
72 Image Processing for Cinema
S(λ
R
) =
R
I(λ
R
)
, S(λ
G
) =
G
I(λ
G
)
, S(λ
B
) =
B
I(λ
B
)
. (3.6)
The white balance process by which a color corrected triplet (R
0
, G
0
, B
0
)
is obtained from (R, G, B) can be written in matrix form thus:
R
0
G
0
B
0
=
1
I(λ
R
)
0 0
0
1
I(λ
G
)
0
0 0
1
I(λ
B
)
R
G
B
. (3.7)
Some cameras proceed this way although it is usually preferred to apply
the diagonal model of Equation 3.5 not directly on the RGB values but on
their correspondent cone tristimulus values, since color adaptation in the eye
is very much dependent on the sensitivity of the cones [205]. We can convert
RGB values into CIE tristimulus values XY Z with:
X
Y
Z
= A
R
G
B
, (3.8)
where