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Dear Thomas,
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<div class="">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.</div>
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<div class="">Best wishes,</div>
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<div class="">Jan-Mathijs</div>
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<div class="">On 21 Aug 2018, at 10:24, Thomas Hartmann <<a href="mailto:thomas.hartmann@th-ht.de" class="">thomas.hartmann@th-ht.de</a>> wrote:</div>
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<p class="">hi,</p>
<p class="">while diving into the import code for elekta systems, i noticed that coil orientations are imported like this:</p>
<pre class="">t = orig.chs(n).coil_trans;
grad.coilpos(kCoil,:) = t(<span class="pl-c1">1</span>:<span class="pl-c1">3</span>,<span class="pl-c1">4</span>)+<span class="pl-c1">0.0003</span>*t(<span class="pl-c1">1</span>:<span class="pl-c1">3</span>,<span class="pl-c1">3</span>);
grad.coilori(kCoil,:) = t(<span class="pl-c1">1</span>:<span class="pl-c1">3</span>,<span class="pl-c1">3</span>);
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<p class="">(full code is here: <a class="moz-txt-link-freetext" href="https://github.com/fieldtrip/fieldtrip/blob/master/fileio/private/mne2grad.m#L321">
https://github.com/fieldtrip/fieldtrip/blob/master/fileio/private/mne2grad.m#L321</a>)<br class="">
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<p class="">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.</p>
<p class="">if i understand everything correctly, coilori is supposed to store a unit vector pointing in the direction of the coil's orientation.</p>
<p class="">so, why is this the third column?</p>
<p class="">thanks a lot in advance!</p>
<p class="">thomas<br class="">
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<pre class="moz-signature" cols="72">--
Dr. Thomas Hartmann
Centre for Cognitive Neuroscience
FB Psychologie
Universität Salzburg
Hellbrunnerstraße 34/II
5020 Salzburg
Tel: +43 662 8044 5109
Email: <a class="moz-txt-link-abbreviated" href="mailto:thomas.hartmann@th-ht.de">thomas.hartmann@th-ht.de</a>
"I am a brain, Watson. The rest of me is a mere appendix. " (Arthur Conan Doyle)
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