[Insight-users] Using deformation fields when testing registration quality?

[motes motes]

I would like to measure the quality of my modfied BSplineDeformable
transform. I have used one of the ITK examples to create a known
deformation of the fixed image that I use as the moving image.

But how do I use this known deformation in the quality measure? Since
the deformation field is applied to the fixed image and the result is
the moving image I don't see what benefit it does when measuring the
quality of the registration?

How are deformation fields used in this context?