LineWidget2

#include <vtkPolyDataMapper.h>
#include <vtkActor.h>
#include <vtkRenderWindow.h>
#include <vtkRenderer.h>
#include <vtkRenderWindowInteractor.h>
#include <vtkPolyData.h>
#include <vtkSmartPointer.h>
#include <vtkSphereSource.h>
#include <vtkLineWidget2.h>
#include <vtkLineRepresentation.h>
#include <vtkCommand.h>

// This does the actual work.
// Callback for the interaction
class vtkLineCallback : public vtkCommand
{
  public:
    static vtkLineCallback *New()
    {
      return new vtkLineCallback;
    }

    virtual void Execute(vtkObject *caller, unsigned long, void*)
    {

      vtkLineWidget2 *lineWidget = 
          reinterpret_cast<vtkLineWidget2*>(caller);

      // Get the actual box coordinates of the line
      vtkSmartPointer<vtkPolyData> polydata = 
          vtkSmartPointer<vtkPolyData>::New();
      static_cast<vtkLineRepresentation*>(lineWidget->GetRepresentation())->GetPolyData (polydata);

      // Display one of the points, just so we know it's working
      double p[3];
      polydata->GetPoint(0,p);
      std::cout << "P: " << p[0] << " " << p[1] << " " << p[2] << std::endl;
    }
    vtkLineCallback(){}

};


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

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

  // A renderer and render window
  vtkSmartPointer<vtkRenderer> renderer = 
    vtkSmartPointer<vtkRenderer>::New();
  vtkSmartPointer<vtkRenderWindow> renderWindow = 
    vtkSmartPointer<vtkRenderWindow>::New();
  renderWindow->AddRenderer(renderer);
  //renderer->AddActor(actor);

  // An interactor
  vtkSmartPointer<vtkRenderWindowInteractor> renderWindowInteractor = 
    vtkSmartPointer<vtkRenderWindowInteractor>::New();
  renderWindowInteractor->SetRenderWindow(renderWindow);

  vtkSmartPointer<vtkLineWidget2> lineWidget = 
    vtkSmartPointer<vtkLineWidget2>::New();
  lineWidget->SetInteractor(renderWindowInteractor);
  lineWidget->CreateDefaultRepresentation();

  // You could do this if you want to set properties at this point:
  //vtkSmartPointer<vtkLineRepresentation> lineRepresentation = 
    //vtkSmartPointer<vtkLineRepresentation>::New();
  //lineWidget->SetRepresentation(lineRepresentation);

  vtkSmartPointer<vtkLineCallback> lineCallback = 
    vtkSmartPointer<vtkLineCallback>::New();

  lineWidget->AddObserver(vtkCommand::InteractionEvent,lineCallback);

  // Render
  renderWindow->Render();

  renderWindowInteractor->Initialize();
  renderWindow->Render();
  lineWidget->On();

  // Begin mouse interaction
  renderWindowInteractor->Start();

  return EXIT_SUCCESS;
}

cmake_minimum_required(VERSION 3.3 FATAL_ERROR)

project(LineWidget2)

find_package(VTK COMPONENTS 
  vtkCommonCore
  vtkCommonDataModel
  vtkFiltersSources
  vtkInteractionStyle
  vtkInteractionWidgets
  vtkRenderingCore
  vtkRenderingFreeType
  vtkRenderingOpenGL2 QUIET)
if (NOT VTK_FOUND)
  message("Skipping LineWidget2: ${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(LineWidget2 MACOSX_BUNDLE LineWidget2.cxx )
  target_link_libraries(LineWidget2 PRIVATE ${VTK_LIBRARIES})
else ()
  # include all components
  add_executable(LineWidget2 MACOSX_BUNDLE LineWidget2.cxx )
  target_link_libraries(LineWidget2 PRIVATE ${VTK_LIBRARIES})
  # vtk_module_autoinit is needed
  vtk_module_autoinit(
    TARGETS LineWidget2
    MODULES ${VTK_LIBRARIES}
    )
endif () 

cd LineWidget2/build 

cmake ..

cmake -DVTK_DIR:PATH=/home/me/vtk_build ..

make

./LineWidget2