[vtkusers] How save a single vtkRenderer as picture?

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Hi,everyone
   I can save whole vtkRenderWindow as a picture By
vtkWindowToImageFilter and vtkJPEWriter.

  w2i = vtkWindowToImageFilter::New();
  writer = vtkJPEGWriter::New();

  w2i->SetInput(renWin);
  writer->SetInput(w2i->GetOutput());
  writer->SetFileName(PictureFile1);
  writer->Write();

But I don't know how to save a single vtkRenderer as a
picture when several vtkRenderer in one
vtkRenderWindow.Can you help me?
Thanks
echo

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