The class vtkImageResample declared a member called OutputSpacing, even though that member was already present in the superclass vtkImageReslice. In both classes, OutputSpacing is only used by RequestInformation(), and each class has its own RequestInformation() method.
Since vtkImageResample uses "0" as a special value for the OutputSpacing, and because of the interaction between OutputSpacing and MagnificationFactors, it was necessary to define a custom SetOutputSpacing() method for vtkImageResample. A matching SetMagnificationFactors() method was also added.