[vtkusers] How to SetInput for vtkImageMathematics?

[Wolfram H Volpi]

How do I SetInput for vtkImageMathematics?
The man pages have no explination, and I get errors doing it this way:

vtkPNMReader ReaderRefr2
vtkPNMReader ReaderTest2
vtkImageDifference imgDiff2
vtkImageMathematics mult
  mult SetOperationToMultiply
vtkPNMWriter  PNMWriter

	imgDiff2 SetImage [ReaderRefr2 GetOutput]
	imgDiff2 SetInput [ReaderTest2 GetOutput]
	mult SetInput 1 [imgDiff2 GetOutput]
	PNMWriter SetInput [mult GetOutput]
	PNMWriter Write

This produces vtkOutputWindow error:

ERROR: In D:\vtk31CD\vtk\common\vtkImageData.cxx, line 1585
vtkImageData (0x01D7ECB0): Bad Extent, retaining previous values

Thanks,
Wolfv.