Color conversions¶

work under construction.

to do: http://forum.doom9.org/showthread.php?p=1084293#post1084293

Should cover RGB->YUV conversions and lumarange scaling/preservation and when to use which conversion.

Color conversions¶

coefficients	Rec.601	Rec.709	FCC
Kr : Red channel coefficient	0.299	0.2125	0.3
Kg : Green channel coefficient	0.587	0.7154	0.59
Kb : Blue channel coefficient	0.114	0.0721	0.11

(0.0 <= [Y,R,G,B] <= 1.0) ; (-1.0 < [U,V] < 1.0)

Kg = 1 - Kr - Kb

Y = Kr*R + Kg*G + Kb*B
V = (R - Y)/(1-Kr) = R - G * Kg/(1-Kr) - B * Kb/(1-Kr)
U = (B - Y)/(1-Kb) = - R * Kr/(1-Kb) - G * Kg/(1-Kb) + B

R = Y + V*(1-Kr) G = Y - U*(1-Kb)*Kb/Kg - V*(1-Kr)*Kr/Kg B = Y + U*(1-Kb)

Converting to programming values¶

YUV [0,255] <-> RGB [0,255] (0 <= [r,g,b] <= 255, 0 <= y <= 255, 0 < [u,v] < 255)

y = Y * 255
v = V * 127.5 + 128
u = U * 127.5 + 128
r = R * 255
g = G * 255
b = B * 255

Substituting (Y,V,U,R,G,B) in the equations above and multiplying with 127.5 and respectively 255 gives

y = Kr*r + Kg*g + Kb*b
v - 128 = 0.5*(r - y)/(1-Kr) = 0.5 * r - 0.5 * g * Kg/(1-Kr) - 0.5 * b * Kb/(1-Kr)
u - 128 = 0.5*(b - y)/(1-Kb) = - 0.5 * r * Kr/(1-Kb) - 0.5 * g * Kg/(1-Kb) + 0.5 * b

r = y + 2*(v-128)*(1-Kr)
g = y - 2*(u-128)*(1-Kb)*Kb/Kg - 2*(v-128)*(1-Kr)*Kr/Kg
b = y + 2*(u-128)*(1-Kb)

YUV [16,235] <-> RGB [0,255] (0 <= [r,g,b] <= 255, 16 <= y <= 235, 16 <= [u,v] <= 240)

y = Y * 219 + 16
u = U * 112 + 128
v = V * 112 + 128
r = R * 255
g = G * 255
b = B * 255

References¶

http://www.poynton.com/notes/colour_and_gamma/ColorFAQ.html
ITU BT.601 ...
ITU BT.709 ...

$Date: 2010/02/27 14:45:12 $

Color conversions¶