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      SECTION 9.1                                                              Multimedia



9.1.1 Viability

      When playing multimedia content, the viewer application must often make deci-
      sions such as which player software and which options (for example, volume and
      duration) to use. In making these decisions, the viewer must determine the via-
      bility of the objects used. If an object is considered non-viable, the media should
      not be played. If the object is viable, the media should be played, though possibly
      under less than optimum conditions.

      There are several entries in the multimedia object dictionaries whose values have
      an effect on viability. In particular, some of the object dictionaries define two en-
      tries that divide options into one of two categories:

      • MH (“must honor”): The options specified by this entry must be honored; other-
        wise, the containing object is considered non-viable.
      • BE (“best effort”): An attempt should be made to honor the options; however, if
        they cannot be honored, the containing object is still considered viable.

      MH and BE are both dictionaries, and the same entries are defined for both of
      them. In any dictionary where these entries are allowed, both entries may be
      present, or only one, or neither. For example, the media play parameters dictio-
      nary (see Table 9.14) allows the playback volume to be set by means of the V entry
      in its MH and BE dictionaries (see Table 9.15). If the specified volume cannot be
      honored, the object is considered non-viable if V is in the MH dictionary, and
      playback should not occur. If V is in the BE dictionary (and not also in the MH
      dictionary), playback should still occur: the playing software attempts to honor
      the specified option as best it can.

      Using this mechanism, authors can specify minimum requirements (MH) and
      preferred options (BE). They can also specify how entries that are added in the fu-
      ture to the multimedia dictionaries are interpreted by old viewer applications. If
      an entry that is unrecognized by the viewer is in the MH dictionary, the object is
      considered non-viable. If an unrecognized entry is in a BE dictionary, the entry is
      ignored and viability is unaffected. Unless otherwise stated, an object should be
      considered non-viable if its MH dictionary contains an unrecognized key or an
      unrecognized value for a recognized key.

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