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# ExtractOutsideSurface

VTKEx/Cxx/PolyData/ExtractOutsideSurface

### Description

This example extracts the outer surface of a multiple surface vtkPolyData model. The example assumes the outside surface is closed and there is only one outside surface. It also assumes that the center of the model lies inside the outer surface.

The example casts a ray from outside the bounds of the model into the center of the model. It uses a vtkCellLocator to intersect the ray with the model. The intersected cell is used as a seed to the vtkPolyDataConnectivityFilter which is run with the extraction mode set to CellSeededRegions.

If run without arguments, the example generates a small (in area) sphere inside a large (in area) sphere. The inner sphere has more cells than the outer sphere, but should not be extracted.

The gold model is the original vtkPolyData and the blue model is the outer surface.

You can try your own surface by specifying a XML PolyData file (.vtp) on the command line.

Seealso

RemoveOutsideSurface removes the outside surface.

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

ExtractOutsideSurface.cxx

    #include <vtkSmartPointer.h>
    #include <vtkPolyDataConnectivityFilter.h>

    #include <vtkSphereSource.h>
    #include <vtkPolyDataMapper.h>
    #include <vtkActor.h>
    #include <vtkProperty.h>
    #include <vtkRenderer.h>
    #include <vtkRenderWindow.h>
    #include <vtkRenderWindowInteractor.h>
    #include <vtkAppendPolyData.h>
    #include <vtkCellLocator.h>
    #include <vtkXMLPolyDataReader.h>
    #include <vtkTriangleFilter.h>

    #include <vtkNamedColors.h>

    int main(int argc, char *argv[])
    {
      // PolyData to process
      vtkSmartPointer<vtkPolyData> polyData;

      if (argc > 1)
      {
        vtkSmartPointer<vtkXMLPolyDataReader> reader =
          vtkSmartPointer<vtkXMLPolyDataReader>::New();
        reader->SetFileName(argv[1]);
        reader->Update();
        polyData = reader->GetOutput();
      }
      else
      {
        // Small sphere with most polgons
        vtkSmartPointer<vtkSphereSource> sphereSource1 =
          vtkSmartPointer<vtkSphereSource>::New();
        sphereSource1->SetThetaResolution(10);
        sphereSource1->SetPhiResolution(10);
        sphereSource1->SetCenter(5, 5, 5);
        sphereSource1->SetRadius(1.5);

        // Large sphere with least polygons
        vtkSmartPointer<vtkSphereSource> sphereSource2 =
          vtkSmartPointer<vtkSphereSource>::New();
        sphereSource2->SetRadius(10);
        sphereSource2->SetCenter(10,1,10);

        vtkSmartPointer<vtkAppendPolyData> appendFilter =
          vtkSmartPointer<vtkAppendPolyData>::New();
        appendFilter->AddInputConnection(sphereSource1->GetOutputPort());
        appendFilter->AddInputConnection(sphereSource2->GetOutputPort());
        appendFilter->Update();
        polyData = appendFilter->GetOutput();
      }

      double center[3], bounds[6];
      polyData->GetCenter(center);
      std::cout << "Center of data is: "
                << center[0] << ", "
                << center[1] << ", "
                << center[2] << std::endl;
      polyData->GetPoints()->GetBounds(bounds);
      std::cout << "Bounds of data is: "
                << bounds[0] << ", "
                << bounds[1] << ", "
                << bounds[2] << ", "
                << bounds[3] << ", "
                << bounds[4] << ", "
                << bounds[5] << std::endl;

      // Build a cell locator.
      vtkSmartPointer<vtkCellLocator> cellLocator =
        vtkSmartPointer<vtkCellLocator>::New();
      cellLocator->SetDataSet(polyData);
      cellLocator->BuildLocator();

      // Now fire a ray from outside the bounds to the center and find a
      // cell. This cell should be on the outside surface
      double rayStart[3];
      for (unsigned int i = 0; i < 3; i++)
      {
        rayStart[i] = bounds[2 * i + 1] * 1.1;
      }

      vtkIdType cellId = -1;
      double xyz[3], t, pcoords[3];
      int subId;

      cellLocator->IntersectWithLine(
        rayStart,
        center,
        0.0001,
        t,
        xyz,
        pcoords,
        subId,
        cellId);
      std::cout << "Id of cell on outside surface: " << cellId << std::endl;

      vtkSmartPointer<vtkPolyDataConnectivityFilter> connectivityFilter =
          vtkSmartPointer<vtkPolyDataConnectivityFilter>::New();
      connectivityFilter->SetInputData(polyData);
      connectivityFilter->SetExtractionModeToCellSeededRegions();
      connectivityFilter->InitializeSeedList();
      connectivityFilter->AddSeed(cellId);

      // Create a mapper and actor for original data
        vtkSmartPointer<vtkPolyDataMapper> originalMapper =
          vtkSmartPointer<vtkPolyDataMapper>::New();
      originalMapper->SetInputData(polyData);

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

      vtkSmartPointer<vtkActor> originalActor =
          vtkSmartPointer<vtkActor>::New();
      originalActor->SetMapper(originalMapper);
      originalActor->GetProperty()->BackfaceCullingOn();
      originalActor->GetProperty()->SetOpacity(.5);;
      originalActor->GetProperty()->SetColor(colors->GetColor3d("Gold").GetData());

      // Create a mapper and actor for extracted data
      vtkSmartPointer<vtkPolyDataMapper> extractedMapper =
          vtkSmartPointer<vtkPolyDataMapper>::New();
      extractedMapper->SetInputConnection(connectivityFilter->GetOutputPort());

      vtkSmartPointer<vtkActor> extractedActor =
          vtkSmartPointer<vtkActor>::New();
      extractedActor->GetProperty()->SetColor(colors->GetColor3d("Peacock").GetData());
      extractedActor->SetMapper(extractedMapper);
      extractedActor->GetProperty()->SetOpacity(.5);;
      extractedActor->GetProperty()->BackfaceCullingOn();

      // Create a renderer
      vtkSmartPointer<vtkRenderer> renderer =
          vtkSmartPointer<vtkRenderer>::New();
      renderer->AddActor(originalActor);
      renderer->AddActor(extractedActor);

      renderer->GradientBackgroundOn();
      renderer->SetBackground2(colors->GetColor3d("Beige").GetData());
      renderer->SetBackground(colors->GetColor3d("Burlywood").GetData());

      extractedActor->SetPosition((bounds[1] - bounds[0]) / 1.9, 0, 0);
      originalActor->SetPosition(-(bounds[1] - bounds[0]) / 1.9, 0, 0);
      // Create a render window
      vtkSmartPointer<vtkRenderWindow> renwin =
          vtkSmartPointer<vtkRenderWindow>::New();
      renwin->AddRenderer(renderer);
      renwin->SetSize(512, 512);

      // Create an interactor
      vtkSmartPointer<vtkRenderWindowInteractor> iren =
          vtkSmartPointer<vtkRenderWindowInteractor>::New();
      iren->SetRenderWindow(renwin);
      renwin->Render();
      iren->Initialize();
      iren->Start();

      return EXIT_SUCCESS;
    }

### CMakeLists.txt

    cmake_minimum_required(VERSION 3.3 FATAL_ERROR)

    project(ExtractOutsideSurface)

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

### Download and Build ExtractOutsideSurface

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

    cd ExtractOutsideSurface/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:

    ./ExtractOutsideSurface

WINDOWS USERS

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