VTK/Examples/Cxx/PolyData/PointsProjectedHull

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) ).

PointsProjectedHull.cxx

#include <vtkVersion.h>
#include <vtkSmartPointer.h>

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

int main(int, char *[])
{
  vtkSmartPointer<vtkPointSource> pointSource = 
    vtkSmartPointer<vtkPointSource>::New();
  pointSource->SetNumberOfPoints(40);
  pointSource->Update();
  
  //setup actor and mapper
  vtkSmartPointer<vtkPolyDataMapper> pointMapper = 
    vtkSmartPointer<vtkPolyDataMapper>::New();
  pointMapper->SetInputConnection(pointSource->GetOutputPort());
 
  vtkSmartPointer<vtkActor> pointActor = 
    vtkSmartPointer<vtkActor>::New();
  pointActor->SetMapper(pointMapper);
  
  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);
  
  // Setup actor and mapper
  vtkSmartPointer<vtkPolyDataMapper> xHullMapper = 
    vtkSmartPointer<vtkPolyDataMapper>::New();
#if VTK_MAJOR_VERSION <= 5
  xHullMapper->SetInput(polyData);
#else
  xHullMapper->SetInputData(polyData);
#endif
 
  vtkSmartPointer<vtkActor> xHullActor = 
    vtkSmartPointer<vtkActor>::New();
  xHullActor->SetMapper(xHullMapper);
 
  // 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->AddActor(xHullActor);
  renderer->AddActor(pointActor);
 
  // Rotate camera
  renderer->GetActiveCamera()->Azimuth(90);
  renderer->ResetCamera();

  renderWindow->Render();
  renderWindowInteractor->Start();
  
  return EXIT_SUCCESS;
}

Please try the new VTKExamples website.

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)

if(VTK_LIBRARIES)
  target_link_libraries(PointsProjectedHull ${VTK_LIBRARIES})
else()
  target_link_libraries(PointsProjectedHull vtkHybrid vtkWidgets)
endif()

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