onlyCb rgb = Ycbcr.fromYcbcr (128, cb, 0.0)
  where (y, cb, cr) = Ycbcr.toYcbcr rgb
onlyCb rgb = Ycbcr.fromYcbcr (128, cb, 0.0)
  where (y, cb, cr) = Ycbcr.toYcbcr rgb
onlyCb rgb = Ycbcr.fromYcbcr (128, cb, 0.0)
  where (y, cb, cr) = Ycbcr.toYcbcr rgb