cb :: RGB8 -> Double
cb rgb@(r, g, b) = 0.492 * (b' - y rgb) 
  where b' = fromIntegral b
cb :: RGB8 -> Double
cb rgb@(r, g, b) = 0.492 * (b' - y rgb) 
  where b' = fromIntegral b
cb rgb@(r, g, b) = 0.492 * (b' - y rgb) 
  where b' = fromIntegral b