颜色空间转换

介绍几种常用颜色空间之间的转换算法。在转换过程中，设各个参数的变化范围为：

0≤R, G, B≤1;  0≤C, M, Y≤1;  (-1)0°≤H≤360° 0≤S, V, L≤1。

1．RGB←→CMY

(1)  RGB→CMY

C = 1- R         M = 1- G        Y = 1 - B

(2)  CMY→RGB

R = 1- C         G = 1- M        B = 1 - Y

2．RGB←→HSL

(1)     RGB→HSL

设M = max(R, G, B), m = min(R, G, B)，P = M + m，D = M – m，则

L = (M + m)/2

若M = m，则S = H = 0

若M ≠ m，

       若L < 0.5，则S = D / P

       若L ≥ 0.5，则S = D / (2 - P)

若R = M，则H = (G - B) / D

若G = M，则H = 2 + (B - R) / D

若B = M，则H = 4 + (R - G) / D

若H ≥ 0，则H = H * 60

若H < 0，则H = H * 60 + 360

 

(2)    HSL→RGB

若S = 0，则R = G = B = L

若S ≠ 0，则

       若L < 0.5，则T2 = L * (1 + S)

       若L ≥ 0.5，则T2 = L + S – L * S

       T1 = 2 * L – T2

[      H = H / 360

       T3r = H + 1/3,       T3g = H, T3b = H –1/3

       若T3 < 0，则T3 = T3 +1

              若T3 > 1，则T3 = T3 –1

       若6 * T3 < 1，则color = T1 + (T2 – T1) * 6 * T3

              否则，若 2 * T3 < 1，则color = T2

              否则，若 3 * T3 < 2，则color = T1 + (T2 – T1) * (2/3 –T3) * 6

              否则color = T1

]

其中的[ ]段可用下一段代替：

       T3r = H + 120,  T3g = H,  T3b = H –120

       若T3 < 0，则T3 += 360

              若T3 > 360，则T3 -= 360

       若 T3 < 60，则 color = T1 + (T2 – T1) * T3 / 60

       否则，若 T3 < 180，则 color = T2

否则，若 T3 < 240，则 color = T1 + (T2 – T1) * (240 – T3) / 60

       否则color = T1

3．RGB←→YCbCr

(1)    RGB→YCbCr

              Y = 0.299R + 0.587G + 0.114B

              Cb = -0.1687R – 0.3313G + 0.5B +128

              Cr = 0.5R – 0.4187G – 0.0813B + 128

(2)    YCbCr→RGB

              R = Y + 1.402 * (Cr - 128)

              G = Y – 0.3441 * (Cb – 128) – 0.7141 * (Cr - 128)

              B = Y + 1.772 * (Cb - 128)