Hi, June
If I understood correctly, you want to make “disp” from the average surface and each individual surfaces. And then you will put the “disp” as a dependent variable into glm. Did you obtain each surface that is generated by SPHARM-PDM? If so, I thought it is quite simple to get the “disp”. you can calculate the distance between two points ( e.g. between coord from individual and coord from average).