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                                               758
      CHAPTER 9                                                        Multimedia Features



      The following rules apply to the entries in MH and BE dictionaries, which behave
      somewhat differently from other PDF dictionaries:

      • If an entry is required, the requirement is met if the entry is present in either
        the MH dictionary or the BE dictionary.
      • If an optional entry is not present in either dictionary, it is considered to be
        present with its default value (if one is defined) in the BE dictionary.
      • If an instance of the same entry is present in both MH and BE, the instance in
        the BE dictionary is ignored unless otherwise specified.
      • If the value of an entry in an MH or a BE dictionary is a dictionary or array, it is
        treated as an atomic unit when determining viability. That is, all entries within
        the dictionary or array must be honored for the containing object to be viable.

      Note: When determining whether entries can be honored, it is not required that
      each one be evaluated independently, since they may be dependent on one another.
      That is, a viewer application or player may examine multiple entries at once (even
      within different dictionaries) to determine whether their values can be honored.

      The following media objects have MH and BE dictionaries. They function as de-
      scribed above, except where noted in the individual sections:

      • Rendition (Table 9.2)
      • Media clip data (Table 9.11)
      • Media clip section (Table 9.13)
      • Media play parameters (Table 9.15)
      • Media screen parameters (Table 9.18)

9.1.2 Renditions

      There are two types of rendition objects:

      • A media rendition (see “Media Renditions” on page 762) is a basic media object
        that specifies what to play, how to play it, and where to play it.
      • A selector rendition (see “Selector Renditions” on page 763) contains an ordered
        list of renditions. This list may include other selector renditions, resulting in a
        tree whose leaves are media renditions. The viewer application should play the
        first viable media rendition it encounters in the tree (see Section 9.1.1, “Viabili-
        ty”).

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