[vtkusers] creating a negative

[David J. Netherway]

What is a simple way to create a negative image?

I am currently using the vtkImageViewer to map 12 bit images using the
SetColorWindow and SetColorLevel methods.

I had a look for a suitable filter or mapper flag but was unsuccessful.

I could set up specific lookup tables but I thought that there might be
something simple for this purpose.

Thanks, David