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                                                 547
    SECTION 7.5                                             Specifying Transparency in PDF



    G can be any kind of group—isolated or not, knockout or not—producing vari-
    ous effects on the C result in each case. The color C is then converted to luminos-
    ity in one of the following ways, depending on the group’s color space:

    • For CIE-based spaces, convert to the CIE 1931 XYZ space and use the Y com-
      ponent as the luminosity. This produces a colorimetrically correct luminosity.
      In the case of a PDF CalRGB space, the formula is
                    GR              GG                 GB
      Y = YA × A         + YB × B        + YC × C

      using components of the Gamma and Matrix entries of the color space dictio-
      nary (see Table 4.14 on page 248). An analogous computation applies to other
      CIE-based color spaces.
    • For device color spaces, convert the color to DeviceGray by device-dependent
      means and use the resulting gray value as the luminosity, with no compensa-
      tion for gamma or other color calibration. This method makes no pretense of
      colorimetric correctness; it merely provides a numerically simple means to pro-
      duce continuous-tone mask values. Here are some recommended formulas for
      converting from DeviceRGB and DeviceCMYK, respectively:
      Y = 0.30 × R + 0.59 × G + 0.11 × B

      Y = 0.30 × ( 1 – C ) × ( 1 – K )
              + 0.59 × ( 1 – M ) × ( 1 – K )
                + 0.11 × ( 1 – Y ) × ( 1 – K )

    Following this conversion, the result is passed through a separately specified
    transfer function, allowing the masking effect to be customized.

    The backdrop color most likely to be useful is black, which causes any areas out-
    side the group’s shape to have zero luminosity values in the resulting mask. If the
    contents of the group are viewed as a positive mask, this produces the results that
    would be expected with respect to points outside the shape.


7.5 Specifying Transparency in PDF

    The preceding sections have presented the transparent imaging model at an
    abstract level, with little mention of its representation in PDF. This section
    describes the facilities available for specifying transparency in PDF 1.4.

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