Hi

Does anyone know whynibabel.freesurfer.io.read_geometry only allows either[20] or[2, 0, 20] as the first entry in the metadata section of a FreeSurfer surface file (referred to ashead in the metadata dict)? The reason I am asking is that I am interested in being able to tell whether a surface is stored in surface RAS (tkregister space) or scanner RAS. Currently, it seems thatread_geometry only supports reading the metadata when a surface is in surface RAS.

To test this, I tried to convert a surface file (created during arecon-all run) to scanner RAS like this

mris_convert --to-scanner lh.white lh.white.scanner

nibabel reads the metadata oflh.white just fine, however, forlh.white.scanner I get

.../nibabel/freesurfer/io.py:62: UserWarning: Unknown extension code. warnings.warn('Unknown extension code.') .../nibabel/freesurfer/io.py:188: UserWarning: No volume information contained in the file warnings.warn('No volume information contained in the file')

It turns out that it reads thehead property just fine in both cases. Forlh.white it is[2, 0, 20] but forlh.white.scanner it is[2, 1, 20] which, from looking into the freesurfer code (e.g., herehttps://github.com/freesurfer/freesurfer/blob/920f33cade45b901f702192ace64b37ef2c4b3e1/utils/mrisurf_io.cpp#L5348), is what I would expect as it seems that these three numbers signify[TAG_OLD_USEREALRAS (= 2), value associated with this tag (= 0 or 1), TAG_OLD_SURF_GEOM (= 20)]. From the FS code, it is difficult to tell whether this is always the pattern or not but I was thinking that it might make sense to add[2, 1, 20] as an allowed value ofhead metadata - and perhaps even add it as an entry in the returned metadata dict (e.g.,realras = True/False). Any thoughts? I can do a PR if needed.