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         SECTION 8.2                                                   Document-Level Navigation



         items by clicking them with the mouse. When an item is open, its immediate chil-
         dren in the hierarchy become visible on the screen; each child may in turn be
         open or closed, selectively revealing or hiding further parts of the hierarchy.
         When an item is closed, all of its descendants in the hierarchy are hidden. Click-
         ing the text of any visible item activates the item, causing the viewer application to
         jump to a destination or trigger an action associated with the item.

         The root of a document’s outline hierarchy is an outline dictionary specified by
         the Outlines entry in the document catalog (see Section 3.6.1, “Document Cata-
         log”). Table 8.3 shows the contents of this dictionary. Each individual outline item
         within the hierarchy is defined by an outline item dictionary (Table 8.4). The
         items at each level of the hierarchy form a linked list, chained together through
         their Prev and Next entries and accessed through the First and Last entries in the
         parent item (or in the outline dictionary in the case of top-level items). When dis-
         played on the screen, the items at a given level appear in the order in which they
         occur in the linked list. (See also implementation note 74 in Appendix H.)

                              TABLE 8.3 Entries in the outline dictionary
KEY         TYPE               VALUE

Type        name               (Optional) The type of PDF object that this dictionary describes; if present,
                               must be Outlines for an outline dictionary.

First       dictionary         (Required if there are any open or closed outline entries; must be an indirect ref-
                               erence) An outline item dictionary representing the first top-level item in the
                               outline.

Last        dictionary         (Required if there are any open or closed outline entries; must be an indirect ref-
                               erence) An outline item dictionary representing the last top-level item in the
                               outline.

Count       integer            (Required if the document has any open outline entries) The total number of
                               open items at all levels of the outline. This entry should be omitted if there
                               are no open outline items.


                            TABLE 8.4 Entries in an outline item dictionary
KEY         TYPE               VALUE

Title       text string        (Required) The text to be displayed on the screen for this item.

Parent      dictionary         (Required; must be an indirect reference) The parent of this item in the outline
                               hierarchy. The parent of a top-level item is the outline dictionary itself.

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