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SECTION 3.3                                                                  Filters



The JBIG2 compression method can also be used for encoding multiple images
into a single JBIG2 bit stream. Typically, these images are scanned pages of a
multiple-page document. Since a single table of symbol bitmaps is used to match
symbols across multiple pages, this type of encoding can result in higher
compression ratios than if each of the pages had been individually encoded using
JBIG2.

In general, an image may be specified in PDF as either an image XObject or an
inline image (as described in Section 4.8, “Images”); however, the JBIG2Decode
filter can be applied only to image XObjects.

This filter addresses both single-page and multiple-page JBIG2 bit streams by
representing each JBIG2 page as a PDF image, as follows:

• The filter uses the embedded file organization of JBIG2. (The details of this and
  the other types of file organization are provided in an annex of the ISO specifi-
  cation.) The optional 2-byte combination (marker) mentioned in the specifica-
  tion is not used in PDF. JBIG2 bit streams in random-access organization
  should be converted to the embedded file organization. Bit streams in sequen-
  tial organization need no reorganization, except for the mappings described
  below.
• The JBIG2 file header, end-of-page segments, and end-of-file segment are not
  used in PDF. These should be removed before the PDF objects described below
  are created.
• The image XObject to which the JBIG2Decode filter is applied contains all seg-
  ments that are associated with the JBIG2 page represented by that image; that
  is, all segments whose segment page association field contains the page number
  of the JBIG2 page represented by the image. In the image XObject, however, the
  segment’s page number should always be 1; that is, when each such segment is
  written to the XObject, the value of its segment page association field should be
  set to 1.
• If the bit stream contains global segments (segments whose segment page asso-
  ciation field contains 0), these segments must be placed in a separate PDF
  stream, and the filter parameter listed in Table 3.10 should refer to that stream.
  The stream can be shared by multiple image XObjects whose JBIG2 encodings
  use the same global segments.

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