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                                         543
SECTION 7.3                                                       Transparency Groups



where the variables have the meanings shown in Table 7.9. The first formula com-
putes the color and alpha for the group given a transparent backdrop—in effect,
treating P as an isolated group. The second formula composites the results with
the context-dependent backdrop (using the equivalent of the Normal blend
mode).

           TABLE 7.9 Variables used in the page group compositing formulas
VARIABLE             MEANING

P                    The page group, consisting of all elements E1, … , En in the page’s
                     top-level stack

Cg                   Computed color of the page group

fg                   Computed shape of the page group

αg                   Computed alpha of the page group

C                    Computed color of the page

W                    Initial color of the page (nominally white but may vary depending
                     on the properties of the medium or the needs of the application)

U                    An undefined color (which is not used, since the α0 argument of
                     Composite is 0)

If not otherwise specified, the page group’s color space is inherited from the
native color space of the output device—that is, a device color space, such as
DeviceRGB or DeviceCMYK. It is often preferable to specify an explicit color space,
particularly a CIE-based space, to ensure more predictable results of the compos-
iting computations within the page group. In this case, all page-level compositing
is done in the specified color space, with the entire result then converted to the
native color space of the output device before being composited with the context-
dependent backdrop. This case also arises when the page is not actually being
rendered but is converted to a flattened representation in an opaque imaging
model, such as PostScript.

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