[MINC-users] Affine transformation matrix

[Andrew Janke]

Hi Aaryani,

You are asking a pretty loaded question here. If the scaling is small
(say 1.001) then I wouldn't expect removing it will make much
difference, of course there will still be a difference but it probably
will be tolerable.

Remember also that an .xfm is really just an affine transformation
matrix that can be expressed in a many different combinations of
rotations, translations, scales and shears. xfm2param just gives you
one of these solutions based upon a set of assumptions (rotations all
less than 90deg, etc).

As for changing the rotations and translations accordingly, the answer
is definitely yes, but the matrix algebra involved in this is going to
get pretty ugly pretty quick and in any case will not be the same as
the original transformation once the scaling is removed. My suggestion
if you really want to do this is to recover the transformation type
you want using minctracc from the outset. In your case this would be
done via the -lsq6 argument, or perhaps lsq7.

a



On Wed, Jun 22, 2011 at 10:22, Aaryani Tipirneni <atipirne at stanford.edu> wrote:
>  I have a basic question - As we know, If we do affine registration using mritotal to the atlas, we will get a .xfm file with the translations, rotations, scaling and shearing. If I now want to remove the scaling from the .xfm matrix so that the coregistered data has the same scaling as the original data (to avoid stretching and shrinking), do you expect to still coregister well? Do I have to change even the rotations and translations accordingly?