
siena_diff - cannot compile with optimisation turned on (gcc bug)

37744 nonzero voxels in avg152T1_edges
say 20000 real edge voxels (ignoring corner points)

smooth with sigma=10mm=5voxels
thus 1 resel = approx 100 voxels
but intrinsic smoothness factor = 2
-> 200 voxels

thus bonf correct by factor of 100
thus uncorrected 0.05 -> 0.0002 -> z=3.7
