Hi - yes this is due to the interpolation step which internally does an eigenvector decomposition in which obviously there is a sign ambiguity.  Obviously for DTI stuff this doesn’t matter but hopefully for your application it doesn’t matter either.

Cheers,
Saad





On 26 Oct 2016, at 15:53, Justin <[log in to unmask]> wrote:

These aren't eigenvectors, but they are unit vectors. I guess it does not matter if they are totally flipped for my case, but it is a little bit disconcerting, especially since I'm only applying an identity transformation.

If this is well-known behavior of whatever interpolation algorithm is used by vecreg then I'll just get over it.