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                                                   955
       SECTION 10.9                                                                  Web Capture




                 TABLE 10.40 Additional entries specific to a Web Capture image set
KEY   TYPE        VALUE

S     name        (Required) The subtype of content set that this dictionary describes; must be SIS (“Spider
                  image set”) for an image set.

R     integer     (Required) The reference counts (see below) for the image XObjects belonging to the im-
      or array    age set. For an image set containing a single XObject, the value is simply the integer
                  reference count for that XObject. If the image set contains multiple XObjects, the value is
                  an array of reference counts parallel to the O array (see Table 10.38 on page 953); that is,
                  each element in the R array holds the reference count for the image XObject at the corre-
                  sponding position in the O array.

       Each image XObject in an image set has a reference count indicating the number
       of PDF pages referring to that XObject. The reference count is incremented
       whenever Web Capture creates a new page referring to the XObject (including
       copies of already existing pages) and decremented whenever such a page is
       destroyed. (The reference count is incremented or decremented only once per
       page, regardless of the number of times the XObject may be referenced by that
       same page.) When the reference count reaches 0, it is assumed that there are no
       remaining pages referring to the XObject and that it can be removed from the im-
       age set’s O array. (See implementation note 165 in Appendix H.)


10.9.4 Source Information

       The SI entry in a content set dictionary (see Table 10.38 on page 953) identifies
       one or more source information dictionaries containing information about the lo-
       cations from which the source data for the content set was retrieved. Table 10.41
       shows the contents of this type of dictionary.

                        TABLE 10.41 Entries in a source information dictionary
KEY   TYPE            VALUE

AU    ASCII string    (Required) An ASCII string or URL alias dictionary (see “URL Alias Dictionaries,” be-
      or dictionary   low) identifying the URLs from which the source data was retrieved.

TS    date            (Optional) A time stamp giving the most recent date and time at which the content set’s
                      contents were known to be up to date with the source data.

E     date            (Optional) An expiration stamp giving the date and time at which the content set’s con-
                      tents should be considered out of date with the source data.

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