Elastix displacement vector field output

Operating system: Win7
Slicer version: 4.11.0-2019-08-13 r28438

Hi all,

I am registering two ct images using SlicerElastix (Preset-generic (all)). Now I want to get the displacement field merely indicating bspline transform. Can I just subtract the linear part and how to accomplish this using python interactor? Besides, I would like to get the displacement of specific points in moving image. Could I invert displacement field and save it to nrrd file? I have looked through the discussion thread but still have no idea how to do that.

Hope for some advice! Your help is highly appreciated!!

Crayon