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# ExternalContour

VTKEx/Cxx/PolyData/ExternalContour

### Description

Compute the external contour of a polydata.
At first, it creates a black and white image of a scene containing the polydata and then extracts the contour of the black shape from the image.

Question

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### Code

ExternalContour.cxx

    #include <vtkSmartPointer.h>
    #include <vtkSphereSource.h>
    #include <vtkPolyData.h>
    #include <vtkWindowToImageFilter.h>
    #include <vtkContourFilter.h>
    #include <vtkTransform.h>
    #include <vtkTransformPolyDataFilter.h>

    #include <vtkPolyDataMapper.h>
    #include <vtkActor.h>
    #include <vtkProperty.h>
    #include <vtkCamera.h>
    #include <vtkRenderWindow.h>
    #include <vtkRenderer.h>
    #include <vtkRenderWindowInteractor.h>
    #include <vtkInteractorStyleTrackballCamera.h>
    #include <vtkVersion.h>

    #include <vtkXMLPolyDataReader.h>

    int main(int argc, char *argv[])
    {
      vtkSmartPointer<vtkPolyData> data3d;

      if ( argc > 1 )
      {
        vtkSmartPointer<vtkXMLPolyDataReader> reader =
          vtkSmartPointer<vtkXMLPolyDataReader>::New();
        reader->SetFileName (argv[1]);
        reader->Update();

        data3d = reader->GetOutput();
      }
      else
      {
        vtkSmartPointer<vtkSphereSource> source =
          vtkSmartPointer<vtkSphereSource>::New();
        source->SetCenter(0.0, 0.0, 5.0);
        source->SetRadius(2.0);
        source->SetPhiResolution(20.);
        source->SetThetaResolution(20.);
        source->Update();

        data3d = source->GetOutput();
      }

      double bounds_data[6], center_data[3];
      data3d->GetBounds(bounds_data);
      data3d->GetCenter(center_data);

      // Black and white scene with the data in order to print the view
      vtkSmartPointer<vtkPolyDataMapper> mapper_data =
        vtkSmartPointer<vtkPolyDataMapper>::New();
      mapper_data->SetInputData(data3d);

      vtkSmartPointer<vtkActor> actor_data =
        vtkSmartPointer<vtkActor>::New();
      actor_data->SetMapper(mapper_data);
      actor_data->GetProperty()->SetColor(0,0,0);

      vtkSmartPointer<vtkRenderer> tmp_rend =
        vtkSmartPointer<vtkRenderer>::New();
      tmp_rend->SetBackground(1,1,1);

      tmp_rend->AddActor(actor_data);
      tmp_rend->ResetCamera();
      tmp_rend->GetActiveCamera()->SetParallelProjection(1);

      vtkSmartPointer<vtkRenderWindow> tmp_rW =
        vtkSmartPointer<vtkRenderWindow>::New();
      tmp_rW->SetOffScreenRendering( 1 );
      tmp_rW->AddRenderer(tmp_rend);

      tmp_rW->Render();

      // Get a print of the window
      vtkSmartPointer<vtkWindowToImageFilter> windowToImageFilter =
        vtkSmartPointer<vtkWindowToImageFilter>::New();
      windowToImageFilter->SetInput(tmp_rW);
    #if VTK_MAJOR_VERSION >= 8 || VTK_MAJOR_VERSION == 8 && VTK_MINOR_VERSION >= 90
      windowToImageFilter->SetScale(2); // image quality
    #else
      windowToImageFilter->SetMagnification(2); //image quality
    #endif
      windowToImageFilter->Update();

      // Extract the silhouette corresponding to the black limit of the image
      vtkSmartPointer<vtkContourFilter> ContFilter = vtkSmartPointer<vtkContourFilter>::New();
      ContFilter->SetInputConnection(windowToImageFilter->GetOutputPort());
      ContFilter->SetValue(0,255);
      ContFilter->Update();

      // Make the contour coincide with the data.
      vtkSmartPointer<vtkPolyData> contour = ContFilter->GetOutput();

      double bounds_contour[6], center_contour[3];
      double trans_x=0., trans_y=0., trans_z=0., ratio_x=0., ratio_y=0.;
      contour->GetBounds(bounds_contour);

      ratio_x = (bounds_data[1]-bounds_data[0])/(bounds_contour[1]-bounds_contour[0]);
      ratio_y = (bounds_data[3]-bounds_data[2])/(bounds_contour[3]-bounds_contour[2]);

      // Rescale the contour so that it shares the same bounds as the
      // input data
      vtkSmartPointer<vtkTransform>transform1 =
        vtkSmartPointer<vtkTransform>::New();
      transform1->Scale( ratio_x, ratio_y, 1.);

      vtkSmartPointer<vtkTransformPolyDataFilter> tfilter1 =
        vtkSmartPointer<vtkTransformPolyDataFilter>::New();
      tfilter1->SetInputData(contour);
      tfilter1->SetTransform(transform1);
      tfilter1->Update();

      contour = tfilter1->GetOutput();

      // Translate the contour so that it shares the same center as the
      // input data
      contour->GetCenter(center_contour);
      trans_x = center_data[0]-center_contour[0];
      trans_y = center_data[1]-center_contour[1];
      trans_z = center_data[2]-center_contour[2];

      vtkSmartPointer<vtkTransform> transform2 =
        vtkSmartPointer<vtkTransform>::New();
      transform2->Translate( trans_x, trans_y, trans_z);

      vtkSmartPointer<vtkTransformPolyDataFilter> tfilter2 =
        vtkSmartPointer<vtkTransformPolyDataFilter>::New();
      tfilter2->SetInputData(contour);
      tfilter2->SetTransform(transform2);
      tfilter2->Update();

      contour = tfilter2->GetOutput();

      // Render the result : Input data + resulting silhouette

      // Updating the color of the data
      actor_data->GetProperty()->SetColor(0.9,0.9,0.8);

      // Create a mapper and actor of the silhouette
      vtkSmartPointer<vtkPolyDataMapper> mapper_contour =
        vtkSmartPointer<vtkPolyDataMapper>::New();
      mapper_contour->SetInputData(contour);

      vtkSmartPointer<vtkActor> actor_contour =
        vtkSmartPointer<vtkActor>::New();
      actor_contour->SetMapper(mapper_contour);
      actor_contour->GetProperty()->SetLineWidth(2.);

      // 2 renderers and a render window
      vtkSmartPointer<vtkRenderer> renderer1 =
        vtkSmartPointer<vtkRenderer>::New();
      renderer1->AddActor(actor_data);

      vtkSmartPointer<vtkRenderer> renderer2 =
        vtkSmartPointer<vtkRenderer>::New();
      renderer2->AddActor(actor_contour);

      vtkSmartPointer<vtkRenderWindow> renderwindow =
        vtkSmartPointer<vtkRenderWindow>::New();
      renderwindow->SetSize(400,400);

      renderwindow->AddRenderer(renderer1);
      renderer1->SetViewport(0., 0., 0.5, 1.);

      renderwindow->AddRenderer(renderer2);
      renderer2->SetViewport(0.5, 0., 1., 1.);

      vtkSmartPointer<vtkInteractorStyleTrackballCamera> style =
        vtkSmartPointer<vtkInteractorStyleTrackballCamera>::New();
      vtkSmartPointer<vtkRenderWindowInteractor> iren =
        vtkSmartPointer<vtkRenderWindowInteractor>::New();

      iren->SetRenderWindow( renderwindow);
      iren->SetInteractorStyle(style);

      renderwindow->Render();
      iren->Start();

      return EXIT_SUCCESS;
    }

### CMakeLists.txt

    cmake_minimum_required(VERSION 3.3 FATAL_ERROR)

    project(ExternalContour)

    find_package(VTK COMPONENTS
      vtkCommonCore
      vtkCommonDataModel
      vtkCommonTransforms
      vtkFiltersCore
      vtkFiltersGeneral
      vtkFiltersSources
      vtkIOXML
      vtkInteractionStyle
      vtkRenderingContextOpenGL2
      vtkRenderingCore
      vtkRenderingFreeType
      vtkRenderingGL2PSOpenGL2
      vtkRenderingOpenGL2 QUIET)
    if (NOT VTK_FOUND)
      message("Skipping ExternalContour: ${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(ExternalContour MACOSX_BUNDLE ExternalContour.cxx )
      target_link_libraries(ExternalContour PRIVATE ${VTK_LIBRARIES})
    else ()
      # include all components
      add_executable(ExternalContour MACOSX_BUNDLE ExternalContour.cxx )
      target_link_libraries(ExternalContour PRIVATE ${VTK_LIBRARIES})
      # vtk_module_autoinit is needed
      vtk_module_autoinit(
        TARGETS ExternalContour
        MODULES ${VTK_LIBRARIES}
        )
    endif ()

### Download and Build ExternalContour

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

    cd ExternalContour/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:

    ./ExternalContour

WINDOWS USERS

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