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250
CHAPTER 4 Graphics
chromaticity coordinates and uppercase letters to specify tristimulus values. Giv-
en this information, Matrix and WhitePoint can be found as follows:
z = yW × ( ( xG – xB ) × yR – ( xR – xB ) × yG + ( xR – xG ) × yB )
yR ( xG – xB ) × yW – ( xW – xB ) × yG + ( xW – xG ) × yB
-
Y A = ----- × --------------------------------------------------------------------------------------------------------------------------------
R z
xR ⎛ 1 – xR ⎞
-
X A = Y A × ----- -
Z A = Y A × ⎜ -------------- – 1 ⎟
yR ⎝ yR ⎠
yG ( xR – xB ) × yW – ( xW – xB ) × yR + ( xW – xR ) × yB
Y B = – ----- × -------------------------------------------------------------------------------------------------------------------------------
- -
G z
xG ⎛ 1 – xG ⎞
-
X B = Y B × ----- -
Z B = Y B × ⎜ -------------- – 1 ⎟
yG ⎝ yG ⎠
yB ( xR – xG ) × yW – ( xW – xG ) × yR + ( xW – xR ) × yG
Y C = ----- × ---------------------------------------------------------------------------------------------------------------------------------
B z
xB ⎛ 1 – xB ⎞
-
X C = Y C × ----- Z C = Y C × ⎜ -------------- – 1 ⎟
yB ⎝ yB ⎠
XW = XA × R + XB × G + XC × B
YW = YA × R + YB × G + YC × B
ZW = ZA × R + ZB × G + ZC × B
Lab Color Spaces
A Lab color space is a CIE-based ABC color space with two transformation stages
(see Figure 4.14 on page 245). In this type of space, A, B, and C represent the L*,
a*, and b* components of a CIE 1976 L*a*b* space. The range of the first (L*)
component is always 0 to 100; the ranges of the second and third (a* and b*)
components are defined by the Range entry in the color space dictionary (see
Table 4.15).
Plate 3 illustrates the coordinates of a typical Lab color space; Plate 4 compares
the gamuts (ranges of representable colors) for L*a*b*, RGB, and CMYK spaces.
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