[Freesurfer] mri_vol2surf

We need the description of sampling from 3D data on the surface (mri_vol2surf). We are specifically interested how the position is determined in 3D, when the fraction/distance from wm surface is announced. Is it e.g. the normal from one node (resulting in some overlap of 3D projections at the bottom of sulci), or is the neigbourhood somehow interpolated to get a better approximation of the normal direction. Is the projection always a point projection (nearest neighbour or symmetrical interpolation)? Is mri_vol2surf using the voxels only once or is it possible that voxels are used multiple times?

If the method has been described in any publication, that would be helpful.

-Tuomas