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