From the help for 3dAllineate
https://afni.nimh.nih.gov/pub/dist/doc/program_help/3dAllineate.html:
* The option '-nwarp_save sss' lets you save a 3D dataset of the
the displacement field used to create the output dataset. This
dataset can be used in program 3dNwarpApply to warp other datasets.
++ If the warp is symbolized by x -> w(x) [here, x is a DICOM 3-vector],
then the '-nwarp_save' dataset contains w(x)-x; that is, it contains
the warp displacement of each grid point from its grid location.
++ Also see program 3dNwarpCalc for other things you can do with this file:
warp inversion, catenation, square root, ...
It has been a long time since this option was implemented, so I can't be sure if it still works as advertised. You'll have to try it and see.
However, I'm not sure why you would *want* to combine bilinear warps (a very old idea of mine that didn't prove useful, IMHO) with much more general nonlinear warps.
