SECTION 10.6
857
Logical Structure
Structure elements are represented by a dictionary, whose entries are shown in
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
and annotations. These content items represent the graphical content, if any,
associated with a structure element. Content items are discussed in detail in
TABLE 10.9 Entries in the structure tree root
KEY
TYPE
VALUE
Type
name
dictionary
or array
name tree
(Required)
The type of PDF object that this dictionary describes; must be
StructTreeRoot
for a structure tree root.
(Optional)
The immediate child or children of the structure tree root in the
structure hierarchy. The value may be either a dictionary representing a sin-
gle structure element or an array of such dictionaries.
(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.
(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
K
IDTree
ParentTree
number tree
•
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.