SECTION 10.10
975
Prepress Support
Trapping zones and trapping parameters are discussed fully in Sections 6.3.2 and
6.3.3, respectively, of the
PostScript Language Reference,
Third Edition. Trapping
instructions are not directly specified in a PDF file (as they are in a PostScript
file). Instead, they are specified in a
job ticket
that accompanies the PDF file or
can be embedded within it. Various standards exist for the format of job tickets;
two of them, JDF (Job Definition Format) and PJTF (Portable Job Ticket For-
mat), are described in the CIP4 document
JDF Specification
and in Adobe Tech-
nical Note #5620,
Portable Job Ticket Format
(see the Bibliography).
When trapping is performed before the production of final output, the resulting
traps are placed in the PDF file for subsequent use. The traps themselves are
described as a content stream in a trap network annotation (see below). The
stream dictionary can include additional entries describing the method that was
used to produce the traps and other information about their appearance.
Trap Network Annotations
A complete set of traps generated for a given page under a specified set of trap-
ping instructions is called a
trap network (PDF 1.3).
It is a form XObject contain-
ing graphics objects for painting the required traps on the page. A page may have
more than one trap network based on different trapping instructions, presumably
intended for different output devices. All of the trap networks for a given page are
contained in a single
trap network annotation
(see Section 8.4, “Annotations”).
There can be at most one trap network annotation per page, which must be the
last element in the page’s
Annots
array (see “Page Objects” on page 144). This en-
sures that the trap network is printed after all of the page’s other contents. (See
The form XObject defining a trap network is specified as an appearance stream in
the
N
(normal) entry of the trap network annotation’s appearance dictionary (see
for the same page, the
N
entry holds a subdictionary containing the alternate trap
networks, each identified by an arbitrary key. The
AS
(appearance state) entry in
the annotation dictionary designates one of them as the
current trap network
to
be displayed or printed.
Note:
The trap network annotation’s appearance dictionary may include
R
(rollover)
or
D
(down) entries, but appearances defined in either of these entries are never
printed.