how to extract the foreground from vtkWindowToImageFilter?

Question

I'm integrating vtk with qt, i have vtkWindowToImageFilter with input set to vtkGenericOpenGLRenderWindow, how come i extract the image foreground only?

vtkWindowToImageFilter *w2if = vtkWindowToImageFilter::New();
w2if->ReadFrontBufferOff();
w2if->SetInput(renderWindow);
w2if->Update();
vtkImageData *img = w2if->GetOutput();

the vtkImageData include the background how can i get ride of it?

Answer 1

My recommendation is to render everything again except for the background. Something like the following

auto oldSB = renderWindow->GetSwapBuffers();
renderWindow->SwapBuffersOff();

// Hide the background (set visibility to false or whatever)
...
auto windowToImageFilter = vtkSmartPointer<vtkWindowToImageFilter>::New();
windowToImageFilter->SetInput(renderWindow);

windowToImageFilter->SetScale(1);
windowToImageFilter->SetInputBufferTypeToRGBA();

windowToImageFilter->ReadFrontBufferOff();
windowToImageFilter->Update(); // Issues a render on input

renderWindow->SetSwapBuffers(oldSB);
renderWindow->SwapBuffersOn();

// Show background again (set visibility to true or whatever)
...

auto img = windowToImageFilter->GetOutput();

The following render call will show the background again.