[vtkusers] vtkWindowToImageFilter - black image

[ingvar john]

Hello everyone.
This is my first time with VTK and first time I am posting to this list. 
Does anyone have an idea why this code produces black image as a screenshot (attached). I installed vtk 6.1 on Ubuntu 14.04 (on virtual machine). This is just the example I found on http://www.cmake.org/Wiki/VTK/Examples/Cxx/Utilities/Screenshot for saving a screenshot of the render window. I looked around during 2 days but I haven't find a solution.
  
Thanks a lot.

#include <vtkPolyDataMapper.h>
#include <vtkActor.h>
#include <vtkSmartPointer.h>
#include <vtkRenderWindow.h>
#include <vtkRenderer.h>
#include <vtkRenderWindowInteractor.h>
#include <vtkPolyData.h>
#include <vtkSphereSource.h>
#include <vtkWindowToImageFilter.h>
#include <vtkPNGWriter.h>
 


int main(int, char *[])
{
  vtkSmartPointer<vtkSphereSource> sphereSource = 
    vtkSmartPointer<vtkSphereSource>::New();
  sphereSource->SetCenter(0.0, 0.0, 0.0);
  sphereSource->SetRadius(5.0);
  sphereSource->Update();
 
  // Visualize
  vtkSmartPointer<vtkPolyDataMapper> mapper = 
    vtkSmartPointer<vtkPolyDataMapper>::New();
  mapper->SetInputConnection(sphereSource->GetOutputPort());
 
  vtkSmartPointer<vtkActor> actor = 
    vtkSmartPointer<vtkActor>::New();
  actor->SetMapper(mapper);
 
  vtkSmartPointer<vtkRenderer> renderer = 
    vtkSmartPointer<vtkRenderer>::New();
  vtkSmartPointer<vtkRenderWindow> renderWindow = 
    vtkSmartPointer<vtkRenderWindow>::New();
  renderWindow->AddRenderer(renderer);
  renderWindow->SetAlphaBitPlanes(1); //enable usage of alpha channel
 
  vtkSmartPointer<vtkRenderWindowInteractor> renderWindowInteractor = 
    vtkSmartPointer<vtkRenderWindowInteractor>::New();
  renderWindowInteractor->SetRenderWindow(renderWindow);
 
  renderer->AddActor(actor);
  renderer->SetBackground(1,1,1); // Background color white
 
  renderWindow->Render();
 
  // Screenshot  
  vtkSmartPointer<vtkWindowToImageFilter> windowToImageFilter = 
    vtkSmartPointer<vtkWindowToImageFilter>::New();
  windowToImageFilter->SetInput(renderWindow);
  windowToImageFilter->SetMagnification(1); //set the resolution of the output image (3 times the current resolution of vtk render window)
  windowToImageFilter->SetInputBufferTypeToRGBA(); //also record the alpha (transparency) channel
  windowToImageFilter->Update();
 
  vtkSmartPointer<vtkPNGWriter> writer = 
    vtkSmartPointer<vtkPNGWriter>::New();
  writer->SetFileName("screenshot2.png");
  writer->SetInputConnection(windowToImageFilter->GetOutputPort());
  writer->Write();
 
  renderWindow->Render();  
  renderer->ResetCamera();
  renderWindow->Render();
  renderWindowInteractor->Start();
 
  return EXIT_SUCCESS;
}

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