def blend_channels_screen(bottom_chan, top_chan):
 return (1 - ((1 - bottom_chan[:, :]) * (1 - top_chan[:, :])))

def blend_channels_screen ( bottom_chan , top_chan ) : return ( 1 - ( ( 1 - bottom_chan [ : , : ] ) * ( 1 - top_chan [ : , : ] ) ) )
return combination of bottom and top channels .