# ExtractEdges

VTKExamples/Cxx/Meshes/ExtractEdges

### Description

This example shows how to extract the edges from a PolyData. In this case, we convert a sphere into a wireframe sphere. We demonstrate how to traverse the resulting edges.

Other Languages

See (CSharp)

### Code

ExtractEdges.cxx

    #include <vtkSmartPointer.h>
    #include <vtkPolyData.h>
    #include <vtkExtractEdges.h>
    #include <vtkSphereSource.h>
    #include <vtkCellArray.h>
    #include <vtkPoints.h>
    #include <vtkPolyDataMapper.h>
    #include <vtkActor.h>
    #include <vtkRenderWindow.h>
    #include <vtkRenderer.h>
    #include <vtkRenderWindowInteractor.h>
    #include <vtkLine.h>

    int main(int, char *[])
    {
      vtkSmartPointer<vtkSphereSource> sphereSource =
        vtkSmartPointer<vtkSphereSource>::New();
      sphereSource->Update();

      std::cout << "Sphere" << endl << "----------" << endl;
      std::cout << "There are " << sphereSource->GetOutput()->GetNumberOfCells() << " cells." << std::endl;
      std::cout << "There are " << sphereSource->GetOutput()->GetNumberOfPoints() << " points." << std::endl;

      vtkSmartPointer<vtkExtractEdges> extractEdges =
        vtkSmartPointer<vtkExtractEdges>::New();
      extractEdges->SetInputConnection(sphereSource->GetOutputPort());
      extractEdges->Update();

      vtkCellArray* lines= extractEdges->GetOutput()->GetLines();
      vtkPoints* points = extractEdges->GetOutput()->GetPoints();

      std::cout << std::endl << "Edges" << endl << "----------" << std::endl;
      std::cout << "There are " << lines->GetNumberOfCells() << " cells." << std::endl;
      std::cout << "There are " << points->GetNumberOfPoints() << " points." << std::endl;

      // Traverse all of the edges
      for(vtkIdType i = 0; i < extractEdges->GetOutput()->GetNumberOfCells(); i++)
      {
        //std::cout << "Type: " << extractEdges->GetOutput()->GetCell(i)->GetClassName() << std::endl;
        vtkSmartPointer<vtkLine> line = vtkLine::SafeDownCast(extractEdges->GetOutput()->GetCell(i));
        std::cout << "Line " << i << " : " << *line << std::endl;
      }

      // Visualize the edges

      // Create a mapper and actor
      vtkSmartPointer<vtkPolyDataMapper> mapper =
        vtkSmartPointer<vtkPolyDataMapper>::New();
      mapper->SetInputConnection(extractEdges->GetOutputPort());
      vtkSmartPointer<vtkActor> actor =
        vtkSmartPointer<vtkActor>::New();
      actor->SetMapper(mapper);

      // Create a renderer, render window, and interactor
      vtkSmartPointer<vtkRenderer> renderer =
        vtkSmartPointer<vtkRenderer>::New();
      vtkSmartPointer<vtkRenderWindow> renderWindow =
        vtkSmartPointer<vtkRenderWindow>::New();
      renderWindow->AddRenderer(renderer);
      vtkSmartPointer<vtkRenderWindowInteractor> renderWindowInteractor =
        vtkSmartPointer<vtkRenderWindowInteractor>::New();
      renderWindowInteractor->SetRenderWindow(renderWindow);

      // Add the actor to the scene
      renderer->AddActor(actor);
      renderer->SetBackground(1,1,1); // Background color white

      // Render and interact
      renderWindow->Render();
      renderWindowInteractor->Start();

      return EXIT_SUCCESS;
    }

### CMakeLists.txt

    cmake_minimum_required(VERSION 3.3 FATAL_ERROR)

    project(ExtractEdges)

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

### Download and Build ExtractEdges

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

    cd ExtractEdges/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:

    ./ExtractEdges

WINDOWS USERS

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