[vtkusers] vtkWindowToImageFilter and vtkImageViewer

[Wolfram H Volpi]

Please help me find why the viewer displays all black.
I suspect I a doing something wrong with vtkWindowToImageFilter.
The code is below:

#iren and iren_sub look good
vtkRenderWindowInteractor iren
	iren SetRenderWindow renWin2
	iren Initialize
vtkRenderWindowInteractor iren_sub
	iren_sub SetRenderWindow renWin2_sub
	iren_sub Initialize

#modified from rtImageTclExamples.tcl and stCone.tcl
vtkWindowToImageFilter wtif
	wtif SetInput renWin2

vtkWindowToImageFilter wtif_sub
	wtif_sub SetInput renWin2_sub

#vtkImagePSNR imgDiff2
vtkImageDifference imgDiff2
	imgDiff2 SetImage [wtif_sub GetOutput]
	imgDiff2 SetInput [wtif GetOutput]

#view is black
vtkImageViewer view
        view SetColorWindow 256
        view SetColorLevel 128
	view SetInput [imgDiff2 GetOutput]
	view Render

Thank you for your help,
Wolfv.