[Mne_analysis] raw.info.dev_head_t.trans

[Matti Hamalainen]

Hi Elena,

On Sep 29, 2008, at 4:20 PM, elena at nmr.mgh.harvard.edu wrote:

> Hi
>
> In raw data Matlab structure
> raw.info.dev_head_t.trans
> have 4 raws and 4 columns:
>
>     0.9974   -0.0710    0.0093   -0.0008
>     0.0704    0.9964    0.0482   -0.0100
>    -0.0127   -0.0474    0.9988    0.0659
>              0              0             0    1.0000
>
> What is the 4th column? Is it the head origin in device coordinates  
> (x,y,z
> in meters)?
>
> Thanks,
> Elena

It is actually the device coordinate origin in head coordinates. The  
name of the field indicates that it is the transformation from device  
to head coordinates. This is, in fact, also coded in the from and to  
fields of dev_head_t. If your head coordinate location is x y z given  
in meters, you augment this location vector with one to get

r_dev = [x y z 1]'

and then you get

r_head = raw.info.dev_head_t.trans*r_dev;

If you want to transform an orientation with orientation cosines cx,  
cy, and cz, you do

ori_dev = [ cx cy cz 0]';

and

ori_head = raw.info.dev_head_t.trans*ori_dev;

because orientation vectors should not be translated.

- Matti