[FieldTrip] question on coil orientations for elekta systems

[Thomas Hartmann]

dear jan-mathijs,

that is indeed the case. thanks for the quick reply!

best wishes,
thomas

Am 21.08.18 um 11:11 schrieb Schoffelen, J.M. (Jan Mathijs):
> Dear Thomas,
>
> I would guess that the 3x3 ‘rotation’ part of the affine 
> transformation matrix here describes the direction plane of the sensor 
> in the first two columns (x/y axis), and the normal to this plane in 
> the third column, which is the vector perpendicular to the coil. Try 
> and verify whether the cross-product between the first two columns 
> gives the the third.
>
> Best wishes,
>
> Jan-Mathijs
>
>> On 21 Aug 2018, at 10:24, Thomas Hartmann <thomas.hartmann at th-ht.de 
>> <mailto:thomas.hartmann at th-ht.de>> wrote:
>>
>> hi,
>>
>> while diving into the import code for elekta systems, i noticed that 
>> coil orientations are imported like this:
>>
>> t = orig.chs(n).coil_trans;
>>
>> grad.coilpos(kCoil,:) = t(1:3,4)+0.0003*t(1:3,3);
>> grad.coilori(kCoil,:) = t(1:3,3);
>>
>> (full code is here: 
>> https://github.com/fieldtrip/fieldtrip/blob/master/fileio/private/mne2grad.m#L321)
>>
>> t (or coil_trans) is a 4x4 matrix, that if i understood it correctly, 
>> is a 3x3 rotation matrix combined with xyz coordinates in the last 
>> column.
>>
>> if i understand everything correctly, coilori is supposed to store a 
>> unit vector pointing in the direction of the coil's orientation.
>>
>> so, why is this the third column?
>>
>> thanks a lot in advance!
>>
>> thomas
>>
>> -- 
>> Dr. Thomas Hartmann
>>
>> Centre for Cognitive Neuroscience
>> FB Psychologie
>> Universität Salzburg
>> Hellbrunnerstraße 34/II
>> 5020 Salzburg
>>
>> Tel: +43 662 8044 5109
>> Email:thomas.hartmann at th-ht.de
>>
>> "I am a brain, Watson. The rest of me is a mere appendix. " (Arthur Conan Doyle)
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-- 
Dr. Thomas Hartmann

Centre for Cognitive Neuroscience
FB Psychologie
Universität Salzburg
Hellbrunnerstraße 34/II
5020 Salzburg

Tel: +43 662 8044 5109
Email: thomas.hartmann at th-ht.de

"I am a brain, Watson. The rest of me is a mere appendix. " (Arthur Conan Doyle)

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