# RemoveOutsideSurface

VTKExamples/Cxx/PolyData/RemoveOutsideSurface

### Description

This example removes the outer surface of a multiple surface vtkPolyData model. The examples 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 proceeds as follows:

1. Use vtkConnectivityFilter to label all of the regions in the model with a region id.

2. Cast a ray from outside the bounds of the model into the center of the model. Use a vtkCellLocator to intersect the ray with the model. The intersected cell will be on the outside surface of the model.

3. The vtkCellData of the intersected cell will have the region id of the cell.

4. Use a vtkCellIterator to visit each cell of the model. Cells whose region id is not equal to the interesected cell's region io are retained on a new vtkPolyData

The resulting vtkPolyData will contain all but the outside surface.

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

The gold model is the original polydata and the blue model is the original with tghe outer surface removed.

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

Seealso

ExtractOutsideSurface extracts only the outside surface.

### Code

RemoveOutsideSurface.cxx

    #include <vtkSmartPointer.h>
    #include <vtkConnectivityFilter.h>

    #include <vtkSphereSource.h>
    #include <vtkDataSetMapper.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 <vtkUnstructuredGrid.h>
    #include <vtkIdTypeArray.h>
    #include <vtkCellIterator.h>
    #include <vtkGenericCell.h>
    #include <vtkCellData.h>

    #include <vtkNamedColors.h>
    #include <vtkMath.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
      {
        vtkMath::RandomSeed(5070); // for testing
        vtkSmartPointer<vtkAppendPolyData> appendFilter =
          vtkSmartPointer<vtkAppendPolyData>::New();

        for (int i = 0; i < 20; ++i)
        {
          // Small spheres with most polgons
          vtkSmartPointer<vtkSphereSource> sphereSource1 =
            vtkSmartPointer<vtkSphereSource>::New();
          sphereSource1->SetThetaResolution(10);
          sphereSource1->SetPhiResolution(10);
          sphereSource1->SetCenter(vtkMath::Random(4.0, 14.0), vtkMath::Random(-3.0, 5.0), vtkMath::Random(4.0, 14.0));
          sphereSource1->SetRadius(1.0);
          appendFilter->AddInputConnection(sphereSource1->GetOutputPort());
        }

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

        appendFilter->AddInputConnection(sphereSource2->GetOutputPort());
        appendFilter->Update();
        polyData = appendFilter->GetOutput();
      }

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

      // Extract all regions and label cells with region number
      vtkSmartPointer<vtkConnectivityFilter> connectivityFilter =
          vtkSmartPointer<vtkConnectivityFilter>::New();
      connectivityFilter->SetInputData(polyData);
      connectivityFilter->SetExtractionModeToAllRegions();
      connectivityFilter->ColorRegionsOn();
      connectivityFilter->Update();

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

      vtkIdTypeArray *cd =
        vtkIdTypeArray::SafeDownCast(connectivityFilter->GetOutput()->GetCellData()->GetScalars());
      vtkIdType outsideRegionId = cd->GetTuple1(cellId);
      std::cout << "Id of cell on outside surface: " << cellId << std::endl;
      std::cout << "CellData at " << cellId << ": " << outsideRegionId << std::endl;

      // Build a polydata from cells that are not in the outside surface
      // Iterate over the original cells
      vtkSmartPointer<vtkPolyData> insidePolyData =
        vtkSmartPointer<vtkPolyData>::New();
      insidePolyData->Allocate();
      insidePolyData->SetPoints(connectivityFilter->GetOutput()->GetPoints());

      vtkSmartPointer<vtkGenericCell> cell =
        vtkSmartPointer<vtkGenericCell>::New();
      vtkSmartPointer<vtkCellIterator> it =
        connectivityFilter->GetOutput()->NewCellIterator();
      vtkIdType originalCellId = 0;
      for (it->InitTraversal();
           !it->IsDoneWithTraversal();
           it->GoToNextCell(), ++originalCellId)
      {
        //  Retain cell if it is NOT an outside cell
        if (cd->GetTuple1(originalCellId) != outsideRegionId)
        {
          it->GetCell(cell);
          insidePolyData->InsertNextCell(it->GetCellType(), cell->GetPointIds());
        }
      }

      // 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->SetInputData(insidePolyData);

      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(RemoveOutsideSurface)

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

### Download and Build RemoveOutsideSurface

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

    cd RemoveOutsideSurface/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:

    ./RemoveOutsideSurface

WINDOWS USERS

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