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                                                   542
     CHAPTER 7                                                              Transparency



     Note: When a non-isolated group is nested within a knockout group, the initial
     backdrop of the inner group is the same as that of the outer group; it is not the im-
     mediate backdrop of the inner group. This behavior, although perhaps unexpected,
     is a consequence of the group compositing formulas when b = 0.

7.3.6 Page Group

     All of the elements painted directly onto a page—both top-level groups and top-
     level objects that are not part of any group—are treated as if they were contained
     in a transparency group P, which in turn is composited with a context-dependent
     backdrop. This group is called the page group.

     The page group can be treated in two distinctly different ways:

     • Ordinarily, the page is imposed directly on an output medium, such as paper or
       a display screen. The page group is treated as an isolated group, whose results
       are then composited with a backdrop color appropriate for the medium. The
       backdrop is nominally white, although varying according to the actual proper-
       ties of the medium. However, some applications may choose to provide a dif-
       ferent backdrop, such as a checkerboard or grid to aid in visualizing the effects
       of transparency in the artwork.
     • A “page” of a PDF file can be treated as a graphics object to be used as an ele-
       ment of a page of some other document. This case arises, for example, when
       placing a PDF file containing a piece of artwork produced by Illustrator into a
       page layout produced by InDesign®. In this situation, the PDF “page” is not
       composited with the media color; instead, it is treated as an ordinary transpar-
       ency group, which can be either isolated or non-isolated and is composited
       with its backdrop in the normal way.

     The remainder of this section pertains only to the first use of the page group,
     where it is to be imposed directly on the medium.

     The color C of the page at a given point is defined by a simplification of the gen-
     eral group compositing formula:
                                       ·
      〈 C g , f g , α g 〉 = Composite (U, 0, P )
                  C = ( 1 – αg ) × W + αg × Cg

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