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SECTION 4.5                                                               Color Spaces



The transformation defined by the Gamma and Matrix entries in the CalRGB color
space dictionary is
                    GR              GG              GB
X = L = XA × A    + XB × B    + XC × C
               GR          GG          GB
Y = M = YA × A    + YB × B    + YC × C
                    GR              GG              GB
Z = N = ZA × A           + ZB × B        + ZC × C

In other words, the A, B, and C components are first decoded individually by the
gamma functions. The results are treated as a three-element vector and multi-
plied by Matrix (a 3-by-3 matrix) to obtain the L, M, and N components of the in-
termediate representation. Since there is no second stage, these are also the X, Y,
and Z components of the final representation.

Example 4.7 shows an example of a CalRGB color space for the CCIR XA/11–
recommended D65 white point with 1.8 gammas and Sony Trinitron phosphor
chromaticities.

Example 4.7

   [ /CalRGB
          << /WhitePoint [ 0.9505 1.0000 1.0890 ]
             /Gamma [ 1.8000 1.8000 1.8 000]
             /Matrix [ 0.4497 0.2446 0.0252
                       0.3163 0.6720 0.1412
                       0.1845 0.0833 0.9227
                     ]
          >>
   ]

In some cases, the parameters of a CalRGB color space may be specified in terms
of the CIE 1931 chromaticity coordinates (xR , yR ), (xG , yG ), (xB , yB ) of the red,
green, and blue phosphors, respectively, and the chromaticity (xW, yW ) of the dif-
fuse white point corresponding to some linear RGB value (R, G, B), where usually
R = G = B = 1.0. Note that standard CIE notation uses lowercase letters to specify

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