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                                                 366
       CHAPTER 4                                                                         Graphics



       an optional content group but rather to an optional content membership dictio-
       nary, whose entries are shown in Table 4.49. (Section 4.10.2 describes how con-
       tent declares its membership in a group or membership dictionary.)

                 TABLE 4.49 Entries in an optional content membership dictionary
KEY     TYPE            VALUE

Type    name            (Required) The type of PDF object that this dictionary describes; must be OCMD
                        for an optional content membership dictionary.

OCGs    dictionary or   (Optional) A dictionary or array of dictionaries specifying the optional content
        array           groups whose states determine the visibility of content controlled by this member-
                        ship dictionary.
                        Note: Null values or references to deleted objects are ignored. If this entry is not
                        present, is an empty array, or contains references only to null or deleted objects, the
                        membership dictionary has no effect on the visibility of any content.

P       name            (Optional) A name specifying the visibility policy for content belonging to this
                        membership dictionary. Valid values are:
                        • AllOn: visible only if all of the entries in OCGs are ON
                        • AnyOn: visible if any of the entries in OCGs are ON
                        • AnyOff: visible if any of the entries in OCGs are OFF
                        • AllOff: visible only if all of the entries in OCGs are OFF
                        Default value: AnyOn

VE      array           (Optional; PDF 1.6) An array specifying a visibility expression, used to compute
                        visibility of content based on a set of optional content groups; see discussion be-
                        low.

       An optional content membership dictionary can express its visibility policy in
       two ways:

       • The P entry specifies a simple boolean expression indicating how the optional
         content groups specified by the OCGs entry determine the visibility of content
         controlled by the membership dictionary.
       • PDF 1.6 introduces the VE entry, which is a visibility expression that can specify
         an arbitrary boolean expression for computing the visibility of content from the
         states of optional content groups.

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