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                                                558
      CHAPTER 7                                                                  Transparency



KEY   TYPE               VALUE

I     boolean            (Optional) A flag specifying whether the transparency group is isolated (see Sec-
                         tion 7.3.4, “Isolated Groups”). If this flag is true, objects within the group are
                         composited against a fully transparent initial backdrop; if false, they are com-
                         posited against the group’s backdrop. Default value: false.
                         In the group attributes dictionary for a page, the interpretation of this entry is
                         slightly altered. In the normal case in which the page is imposed directly on the
                         output medium, the page group is effectively isolated and the specified I value is
                         ignored. But if the page is in turn used as an element of some other page, it is
                         treated as if it were a transparency group XObject; the I value is interpreted in
                         the normal way to determine whether the page group is isolated.

K     boolean            (Optional) A flag specifying whether the transparency group is a knockout
                         group (see Section 7.3.5, “Knockout Groups”). If this flag is false, later objects
                         within the group are composited with earlier ones with which they overlap; if
                         true, they are composited with the group’s initial backdrop and overwrite
                         (“knock out”) any earlier overlapping objects. Default value: false.

      The transparency group XObject’s content stream defines the graphics objects be-
      longing to the group. Invoking the Do operator on the XObject executes its con-
      tent stream and composites the resulting group color, shape, and opacity into the
      group’s parent group or page as if they had come from an elementary graphics ob-
      ject. When applied to a transparency group XObject, Do performs the following
      actions in addition to the normal ones for a form XObject (as described in Sec-
      tion 4.9, “Form XObjects”):

      • If the transparency group is non-isolated (the value of the I entry in its group
        attributes dictionary is false), its initial backdrop, within the bounding box
        specified by the XObject’s BBox entry, is defined to be the accumulated color
        and alpha of the parent group or page—that is, the result of everything that has
        been painted in the parent up to that point. (However, if the parent is a knock-
        out group, the initial backdrop is the same as that of the parent.) If the group is
        isolated (I is true), its initial backdrop is defined to be transparent.
      • Before execution of the transparency group XObject’s content stream, the cur-
        rent blend mode in the graphics state is initialized to Normal, the current strok-
        ing and nonstroking alpha constants to 1.0, and the current soft mask to None.
        Note: The purpose of initializing these graphics state parameters at the beginning
        of execution is to ensure that they are not applied twice: once when member ob-
        jects are painted into the group and again when the group is painted into the par-
        ent group or page.

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