I have a volumetric ROI (integer valued) in MNI space. I'd really like to display them on the fsaverage brain as a surface overlay so that it's clear what regions I'm looking at.

Using something like this, I get pretty good results:

mri_vol2surf --mov ROI_integers.nii --regheader fsaverage --hemi lh --out left_overlay.mgh --projfrac-max 0.15 .17 .01

but the edges are kind of a mess, see: https://imgur.com/a/53RVGCM

Is there any way to smooth these edges while maintaining the "lookup table' nature of the surface overlay? Any kind of interpolation ruins that, so it's a bit hard (you can't see in that image, but there are multiple regions shown).

Alternatively, is there any way I can do arithmetic with surfaces? If there were a tool like fslmaths for surface overlays, solving this problem would be relatively straightforward.

Thanks for any help you can provide!

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