vtkSelectVisiblePoints filter output has 0 points - c++

Question

I’m trying to obtain a polyData with only the visible part of a 3D model.
To that extent, I’m passing the original data through a vtkSelectVisiblePoints filter.
I’m using a mock renderer, mapper and actor because I want to post-process the visible points before displaying them.
However, the output of the vtkSelectVisiblePoints filter contains “0” points for some reason…
Using the following example:
http://www.vtk.org/Wiki/VTK/Examples/Cxx/PolyData/SelectVisiblePoints
I came up with the following piece of code:
// Render window and interactor
vtkSmartPointer<vtkRenderWindow> renderWindow =
vtkSmartPointer<vtkRenderWindow>::New();
renderWindow->SetSize(800, 800);
vtkSmartPointer<vtkRenderWindowInteractor> renderWindowInteractor =
vtkSmartPointer<vtkRenderWindowInteractor>::New();
renderWindowInteractor->SetRenderWindow(renderWindow);
// Mock renderer, mapper and actor
vtkSmartPointer<vtkRenderer> mockRenderer =
vtkSmartPointer<vtkRenderer>::New();
renderWindow->AddRenderer(mockRenderer);
vtkSmartPointer<vtkPolyDataMapper> mockMapper =
vtkSmartPointer<vtkPolyDataMapper>::New();
mockMapper->SetInput(reader->GetOutput());
vtkSmartPointer<vtkActor> mockActor = vtkSmartPointer<vtkActor>::New();
mockActor->SetMapper(mockMapper);
mockRenderer->AddActor(mockActor);
// Set camera to the correct position
mockRenderer->GetActiveCamera()->SetPosition(0, -1, 0);
mockRenderer->GetActiveCamera()->SetFocalPoint(0, 0, 0);
mockRenderer->GetActiveCamera()->SetViewUp(0, 1, 0);
mockRenderer->ResetCamera();
vtkSmartPointer<vtkSelectVisiblePoints> selectVisiblePoints =
vtkSmartPointer<vtkSelectVisiblePoints>::New();
selectVisiblePoints->SetInput(reader->GetOutput());
selectVisiblePoints->SetRenderer(mockRenderer);
selectVisiblePoints->Update();
std::cout << "Visible nr of points = " << selectVisiblePoints->GetOutput()->GetNumberOfPoints() << std::endl;
renderWindow->RemoveRenderer(mockRenderer);
… prints 0.
However, if I call renderWindow->Render(), the visible part of the model is correctly displayed...
Am I missing anything...?

Answer 1

The answer was right there, in the documentation of the filter:
Warning You must carefully synchronize the execution of this filter.
The filter refers to a renderer, which is modified every time a render
occurs. Therefore, the filter is always out of date, and always
executes. You may have to perform two rendering passes
It worked as expected if I added the //new lines, like Arnas also suggested in his comment:
vtkSmartPointer<vtkSelectVisiblePoints> selectVisiblePoints =
vtkSmartPointer<vtkSelectVisiblePoints>::New();
selectVisiblePoints->SetInput(originalData);
selectVisiblePoints->SetRenderer(renderer);
selectVisiblePoints->Update();
renderWindow->Render(); // new
selectVisiblePoints->Update(); // new
std::cout << "Visible nr of points = " << selectVisiblePoints->GetOutput()->GetNumberOfPoints() << std::endl;