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      SECTION 7.5                                            Specifying Transparency in PDF



      • Objects painted by operators in the transparency group XObject’s content
        stream are composited into the group according to the rules described in Sec-
        tion 7.2.2, “Basic Compositing Formula.” The knockout flag (K) in the group at-
        tributes dictionary and the transparency-related parameters of the graphics
        state contribute to this computation.
      • If a group color space (CS) is specified in the group attributes dictionary, all
        painting operators convert source colors to that color space before compositing
        objects into the group, and the resulting color at each point is interpreted in
        that color space. If no group color space is specified, the prevailing color space
        is dynamically inherited from the parent group or page. (If not otherwise spec-
        ified, the page group’s color space is inherited from the native color space of the
        output device.)
      • After execution of the transparency group XObject’s content stream, the graph-
        ics state reverts to its former state before the invocation of the Do operator (as it
        does for any form XObject). The group’s shape—the union of all objects paint-
        ed into the group, clipped by the group XObject’s bounding box—is then paint-
        ed into the parent group or page, using the group’s accumulated color and
        opacity at each point.

      Note: If the Do operator is invoked more than once for a given transparency group
      XObject, each invocation is treated as a separate transparency group. That is, the
      result is as if the group were independently composited with the backdrop on each
      invocation. Applications that perform caching of rendered form XObjects must take
      this requirement into account.

      The actions described above occur only for a transparency group XObject—a
      form XObject having a Group entry that designates a group attributes subdic-
      tionary whose group subtype (S) is Transparency. An ordinary form XObject—
      one having no Group entry—is not subject to any grouping behavior for transpar-
      ency purposes. That is, the graphics objects it contains are composited individu-
      ally, just as if they were painted directly into the parent group or page.

7.5.6 Patterns and Transparency

      In the transparent imaging model, the graphics objects making up the pattern cell
      of a tiling pattern (see Section 4.6.2, “Tiling Patterns”) can include transparent
      objects and transparency groups. Transparent compositing can occur both within
      the pattern cell and between it and the backdrop wherever the pattern is painted.

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