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                                                        857
             SECTION 10.6                                                             Logical Structure



             Structure elements are represented by a dictionary, whose entries are shown in
             Table 10.10. The K entry specifies the children of the structure element, which
             can be zero or more items of the following kinds:

             • Other structure elements
             • References to content items, which are either marked-content sequences (see
               Section 10.5, “Marked Content”) or complete PDF objects such as XObjects
               and annotations. These content items represent the graphical content, if any,
               associated with a structure element. Content items are discussed in detail in
               Section 10.6.3, “Structure Content.”

                                   TABLE 10.9 Entries in the structure tree root
KEY                   TYPE            VALUE

Type                  name            (Required) The type of PDF object that this dictionary describes; must be
                                      StructTreeRoot for a structure tree root.

K                     dictionary      (Optional) The immediate child or children of the structure tree root in the
                      or array        structure hierarchy. The value may be either a dictionary representing a sin-
                                      gle structure element or an array of such dictionaries.

IDTree                name tree       (Required if any structure elements have element identifiers) A name tree that
                                      maps element identifiers (see Table 10.10) to the structure elements they
                                      denote.

ParentTree            number tree     (Required if any structure element contains content items) A number tree
                                      (see Section 3.8.6, “Number Trees”) used in finding the structure elements
                                      to which content items belong. Each integer key in the number tree corre-
                                      sponds to a single page of the document or to an individual object (such as
                                      an annotation or an XObject) that is a content item in its own right. The in-
                                      teger key is given as the value of the StructParent or StructParents entry in
                                      that object (see “Finding Structure Elements from Content Items” on page
                                      868). The form of the associated value depends on the nature of the object:
                                      • For an object that is a content item in its own right, the value is an indi-
                                         rect reference to the object’s parent element (the structure element that
                                         contains it as a content item).
                                      • For   a page object or content stream containing marked-content
                                         sequences that are content items, the value is an array of references to the
                                         parent elements of those marked-content sequences.
                                      See “Finding Structure Elements from Content Items” on page 868 for fur-
                                      ther discussion.

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