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                                                  362
       CHAPTER 4                                                                         Graphics



       plementation note 56 in Appendix H). Those that do implement reference XOb-
       jects can use the proxy in place of the imported content if the latter is unavailable.
       An application may also provide a user interface to allow editing and updating of
       imported content links.

       The imported content consists of a single, complete PDF page in the target docu-
       ment. It is designated by a reference dictionary, which in turn is the value of the
       Ref entry in the reference XObject’s form dictionary (see Section 4.9.1, “Form
       Dictionaries”). The presence of the Ref entry distinguishes reference XObjects
       from other types of form XObjects. Table 4.47 shows the contents of the reference
       dictionary.

                             TABLE 4.47 Entries in a reference dictionary
KEY     TYPE                  VALUE

F       file specification    (Required) The file containing the target document.

Page    integer or            (Required) A page index or page label (see Section 8.3.1, “Page Labels”) iden-
        text string           tifying the page of the target document containing the content to be
                              imported. Note that the reference is a weak one and can be inadvertently in-
                              validated if the referenced page is changed or replaced in the target document
                              after the reference is created.

ID      array                 (Optional) An array of two byte strings constituting a file identifier (see Sec-
                              tion 10.3, “File Identifiers”) for the file containing the target document. The
                              use of this entry improves an application’s chances of finding the intended file
                              and allows it to warn the user if the file has changed since the reference was
                              created.

       When the imported content replaces the proxy, it is transformed according to the
       proxy object’s transformation matrix and clipped to the boundaries of its bound-
       ing box, as specified by the Matrix and BBox entries in the proxy’s form dictionary
       (see Section 4.9.1, “Form Dictionaries”). The combination of the proxy object’s
       matrix and bounding box thus implicitly defines the bounding box of the import-
       ed page. This bounding box typically coincides with the imported page’s crop box
       or art box (see Section 10.10.1, “Page Boundaries”), but it is not required to corre-
       spond to any of the defined page boundaries. If the proxy object’s form dictionary
       contains a Group entry, the specified group attributes apply to the imported page
       as well, which allows the imported page to be treated as a group without further
       modification.

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