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     CHAPTER 10                                                   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
     Content”). Table 10.7 summarizes the marked-content operators.

     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.

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