SECTION 10.10
979
Prepress Support
ating low-resolution placeholders, or
proxies,
for such high-resolution images.
The proxy typically consists of a downsampled version of the full-resolution
image, to be used for screen display and proofing. Before the document is print-
ed, it passes through a filter known as an
OPI server,
which replaces the proxies
with the original full-resolution images.
In PostScript programs, OPI proxies are defined by PostScript code surrounded
by special
OPI comments,
which specify such information as the placement and
cropping of the image and adjustments to its size, rotation, color, and other
attributes. In PDF, proxies are embedded in a document as image or form
XObjects with an associated
OPI dictionary (PDF 1.2)
containing the same infor-
mation conveyed in PostScript by the OPI comments. Two versions of OPI are
supported, versions 1.3 and 2.0. In OPI 1.3, a proxy consisting of a single image,
with no changes in the graphics state, may be represented as an image XObject;
otherwise it must be a form XObject. In OPI 2.0, the proxy always entails changes
in the graphics state and hence must be represented as a form XObject. (See
implementation notes 173 and 174 in Appendix H.)
An XObject representing an OPI proxy must contain an
OPI
entry in its image or
form dictionary (see Table 4.39 on page 340 and Table 4.45 on page 358). The val-
ue of this entry is an
OPI version dictionary
of OPI to which the proxy corresponds. This dictionary consists of a single entry,
whose key is the name
1.3
or
2.0
and whose value is the OPI dictionary defining
the proxy’s OPI attributes.
TABLE 10.54 Entry in an OPI version dictionary
KEY
TYPE
VALUE
version number
dictionary
(Required; PDF 1.2)
An OPI dictionary specifying the attributes of this proxy
(see Tables 10.55 and 10.56). The key for this entry must be the name
1.3
or
2.0
, identifying the version of OPI to which the proxy corresponds.
Note:
As in any other PDF dictionary, the key in an OPI version dictionary must be
a name object. The OPI version dictionary would thus be written in the PDF file in
either the form
<< /1 . 3
d
0 R >>
% OPI 1 . 3 dictionary
or
<< /2 . 0
d
0 R >>
% OPI 2 . 0 dictionary
where
d
is the object number of the corresponding OPI dictionary.