Thanks. The options you are planning to add make sense. Looks like I may not even
need to write the fillin script, because when the grid_parent and the anat volume are
aligned correctly, I get values for all the nodes. The were not alighned properly in the previous case.

I have a separate question about 3dVol2Surf, if you will. 3dVol2Surf works with
two surfaces (say whitematter and pial). It uses line segments between the nodes
on one surface and the corresponding nodes on the another surface. I was wondering how these line segments compare to surface normals on the inner surface. I am pretty new at this, but I have noticed that white matter and gray matter do not always follow each other cleanly. Sometimes you get chunks of gray matter tha do not clearly surround any white matter. I am guessing that in these situations, the line segments will not be the
same as surface normals, but in most other cases they will be the same. Is this true?
They have a program called mri_vol2surf with FreeSurfer, as you probably know. It works with surface normals whose height is adjusted by the gray matter thickness at that point. Do you think mri_vol2surf and 3dVol2Surf would give the same results (although 3dVol2Surf seems much more flexible in terms of what you can do with the line segments)? Thanks.