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                                                          246
             CHAPTER 4                                                                             Graphics




                            Decode A                            Decode LMN


                                                           L                                   X

                      A                    Matrix A        M                 Matrix LMN        Y

                                                           N                                   Z




                     FIGURE 4.15 Component transformations in a CIE-based A color space


             A CalGray color space is a CIE-based A color space with only one transformation
             stage instead of two. In this type of space, A represents the gray component of a
             calibrated gray space. This component must be in the range 0.0 to 1.0. The decod-
             ing function (denoted by “Decode A” in Figure 4.15) is a gamma function whose
             coefficient is specified by the Gamma entry in the color space dictionary (see Ta-
             ble 4.13). The transformation matrix denoted by “Matrix A” in the figure is de-
             rived from the dictionary’s WhitePoint entry, as described below. Since there is no
             second transformation stage, “Decode LMN” and “Matrix LMN” are implicitly
             taken to be identity transformations.

                               TABLE 4.13 Entries in a CalGray color space dictionary
KEY                 TYPE               VALUE

WhitePoint          array              (Required) An array of three numbers [ XW YW ZW ] specifying the tri-
                                       stimulus value, in the CIE 1931 XYZ space, of the diffuse white point; see
                                       “CalRGB Color Spaces,” below, for further discussion. 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 “CalRGB
                                       Color Spaces,” below, for further discussion. All three of these numbers must
                                       be non-negative. Default value: [ 0.0 0.0 0.0 ].

Gamma               number             (Optional) A number G defining the gamma for the gray (A) component. G
                                       must be positive and is generally greater than or equal to 1. Default value: 1.

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