CHAPTER 7
542
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
–
α
g
) ×
W
+
α
g
×
C
g