SECTION 4.10
367
Optional Content
Note:
Since the
VE
entry is more general, if it is present and supported by the PDF
consumer software, it should be used in preference to
OCGs
and
P
. However, for
compatibility purposes, PDF creators should use
OCGs
and
P
entries where possible.
When the use of
VE
is necessary to express the intended behavior,
OCGs
and
P
en-
tries should also be provided to approximate the behavior in older consumer soft-
ware.
A visibility expression is an array with the following characteristics:
Its first element is a name representing a boolean operator (
And
,
Or
, or
Not
).
Subsequent elements are either optional content groups or other visibility ex-
pressions.
If the first element is
Not
, it should have only one subsequent element. If the
first element is
And
or
Or
, it may have one or more subsequent elements.
In evaluating a visibility expression, the
ON
state of an optional content group is
equated to the boolean value
true
;
OFF
is equated to
false
.
Examples 4.33 and 4.34 illustrate the use of visibility expressions.
Membership dictionaries are useful in cases such as these:
Some content may choose to be
invisible
when a group is
ON
and
visible
when it
is
OFF
. In this case, the content would belong to a membership dictionary
whose
OCGs
entry consists of a single optional content group and whose
P
en-
try is
AnyOff
or
AllOff
.
Note:
It is legal to have an
OCGs
entry consisting of a single group and a
P
entry
that is
AnyOn
or
AllOn
. However, in this case it is preferable to use an optional
content group directly because it uses fewer objects.
Some content may belong to more than one group and must specify its policy
when the groups are in conflicting states. In this case, the content would belong
to a membership dictionary whose
OCGs
entry consists of an array of optional
content groups and whose
P
entry specifies the visibility policy, as illustrated in
Example 4.32 below. (Example 4.33 shows the equivalent policy using visibility
expressions.)
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