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                                                      248
             CHAPTER 4                                                                      Graphics



             cients are specified by the Gamma entry in the color space dictionary (see Table
             4.14). The transformation matrix denoted by “Matrix ABC” in Figure 4.14 is de-
             fined by the dictionary’s Matrix entry. Since there is no second transformation
             stage, “Decode LMN” and “Matrix LMN” are implicitly taken to be identity trans-
             formations.

                            TABLE 4.14 Entries in a CalRGB color space dictionary
KEY              TYPE      VALUE

WhitePoint       array     (Required) An array of three numbers [ XW YW ZW ] specifying the tristimulus value,
                           in the CIE 1931 XYZ space, of the diffuse white point; see below for further discus-
                           sion. The numbers XW and ZW must be positive, and YW must be equal to 1.0.

BlackPoint       array     (Optional) An array of three numbers [ XB YB ZB ] specifying the tristimulus value, in
                           the CIE 1931 XYZ space, of the diffuse black point; see below for further discussion.
                           All three of these numbers must be non-negative. Default value: [ 0.0 0.0 0.0 ].

Gamma            array     (Optional) An array of three numbers [ GR GG GB ] specifying the gamma for the red,
                           green, and blue (A, B, and C) components of the color space. Default value:
                           [ 1.0 1.0 1.0 ].

Matrix           array     (Optional) An array of nine numbers [ XA YA ZA XB YB ZB XC YC ZC ] specifying
                           the linear interpretation of the decoded A, B, and C components of the color space
                           with respect to the final XYZ representation. Default value: the identity matrix
                           [ 1 0 0 0 1 0 0 0 1 ].


             The WhitePoint and BlackPoint entries in the color space dictionary control the
             overall effect of the CIE-based gamut mapping function described in Section 6.1,
             “CIE-Based Color to Device Color.” Typically, the colors specified by WhitePoint
             and BlackPoint are mapped to the nearly lightest and nearly darkest achromatic
             colors that the output device is capable of rendering in a way that preserves color
             appearance and visual contrast.

             WhitePoint is assumed to represent the diffuse achromatic highlight, not a specu-
             lar highlight. Specular highlights, achromatic or otherwise, are often reproduced
             lighter than the diffuse highlight. BlackPoint is assumed to represent the diffuse
             achromatic shadow; its value is typically limited by the dynamic range of the in-
             put device. In images produced by a photographic system, the values of
             WhitePoint and BlackPoint vary with exposure, system response, and artistic in-
             tent; hence, their values are image-dependent.

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