grady   : vertical gradient filter.
Computes the vertical first derivative.
out-img = grady(in-img) * (mul/1024) + add.
cmnd: grady in-img,out-img, mul  ,  add
dflt: grady    A  ,   B  , 1024  ,  128
range:       <A-D>,<A-D> ,<10000>,<+/-255>