def center_surface():
"""Center the stl model."""
if not check_surface():
return
updateGUI()
nodes,elems = PF['old_surface'] = PF['surface']
F = Formex(nodes.reshape((-1,1,3)))
center = F.center()
nodes = F.translate(-center).f
PF['surface'] = nodes,elems
clear()
show_changes(PF['old_surface'],PF['surface'])
