[MINC-users] register (and tagtoxfm) can produce reflections in the output transformations
Matthijs van Eede
matthijs.vaneede at sickkids.ca
Tue Oct 4 15:50:04 EDT 2016
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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