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                                                   556
          CHAPTER 7                                                                Transparency



          When preblended image data is used in transparency blending and compositing
          computations, the results are the same as if the original, unblended image data
          were used and no matte color were specified. In particular, the inputs to the blend
          function are the original color values. To derive c from c′, the application may
          sometimes need to invert the formula shown above. If the resulting c value lies
          outside the range of color component values for the image color space, the results
          are unpredictable.

          The preblending computation is done in the color space specified by the parent
          image’s ColorSpace entry. This is independent of the group color space into which
          the image may be painted. If a color conversion is required, inversion of the preb-
          lending must precede the color conversion. If the image color space is an Indexed
          space (see “Indexed Color Spaces” on page 262), the color values in the color table
          (not the index values themselves) are preblended.


    7.5.5 Transparency Group XObjects

          A transparency group is represented in PDF as a special type of group XObject
          (see Section 4.9.2, “Group XObjects”) called a transparency group XObject. A
          group XObject is in turn a type of form XObject, distinguished by the presence of
          a Group entry in its form dictionary (see Section 4.9.1, “Form Dictionaries”). The
          value of this entry is a subsidiary group attributes dictionary defining the proper-
          ties of the group. The format and meaning of the dictionary’s contents are deter-
          mined by its group subtype, which is specified by the dictionary’s S entry. The
          entries for a transparency group (subtype Transparency) are shown in Table 7.13.

          Note: A page object (see “Page Objects” on page 144) may also have a Group entry,
          whose value is a group attributes dictionary specifying the attributes of the page
          group (see Section 7.3.6, “Page Group”). Some of the dictionary entries are inter-
          preted slightly differently for a page group than for a transparency group XObject;
          see their descriptions in the table for details.

           TABLE 7.13 Additional entries specific to a transparency group attributes dictionary
KEY       TYPE               VALUE

S         name               (Required) The group subtype, which identifies the type of group whose at-
                             tributes this dictionary describes; must be Transparency for a transparency
                             group.

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