(define-record-type yuv 
  (fields y u v))

;; rgb -> yuv
(define rgb->yuv
  (lambda (c)
    (let ((r (rgb-r c))
	  (g (rgb-g c))
	  (b (rgb-b c)))
      (make-yuv (+ (* +0.299 r) (* +0.587 g) (* +0.114 b))
		(+ (* -0.147 r) (* -0.289 g) (* +0.436 b))
		(+ (* +0.615 r) (* -0.515 g) (* -0.100 b))))))

#|
 Y  = + 0.299R + 0.587G + 0.114B
 U  = + 0.492(B - Y)
    = - 0.147R - 0.289G + 0.436B
 V  = + 0.877(R - Y)
    = + 0.615R - 0.515G - 0.100B
|#