[MINC-users] register (and tagtoxfm) can produce reflections in the output transformations

[Matthijs van Eede]

Hi Andrew,

I think I wasn't specific enough in the example. Using register for rotations/inversions is okay when creating a 12 parameter transformation, but when you've selected 6 parameters, shouldn't you only get translations and rotations? That should not be able to give you reflections?

On the other hand, Bert's suggestion of adding a warning message sounds good. Indicating the sign of the determinant and potentially an additional message when it's negative that there is a reflection as part of the transformation (in case users don't know what a negative determinant implies).

Cheers,
Matthijs

________________________________________
From: minc-users-bounces at bic.mni.mcgill.ca [minc-users-bounces at bic.mni.mcgill.ca] on behalf of Robert D. Vincent [robert.d.vincent at mcgill.ca]
Sent: October 5, 2016 7:02 AM
To: MINC users mailing list
Subject: Re: [MINC-users] register (and tagtoxfm) can produce reflections in the output transformations

If it's helpful, we could have register simply indicate the sign of the
determinant as a warning...

On Tue, Oct 4, 2016 at 8:47 PM, Andrew Janke <a.janke at gmail.com> wrote:

> Hi Matthijs,
>
> This is expected behaviour. I often use register and tag points to
> resolve rotation/inversion issues in data.
>
> Thanks
>
> a
>
>
> On 5 October 2016 at 05:50, Matthijs van Eede
> <matthijs.vaneede at sickkids.ca> wrote:
> > Greetings,
> >
> > We were trying to align two brains and found that using register it's
> possible to get two kinds of transforms. If you pick the right tag points,
> you get a proper transformation file. If you happen to flip left and right
> in one of the tag points, a transformation is produced that flips one of
> the axes. The question is whether this is expected behaviour for register.
> Flipping axes is rarely something you want when estimating a transformation
> (In this case, the user that had produced these transformations with
> register ran a registration pipeline where all brains were left-right
> flipped without realizing it). Should there be an additional check in
> register to ensure that the determinant of a rotation matrix is positive?
> >
> > Here's an example of tag points that produces a transformation with a
> negative determinant:
> >
> > ######################
> > MNI Tag Point File
> > Volumes = 2;
> > %VIO_Volume: model.mnc
> > %VIO_Volume: source.mnc
> >
> > Points =
> >  -0.366219818592072 7.60433340072632 2.30845427513123 1.40657377243042
> -4.23010206222534 17.1643028259277 ""
> >  0.120084546506405 -0.405666679143906 2.30845427513123 0.545454323291779
> -4.9874153137207 25.2130146026611 ""
> >  0.174118369817734 -5.15316677093506 1.66004836559296 0.592638909816742
> -4.23246097564697 30.2041149139404 ""
> >  -3.39211368560791 -3.08416676521301 -0.879541099071503
> -3.08776211738586 -1.92041432857513 28.3605556488037 "";
> > ######################
> >
> > Cheers
> > Matthijs & Ben
> >
> > ________________________________
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