erosion : erodes white objects. One layer
of an object is peeled off in 1 iter(ation)
Objects < #iter x 2 disappear completely.
cmd : erosion #iter,bitpl,connec, edge, im 
dflt: erosion    1 ,  1  ,   6  ,  0  , B
range:      <1,512>,<1,8>,<4,6,8><0,1><A-D>
