# PointsProjectedHull

VTKExamples/Cxx/PolyData/PointsProjectedHull

### Description

This example demonstrates how to find the 2D convex hull of a point set projected onto any of the coordinate planes (in this case, we choose the YZ plane (projection along the x axis) ).

### Code

PointsProjectedHull.cxx

    #include <vtkSmartPointer.h>

    #include <vtkActor.h>
    #include <vtkCamera.h>
    #include <vtkCellArray.h>
    #include <vtkGlyph3DMapper.h>
    #include <vtkPointSource.h>
    #include <vtkPoints.h>
    #include <vtkPointsProjectedHull.h>
    #include <vtkPolyData.h>
    #include <vtkPolyDataMapper.h>
    #include <vtkPolyLine.h>
    #include <vtkProperty.h>
    #include <vtkRenderWindow.h>
    #include <vtkRenderWindowInteractor.h>
    #include <vtkRenderer.h>
    #include <vtkSphereSource.h>
    #include <vtkTubeFilter.h>
    #include <vtkXMLPolyDataWriter.h>

    #include <vtkNamedColors.h>

    int main(int, char *[])
    {
      vtkSmartPointer<vtkNamedColors> colors =
        vtkSmartPointer<vtkNamedColors>::New();

      vtkSmartPointer<vtkSphereSource> sphere =
        vtkSmartPointer<vtkSphereSource>::New();
      sphere->SetPhiResolution(21);
      sphere->SetThetaResolution(21);
      sphere->SetRadius(.01);

      vtkSmartPointer<vtkPointSource> pointSource =
        vtkSmartPointer<vtkPointSource>::New();
      pointSource->SetNumberOfPoints(40);
      pointSource->Update();

      // Setup actor and mapper
      vtkSmartPointer<vtkGlyph3DMapper> pointMapper =
        vtkSmartPointer<vtkGlyph3DMapper>::New();
      pointMapper->SetInputConnection(pointSource->GetOutputPort());
      pointMapper->SetSourceConnection(sphere->GetOutputPort());

      vtkSmartPointer<vtkActor> pointActor =
        vtkSmartPointer<vtkActor>::New();
      pointActor->SetMapper(pointMapper);
      pointActor->GetProperty()->SetColor(colors->GetColor3d("Peacock").GetData());

      vtkSmartPointer<vtkPointsProjectedHull> points =
        vtkSmartPointer<vtkPointsProjectedHull>::New();
      points->DeepCopy(pointSource->GetOutput()->GetPoints());

      int xSize = points->GetSizeCCWHullX();
      std::cout << "xSize: " << xSize << std::endl;

      double* pts = new double[xSize*2];

      points->GetCCWHullX(pts,xSize);

      vtkSmartPointer<vtkPoints> xHullPoints =
        vtkSmartPointer<vtkPoints>::New();
      for(int i = 0; i < xSize; i++)
      {
        double yval = pts[2*i];
        double zval = pts[2*i + 1];
        std::cout << "(y,z) value of point " << i << " : ("
                  << yval << " , " << zval << ")" << std::endl;
        xHullPoints->InsertNextPoint(0.0, yval, zval);
      }
      // Insert the first point again to close the loop
      xHullPoints->InsertNextPoint(0.0, pts[0], pts[1]);

      // Display the x hull
      vtkSmartPointer<vtkPolyLine> xPolyLine =
        vtkSmartPointer<vtkPolyLine>::New();
      xPolyLine->GetPointIds()->SetNumberOfIds(xHullPoints->GetNumberOfPoints());

      for(vtkIdType i = 0; i < xHullPoints->GetNumberOfPoints(); i++)
      {
        xPolyLine->GetPointIds()->SetId(i,i);
      }

      // Create a cell array to store the lines in and add the lines to it
      vtkSmartPointer<vtkCellArray> cells =
        vtkSmartPointer<vtkCellArray>::New();
      cells->InsertNextCell(xPolyLine);

      // Create a polydata to store everything in
      vtkSmartPointer<vtkPolyData> polyData =
        vtkSmartPointer<vtkPolyData>::New();

      // Add the points to the dataset
      polyData->SetPoints(xHullPoints);

      // Add the lines to the dataset
      polyData->SetLines(cells);

      // Create tubes around the lines
      vtkSmartPointer<vtkTubeFilter> tubes =
        vtkSmartPointer<vtkTubeFilter>::New();
      tubes->SetInputData(polyData);
      tubes->SetRadius(.005);
      tubes->SetNumberOfSides(21);

      // Setup actor and mapper
      vtkSmartPointer<vtkPolyDataMapper> xHullMapper =
        vtkSmartPointer<vtkPolyDataMapper>::New();
      xHullMapper->SetInputConnection(tubes->GetOutputPort());

      vtkSmartPointer<vtkActor> xHullActor =
        vtkSmartPointer<vtkActor>::New();
      xHullActor->SetMapper(xHullMapper);
      xHullActor->GetProperty()->SetColor(colors->GetColor3d("Tomato").GetData());

      // Setup render window, renderer, 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);

      renderer->SetBackground(colors->GetColor3d("Burlywood").GetData());
      renderer->AddActor(xHullActor);
      renderer->AddActor(pointActor);

      // Rotate camera
      renderer->GetActiveCamera()->Azimuth(90);
      renderer->ResetCamera();

      renderWindow->Render();
      renderWindowInteractor->Start();

      return EXIT_SUCCESS;
    }

### CMakeLists.txt

    cmake_minimum_required(VERSION 2.8)

    PROJECT(PointsProjectedHull)

    find_package(VTK REQUIRED)
    include(${VTK_USE_FILE})

    add_executable(PointsProjectedHull MACOSX_BUNDLE PointsProjectedHull.cxx )

    target_link_libraries(PointsProjectedHull ${VTK_LIBRARIES})

### Download and Build PointsProjectedHull

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

    cd PointsProjectedHull/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:

    ./PointsProjectedHull

WINDOWS USERS

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