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                                               374
       CHAPTER 4                                                                   Graphics



       Optional Content in XObjects and Annotations

       In addition to marked content within content streams, form XObjects and image
       XObjects (see Section 4.7, “External Objects”) and annotations (see Section 8.4,
       “Annotations”) may contain an OC entry, which is an optional content group or
       an optional content membership dictionary.

       A form or image XObject's visibility is determined by the state of the group or
       those of the groups referenced by the membership dictionary in conjunction with
       its P (or VE) entry, along with the current visibility state in the context in which
       the XObject is invoked (that is, whether objects are visible in the contents stream
       at the place where the Do operation occurred).

       Annotations have various flags controlling on-screen and print visibility (see Sec-
       tion 8.4.2, “Annotation Flags”). If an annotation contains an OC entry, it is visible
       for screen or print only if the flags have the appropriate settings and the group or
       membership dictionary indicates it is visible.


4.10.3 Configuring Optional Content

       A PDF document containing optional content can specify the default states for
       the optional content groups in the document and indicate which external factors
       should be used to alter the states. The following sections describe the PDF struc-
       tures that are used to specify this information.

       • “Optional Content Properties Dictionary” on page 375 describes the structure
         that lists all the optional content groups in the document and their possible
         configurations.
       • “Optional Content Configuration Dictionaries” on page 375 describes the
         structures that specify initial state settings and other information about the
         groups in the document.
       • “Usage and Usage Application Dictionaries” on page 380 and “Determining the
         State of Optional Content Groups” on page 385 describe how the states of
         groups can be affected based on external factors.

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