SECTION 10.6
861
Logical Structure
low the chain of associations until it either finds a structure type it recognizes or re-
turns to one it has already encountered.
Note:
In PDF versions earlier than 1.5, standard element types were never
remapped. Beginning with PDF 1.5, an element name is always mapped to its corre-
sponding name in the role map, if there is one, even if the original name is one of the
standard types. This is done to allow the element, for example, to represent a tag
with the same name as a standard role, even though its use differs from the standard
role.
10.6.3 Structure Content
Any structure element may have associated graphical content, consisting of one
or more
content items.
Content items are graphical objects that exist in the docu-
ment independently of the structure tree but are associated with structure ele-
ments as described in the following sections. Content items are of two kinds:
•
Marked-content sequences within content streams (see “Marked-Content Se-
•
Complete PDF objects such as annotations and XObjects (see “PDF Objects as
The
K
entry in a structure element dictionary (see Table 10.10) specifies the chil-
dren of the structure element, which can include any number of content items, as
well as child structure elements that may in turn have content items of their own.
Conceptually, content items must be leaf nodes of the structure tree; that is, they
cannot have other content items nested within them for purposes of logical struc-
ture. The hierarchical relationship among structure elements is represented en-
tirely by the
K
entries of the structure element dictionaries, not by nesting of the
associated content items. Therefore, the following restrictions apply:
•
A marked-content sequence delimiting a structure content item may not have
another marked-content sequence for a content item nested within it (though
non-structural marked content is allowed).
•
A structure content item may not invoke (with the
Do
operator) an XObject
that is itself a structure content item.