CHAPTER 10
848
Document Interchange
same value. If both identifiers match when a file reference is resolved, it is very
likely that the correct file has been found. If only the first identifier matches, a
different version of the correct file has been found.
To help ensure the uniqueness of file identifiers, it is recommend that they be
computed by means of a message digest algorithm such as MD5 (described in In-
ternet RFC 1321,
The MD5 Message-Digest Algorithm;
see the Bibliography), us-
ing the following information (see implementation note 163 in Appendix H):
•
The current time
•
A string representation of the file’s location, usually a pathname
•
The size of the file in bytes
•
The values of all entries in the file’s document information dictionary (see
10.4 Page-Piece Dictionaries
A
page-piece dictionary (PDF 1.3)
can be used to hold private application data.
The data can be associated with a page or form XObject by means of the optional
PieceInfo
entry in the page object (see Table 3.27 on page 145) or form dictionary
(see Table 4.45 on page 358). Beginning with PDF 1.4, private data may also be
associated with the PDF document by means of the
PieceInfo
entry in the docu-
ment catalog (see Table 3.25 on page 139).
Applications can use this dictionary as a place to store private data in connection
with that document, page, or form. Such private data can convey information
meaningful to the application that produces it (such as information on object
grouping for a graphics editor or the layer information used by Adobe Photo-
shop
®
) but is typically ignored by general-purpose PDF viewer applications.
As Table 10.5 shows, a page-piece dictionary may contain any number of entries,
each keyed by the name of a distinct application or of a well-known data type rec-
ognized by a family of applications. The value associated with each key is an
ap-
plication data dictionary
containing the private data to be used by the application.
The
Private
entry may have a value of any data type, but typically it is a dictionary
containing all of the private data needed by the application other than the actual
content of the document, page, or form.