MRG, ENH: Allow saving non-head cf in mon.save

[larsoner]

As discussed with @agramfort, there is no need to have this restriction. Simultaneously makes things more DRY and removes two # XXX comments from our code, which is nice.

[agramfort]

thx @larsoner !

[christian-oreilly]

This won't work...

def write_dig(fname, pts, coord_frame=None):
    return _dig_write_dig(fname, pts, coord_frame=None)

[christian-oreilly]

Also, after correcting the error mentioned below, the following code generate and assert exception

        montage = mne.channels.make_dig_montage(ch_pos=ch_pos,  
                                                nasion=brain_fiducials.loc["Nz"].values,
                                                lpa=brain_fiducials.loc["LPA"].values,
                                                rpa=brain_fiducials.loc["RPA"].values,
                                                hsp=hsp,
                                                coord_frame="mri")    

        montage_fname = fs_subject_path / subject / "{}-montage.fif".format(montage_str)
        
        mne.io.meas_info.write_dig(montage_fname, montage.dig, coord_frame=FIFF.FIFFV_COORD_MRI)
        dig = mne.channels.read_dig_fif(montage_fname).dig[:3]
        for d, want in zip(dig, (FIFF.FIFFV_POINT_LPA,
                                 FIFF.FIFFV_POINT_NASION,
                                 FIFF.FIFFV_POINT_RPA)):
            assert d['kind'] == FIFF.FIFFV_POINT_CARDINAL
            assert d['coord_frame'] == FIFF.FIFFV_COORD_MRI 

WRITING coord_frame in mne.io.write.write_dig_points 5

---------------------------------------------------------------------------
AssertionError                            Traceback (most recent call last)
<ipython-input-14-f785d8f255d5> in <module>
     71                                  FIFF.FIFFV_POINT_RPA)):
     72             assert d['kind'] == FIFF.FIFFV_POINT_CARDINAL
---> 73             assert d['coord_frame'] == FIFF.FIFFV_COORD_MRI
     74 
     75 

AssertionError: 

The printing of "WRITING coord_frame in mne.io.write.write_dig_points 5" indicates that the up to mne.io.write.write_dig_points the coordinate frame seems to be taken into account. Maybe the issue is in mne.channels.read_dig_fif ?

[christian-oreilly]

Apologies. Please disregard the last message, I had not rebased my git when I tested. The previous one is valid though and this slight bug should be corrected for the full functionality of write_dig to work.

[larsoner]

@christian-oreilly the idea of this PR is not use mne.io.meas_info.write_dig (this isn't really meant for public consumption despite its name), this should work fine if you just do montage.save(...) -- your assertions can pass. Can you see if using this more visible method works?

[christian-oreilly]

Sure... I can check... but this code is still broken no?

def write_dig(fname, pts, coord_frame=None):
    return _dig_write_dig(fname, pts, coord_frame=None)

At least, if it is not considered broken, the documentation of write_dig should not say that coord_frame is a dummy attributes that do nothing.

[larsoner]

Sure... I can check... but this code is still broken no?

Yes we should fix this

[christian-oreilly]

FYI, montage.save(...) worked fine