I have seen that the official recommendation for making a mid-gray surface is to use mris_expand. This seems to be a very slow procedure on my machine. I have also tried just loading the ?h.white and ?h.pial surfaces into matlab, averaging their vertices, and exporting the result. This seems to result in a nearly identical surface as mris_expand. On a test subject, only 48 out of 127,998 (0.0375%) vertices differed by more than 0.1mm, and these were almost entirely around the edges of the corpus callosum.
Is there a reason this alternative is a bad idea? I imagine mris_expand does additional topology checks along the way, but if the vertices vary so little, can that be a major problem?
I'm inclined to go with the faster solution, but since I'm relatively new to this procedure, I wanted to see what the community's experience might be.
Thanks!