[vtkusers] using vtkImageDifference on vtkStructuredPoints data

[Wolfram H Volpi]

I would like to use vtkImageDifference on two vtkStructuredPoints image
files. 
I am having no luck with vtkStructuredPointsReader and vtkImageCast (see
tcl script below).  
Is there a way to get the difference between two ASCII base image files?

Thank you,
Wolfv.

vtkStructuredPointsReader Reader1
	Reader1 SetFileName ASCIIimage1.vtk

vtkImageCast imgCast1
	imgCast1 SetInput [Reader1 GetOutput]
	imgCast1 SetOutputScalarTypeToUnsignedChar  

vtkImageDifference imgDiff
	imgDiff SetImage [imgCast1 GetOutput]

vtkOutputWindow
ERROR: In D:\vtk\graphics\vtkDataReader.cxx, line 458
vtkDataReader (0x01D8B060): Unrecognized file type: vtk for
file: ASCIIimage1.vtk