Dear SPMers,
I have a couple of questions about spm_matrix.m. I appreciate your
consideration in advance:
1- While I was working with one of my datasets in SPM2 environment, I
realized by applying a roll of (-pi/2) and a pitch of (pi/2), I get exactly
the same result as a roll of (-pi/2) and a yaw of (-pi/2). I tried it on
several other datasets and they confirmed that. But it shouldn't be! For one
of my datasets, in order to bring it to correct radiological orientation, I
had to apply the roll and pitch as I said and it confirms with what it
should be but the roll and yaw of (-pi/2) doesn't make sense. I appreciate
any help to clear this confusion for me.
2- I know a bit of robotics and in the beginning of working with SPM, I
realized SPM developers define roll as rotation around Y axis, pitch as
rotation around X axis and yaw as rotation around Z axis. In robotics it is
roll for Z axis, pitch for Y axis and yaw for X axis. Is there any
particular reason that the definition is different?
3- Also as I found in spm_matrix.m file, the rotation matrices are defined
differently too. In R1, element r32 has the negitive sign (-sin(P(4))) but
in robotics books, r23 has negative sign. The same thing for R3. Also the
order of multiplication is XYZ (R1R2R3) however in robotics it is usually
ZYX. Any particular reason?
I appreciate any help,
Thanks a lot,
Amir Tahmasebi,
Ph.D. Candidate
School of Computing,
Queen's University,
Kingston, ON,
Canada
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