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CHAPTER 9                                                     Multimedia Features



In a Markup3D ExData dictionary, the 3DA entry identifies the 3D annotation to
which the markup is associated. Even though the markup annotation exists
alongside the associated annotation in the page’s Annots array, the markup can be
thought of as a child of the 3DA annotation.

The 3DV entry specifies the markup’s associated 3D view. The markup will only
be printed and displayed when the specified view is the current view of its parent
3D annotation. This ensures that the proper context is preserved when the mark-
up is displayed. Note that an equivalent view is not sufficient; if more than one
markup specify equivalent views represented by different objects, the markups
will not display simultaneously.

The MD5 entry gives viewer applications a means to detect whether or not the 3D
stream of the 3D annotation specified by 3DA has changed. If the 3D stream has
changed, the context provided by the 3DV entry may no longer apply, and the
markup may no longer be useful. Any action taken as a response to such a situa-
tion is dependent on the viewer application, but it is recommended that a warn-
ing be issued to the user.

Example 9.5 shows how markup annotations can be associated with particular
views.

Example 9.5
   2 0 obj            % 3D stream data with two named views
       <<
           /Type /3D
           /Subtype /U3D
           /VA [4 0 R 5 0 R]
           ...
       >>
   stream
       ...
   endstream
   endobj

   3 0 obj         % 3D annotation
       <<
          /Type /Annot
          /Subtype /3D
          /3DD 2 0 R
          ...
       >>

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