      // Tell the polydata to build 'upward' links from points to cells.
      polydata->BuildLinks();
      // Mark a cell as deleted.
      polydata->DeleteCell(1);
      // Remove the marked cell.
      polydata->RemoveDeletedCells();

      // Visualize
      vtkSmartPointer<vtkNamedColors> colors =
        vtkSmartPointer<vtkNamedColors>::New();

      vtkSmartPointer<vtkPolyDataMapper> mapper =
        vtkSmartPointer<vtkPolyDataMapper>::New();
      mapper->SetInputDataObject(polydata);

      vtkSmartPointer<vtkActor> actor =
        vtkSmartPointer<vtkActor>::New();
      actor->SetMapper(mapper);
      actor->GetProperty()->SetColor(colors->GetColor3d("Peacock").GetData());
      actor->GetProperty()->SetLineWidth(4);

      vtkSmartPointer<vtkRenderer> renderer =
        vtkSmartPointer<vtkRenderer>::New();
      vtkSmartPointer<vtkRenderWindow> renderWindow =
        vtkSmartPointer<vtkRenderWindow>::New();
      renderWindow->AddRenderer(renderer);
      vtkSmartPointer<vtkRenderWindowInteractor> renderWindowInteractor =
          vtkSmartPointer<vtkRenderWindowInteractor>::New();
      renderWindowInteractor->SetRenderWindow(renderWindow);
      renderer->AddActor(actor);

      renderer->SetBackground(colors->GetColor3d("Silver").GetData());
      renderWindow->Render();
      renderWindowInteractor->Start();

      return EXIT_SUCCESS;
    }

### CMakeLists.txt

    cmake_minimum_required(VERSION 3.3 FATAL_ERROR)

    project(DeleteCells)

    find_package(VTK COMPONENTS
      vtkCommonColor
      vtkCommonCore
      vtkCommonDataModel
      vtkInteractionStyle
      vtkRenderingCore
      vtkRenderingFreeType
      vtkRenderingOpenGL2 QUIET)
    if (NOT VTK_FOUND)
      message("Skipping DeleteCells: ${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(DeleteCells MACOSX_BUNDLE DeleteCells.cxx )
      target_link_libraries(DeleteCells PRIVATE ${VTK_LIBRARIES})
    else ()
      # include all components
      add_executable(DeleteCells MACOSX_BUNDLE DeleteCells.cxx )
      target_link_libraries(DeleteCells PRIVATE ${VTK_LIBRARIES})
      # vtk_module_autoinit is needed
      vtk_module_autoinit(
        TARGETS DeleteCells
        MODULES ${VTK_LIBRARIES}
        )
    endif ()

### Download and Build DeleteCells

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

    cd DeleteCells/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:

    ./DeleteCells

WINDOWS USERS

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