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                                                         979
          SECTION 10.10                                                                 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 (Table 10.54) identifying the version
          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.

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