--track0:Top,0,4000,0,1
--track1:Bottom,0,4000,0,1
--track2:Left,0,4000,0,1
--track3:Right,0,4000,0,1

local ow,oh = obj.w/2, obj.h/2
local top,bottom,left,right = obj.track0,obj.track1,obj.track2,obj.track3

obj.effect ("Clipping","Top",top,"Bottom",bottom,"Left",left,"Right",right)

obj.ox = (-left+right)/2
obj.oy = (-top+bottom)/2