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                                                364
       CHAPTER 4                                                                     Graphics



         for the logical structure hierarchy of one document to refer indirectly to that of
         another.


  4.10 Optional Content

       Optional content (PDF 1.5) refers to sections of content in a PDF document that
       can be selectively viewed or hidden by document authors or consumers. This ca-
       pability is useful in items such as CAD drawings, layered artwork, maps, and
       multi-language documents.

       The following sections describe the PDF structures used to implement optional
       content:
       • Section 4.10.1, “Optional Content Groups,” describes the primary structures
         used to control the visibility of content.
       • Section 4.10.2, “Making Graphical Content Optional,” describes how individu-
         al pieces of content in a document may declare themselves as belonging to one
         or more optional content groups.
       • Section 4.10.3, “Configuring Optional Content,” describes how the states of op-
         tional content groups are set.


4.10.1 Optional Content Groups

       An optional content group is a dictionary representing a collection of graphics
       that can be made visible or invisible dynamically by users of viewer applications.
       The graphics belonging to such a group can reside anywhere in the document:
       they need not be consecutive in drawing order, nor even belong to the same con-
       tent stream. Table 4.48 shows the entries in an optional content group dictionary.

                     TABLE 4.48 Entries in an optional content group dictionary
KEY        TYPE            VALUE

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

Name       text string     (Required) The name of the optional content group, suitable for presentation in
                           a viewer application’s user interface.

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