[vtkusers] using vtkImageDifference

[Jens Eickmeyer]

hi all,

I want to compute the difference between two volumes. I found the class 
vtkImageDifference which seems to do what I need. So I take my two 
vtkImageData-sets and call the functions SetInput() and SetImage() of my 
vtkImageDifference-object. But when I execute my programm, I get the 
following error:

vtkImageDifference (0x80c71b0): Execute: Expecting 3 components (RGB)

Obviously I missed something. Can you tell me which filters I have to apply to 
the data-sets?

Thanks in advance

Jens