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This document describes actual and proposed changes to the djvu
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format since the release of the DjVu3 specification by Lizardtech in
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2- ESCAPE SEQUENCES IN ANNOTATION CHUNK STRINGS
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The treatment of escape sequence in annotation chunk strings has
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historically been slightly different in Lizardtech DjVu and
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DjVuLibre. We are expecting that the DjVuLibre solution will
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eventually become the standard.
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Lizardtech DjVu uses the "old" rule described in section 8.3.4.2.
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The sequence of characters BACKSLASH DOUBLEQUOTE represents a
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DOUBLEQUOTE character without terminating the string. There are no
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other escape sequences. All other utf8 characters are written
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directly. The main drawback of this approach is the inability to
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write a string containing the sequence of characters BACKSLASH
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DOUBLEQUOTE since there is no way to escape the first BACKSLASH
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DjVuLibre has introduced a more flexible scheme a few years ago.
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Annotation strings are similar to strings in the C language.
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Character sequences starting with a backslash have special meaning.
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A BACKSLASH followed by "a", "b", "t", "n", "v", "f", "r", or "\"
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stands for the ascii character BEL, BS, HT, LF, VT, FF, CR,
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BACKSLASH or DOUBLEQUOTE. A BACKSLASH followed by one to three
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digits stands for the byte whose octal code is expressed by the
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digits. All other backslash sequences are illegal. Non printable
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ascii characters must be escaped. Multibyte characters should either
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be entered directly, or represented using octal sequences.
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DjVuLibre minimizes the compatibility problems by searching illegal
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escape sequences in the annotation chunk. If any illegal sequence
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is found, the Lizardtech rule is used instead of the DjVuLibre rule.
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It is expected that Lizardtech will at some point adopt the improved
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DjVuLibre rules. We will then be able to state that all DjVu files
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with version greater than some constant use the
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Each page in a DjVu document is identified by three strings named
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the ID, the NAME, and the TITLE. The semantic distinction between
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these three strings is no longer very clear. The current software
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does not work consistently when these strings are different. See
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the comment in the table, section 8.3.2.2 of the specification.
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Recent versions of DjVuLibre still require that the ID and NAME
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string are equal. The TITLE string however can be different and
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should be used to display friendly page names. The djvused program
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now features a command 'set-page-title' to install a TITLE different
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from the ID string. The djview program then displays and recognizes
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these page titles in lieu of the sequential page numbers.
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4- METADATA ANNOTATIONS
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DjVuLibre has introduced metadata annotations a few years ago.
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Metadata entries for each page is represent by key/value pairs
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located in a metadata directive in the annotation chunk.
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The metadata directive has the form
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(metadata ... (key "value") ... )
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Each entry is identified by a symbol <key> representing the nature
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of the meta data entry. Typical keys include 'year', 'booktitle',
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'editor', 'author', etc. It is suggested to use the same key names
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as the BibTeX bibliography system. The string <"value"> represents
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the value associated with the corresponding key.
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5- ANNOTATIONS FOR THE DOCUMENT
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This scheme provides a simple way to specify metadata for each page.
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But it is often useful to provide metadata that applies to the whole
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document. Document wide metadata are represented using one or
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several metadata directives in the shared annotations chunk.
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This scheme has a potential drawback. Since the shared annotations
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is included by all pages, the document wide metadata also appears as
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page metadata for all pages. This might not be adequate for some
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uses. As a workaround, the djview4 viewer (in preparation) only
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displays the page metadata that differ from the document metadata.
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A more definitive answer would be the definition of a document
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annotation chunk located after the DIRM chunk and before any
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component file. This space is already used by the NAVM chunk.
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This is being considered.
added
removed
doc/djvuchanges.txt
