Greg -

Yes, you can use a deformation field created by xform2dfield just like any other transformation on the reformatx command line (you cxan even use "--invert" with it, but then again, why would you?))

As for fit_bspline_dfield - yes, you could use that, but it wouldn't make things any faster (applying a deformation field should be very fast) and you would loose some precision of the dfield due to fitting.

What I meant in my original elaboration was slightly different - you can fit a B-spline to a set of point correspondences. These point correspondences could be a set of pairs (x,T(x)) with T being any transformation you may have. Then if you fit a B-spline transformation to the set of pairs (T(x),x) you will get a fit of the inverse transformation without ever actually doing the inversion (either analytic or numerical).

So that would be what I would strongly encourage you to use, but alas, I have not been able to find the time to actually implement this particular fitting procedure. (It is obviously not very hard to implement, given that I already have the general fitting code running, but there is just too much other stuff that I need to get done first; sorry).

Torsten