Take a look at "mris_decimate":

~$>mris_decimate -x
                                Help

NAME
        mris_decimate

SYNOPSIS
        mris_decimate [<options>] <input surface> <output surface>

DESCRIPTION
        This program reduces the number of triangles in a surface and outputs
        the new surface to a file using the GNU Triangulated Surface (GTS)
        Library.

POSITIONAL ARGUMENTS
        input surface file

        output surface file

REQUIRED FLAGGED ARGUMENTS
        None

OPTIONAL FLAGGED ARGUMENTS
        -d <decimation level>
                target decimation level of new surface (value between
                0<-->1.0, default: 0.5). The resulting surface will have
                approximately triangles = <decimationLevel> * origTriangles

        -m <minimum angle>
                The minimum angle in degrees allowed between faces during
                decimation (default: 1.0).

        --help
                print out information on how to use this program

        --version
                print out version and exit

REPORTING
        Report bugs to <freesurfer@nmr.mgh.harvard.edu>


On 12/20/11 10:47, Ecker, Christine wrote:
Dear FreeSurfer Experts,

I would like to down-sample the pial surface of individual subjects to have the same number of vertices as fsaverage6. I have done so using:

mri_surf2surf --hemi lh --srcsubject 'subjectID' --sval-xyz pial --trgsubject fsaverage6 --trgicoorder 6 --trgsurfval 'outputfilename'

This worked well and the pial output surface has 40962 vertices. I now have an overlay vector, which was computed on the down-sampled pial surface, which I would like to map back to fsaverage6 in order to perform a group comparison.

Please could you advise on how to do this as all scripts I am aware of (e.g. mris_preproc) use sphere.reg, which has ~150000 vertices.

Many thanks,
Christine


_______________________________________________
Freesurfer mailing list
Freesurfer@nmr.mgh.harvard.edu
https://mail.nmr.mgh.harvard.edu/mailman/listinfo/freesurfer


-- 
Rudolph Pienaar, M.Eng, D.Eng / email: rudolph@nmr.mgh.harvard.edu
MGH/MIT/HMS Athinoula A. Martinos Center for Biomedical Imaging
149 (2301) 13th Street, Charlestown, MA 02129 USA