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                                                      767
        SECTION 9.1                                                                       Multimedia



        • If BU is not present and the media is embedded within the document, the URL
          to the PDF file itself should be used as if it were the value of a BU entry in the BE
          dictionary; that is, as an implicit best-effort base URL.

                          TABLE 9.11 Entries in a media clip data MH/BE dictionary
KEY           TYPE          VALUE

BU            ASCII         (Optional) An absolute URL to be used as the base URL in resolving any relative
              string        URLs found within the media data.


        Media Clip Section

        A media clip section dictionary (see Table 9.12) defines a continuous section of
        another media clip object (known as the next-level media clip object). For exam-
        ple, a media clip section could define a 15-minute segment of a media clip data
        object representing a two-hour movie. The next-level media clip object, specified
        by the D entry, can be either a media clip data object or another media clip sec-
        tion object. However, the linked list formed by the D entries of media clip sec-
        tions must terminate in a media clip data object. If the next-level media object is
        non-viable, the media clip section is also non-viable.

                       TABLE 9.12 Additional entries in a media clip section dictionary
KEY   TYPE                  VALUE

D     dictionary            (Required) The media clip section or media clip data object (the next-level media
                            object) of which this media clip section object defines a continuous section.

Alt   array                 (Optional) An array that provides alternate text descriptions for the media clip sec-
                            tion in case it cannot be played; see “Multi-language Text Arrays” on page 942.

MH    dictionary            (Optional) A dictionary whose entries (see Table 9.13) must be honored for the me-
                            dia clip section to be considered viable.

BE    dictionary            (Optional) A dictionary whose entries (see Table 9.13) need only be honored in a
                            “best effort” sense.


        The B and E entries in the media clip section’s MH and BE dictionaries (see Table
        9.13) define a subsection of the next-level media object referenced by D by speci-
        fying beginning and ending offsets into it. Depending on the media type, the off-
        sets may be specified by time, frames, or markers (see “Media Offset Dictionary”
        on page 775). B and E are not required to specify the same type of offset.

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