CHAPTER 10
850
Document Interchange
10.5 Marked Content
Marked-content operators (PDF 1.2)
identify a portion of a PDF content stream as
a
marked-content element
of interest to a particular application or PDF plug-in
extension. Marked-content elements and the operators that mark them fall into
two categories:
•
The
MP
and
DP
operators designate a single
marked-content point
in the con-
tent stream.
•
The
BMC
,
BDC
, and
EMC
operators bracket a
marked-content sequence
of ob-
jects within the content stream. Note that this is a sequence not simply of bytes
in the content stream but of complete graphics objects. Each object is fully
qualified by the parameters of the graphics state in which it is rendered.
A graphics application, for example, might use marked content to identify a set of
related objects as a group to be processed as a single unit. A text-processing
application might use it to maintain a connection between a footnote marker in
the body of a document and the corresponding footnote text at the bottom of the
page. The PDF logical structure facilities use marked-content sequences to asso-
ciate graphical content with structure elements (see Section 10.6.3, “Structure
All marked-content operators except
EMC
take a
tag
operand indicating the role
or significance of the marked-content element to the processing application. All
such tags must be registered with Adobe Systems (see Appendix E) to avoid con-
flicts between different applications marking the same content stream. In addi-
tion to the tag operand, the
DP
and
BDC
operators specify a
property list
containing further information associated with the marked content. Property lists
are discussed further in Section 10.5.1, “Property Lists.”
Marked-content operators may appear only
between
graphics objects in the con-
tent stream. They may not occur within a graphics object or between a graphics
state operator and its operands. Marked-content sequences may be nested one
within another, but each sequence must be entirely contained within a single con-
tent stream; it may not cross page boundaries, for example.
Note:
The
Contents
entry of a page object (see “Page Objects” on page 144), which
may be either a single stream or an array of streams, is considered a single stream
with respect to marked-content sequences.