@pimms.value<EOL><INDENT>def white_to_pial_vectors(white_surface, pial_surface):<DEDENT>
u = pial_surface.coordinates - white_surface.coordinates<EOL>d = np.sqrt(np.sum(u**<NUM_LIT:2>, axis=<NUM_LIT:0>))<EOL>z = np.isclose(d, <NUM_LIT:0>)<EOL>return pimms.imm_array(np.logical_not(z) / (d + z))<EOL>
cortex.white_to_pial_vectors is a (3 x n) matrix of the unit direction vectors that point
  from the n vertices in the cortex's white surface to their equivalent positions in the
  pial surface.