0- INTRODUCTION. This document describes actual and proposed changes to the djvu format since the release of the DjVu3 specification by Lizardtech in november 2005. 1- AMBIGUOUS PARTS IN THE SPECIFICATION 1.1- BACKGROUND REDUCTION RATIO The relation between the size (W,H) of the foreground image and the size (w,h) of the background image is described in page 33 of the specification. The requirement ceil(W/w)=ceil(H/h) does not completely capture the fact that the same reduction factor should be applied to both dimensions. The requirements should be read as: <> 1.2- ENCODING OF URLS IN ANNOTATIONS Page 16 omits to specify that URLs in annotations should be percent encoded. However, consistent with the encoding of strings in the annotation chunk, UTF-8 characters in URL strings will be interpreted properly as if they were percent encoded. 1.3- FLAGS IN DJVUINFO CHUNKS Page 24 specifies that {1,6,2,5} are the only four allowed values of the "flags" field in the INFO chunk. To maximize compatibility with earlier versions of the standard, values different from {1,6,2,5} should be ignored and interpreted as 1 : rightside up orientation. 2- ESCAPE SEQUENCES IN ANNOTATION CHUNK STRINGS. The treatment of escape sequence in annotation chunk strings has historically been slightly different in Lizardtech DjVu and DjVuLibre. We are expecting that the DjVuLibre solution will eventually become the standard. Lizardtech DjVu uses the "old" rule described in section 8.3.4.2. The sequence of characters BACKSLASH DOUBLEQUOTE represents a DOUBLEQUOTE character without terminating the string. There are no other escape sequences. All other utf8 characters are written directly. The main drawback of this approach is the inability to write a string containing the sequence of characters BACKSLASH DOUBLEQUOTE since there is no way to escape the first BACKSLASH character. DjVuLibre has introduced a more flexible scheme a few years ago. Annotation strings are similar to strings in the C language. Character sequences starting with a backslash have special meaning. A BACKSLASH followed by "a", "b", "t", "n", "v", "f", "r", or "\" stands for the ascii character BEL, BS, HT, LF, VT, FF, CR, BACKSLASH or DOUBLEQUOTE. A BACKSLASH followed by one to three digits stands for the byte whose octal code is expressed by the digits. All other backslash sequences are illegal. Non printable ascii characters must be escaped. Multibyte characters should either be entered directly, or represented using octal sequences. DjVuLibre minimizes the compatibility problems by searching illegal escape sequences in the annotation chunk. If any illegal sequence is found, the Lizardtech rule is used instead of the DjVuLibre rule. It is expected that Lizardtech will at some point adopt the improved DjVuLibre rules. We will then be able to state that all DjVu files with version greater than some constant use the new convention. 3- PAGE TITLES. Each page in a DjVu document is identified by three strings named the ID, the NAME, and the TITLE. The semantic distinction between these three strings is no longer very clear. DjVu libraries does not work consistently when these strings are different. See the comment in the table, section 8.3.2.2 of the specification. Recent versions of DjVuLibre still require that the ID and NAME string are equal. The TITLE string however can be different and should be used to display friendly page names. The djvused program now features a command 'set-page-title' to install a TITLE different from the ID string. The djview program then displays and recognizes these page titles in lieu of the sequential page numbers. Things get complicated when some page titles are purely numerical. For instance, the page titled "4" might not be the fourth page of the document, and there might be many pages titled "4". The djview4 viewer uses the following rules: 3.1- DJVU CGI ARGUMENT "PAGE=". The djvu cgi argument "page=X" is interpreted as follows: - It first searches a page whose ID matches X. - Otherwise, if X has the form +N or -N where N is a number, this indicates a displacement relative to the current page. - Otherwise, it searches a page with TITLE X starting from the current page and wrapping around. - Otherwise, if X is numerical and in range, this is the page number. - Otherwise, it searches a page whose NAME matches X. 3.2- PAGE REFERENCES IN MAPAREA AND OUTLINE LINKS Page references in hyperlink annotations always have the form "#X". These references are interpretes as follows. - It first searches a page whose ID matches X. - Otherwise, if X has the form +N or -N where N is a number, this indicates a displacement relative to the page containing the link. - Otherwise, it searches a page with TITLE X starting from the current page and wrapping around. - Otherwise, if X is numerical and in range, this is the page number. - Otherwise, it searches a page whose NAME matches X. 4- METADATA 4.1- DJVULIBRE METADATA DjVuLibre has introduced metadata annotations a few years ago. Metadata entries for each page are represent by key/value pairs located in a metadata directive in the annotation chunk. Metadata entries for the document are represented similarly using the methods described in the next section. The metadata directive has the form (metadata ... (key "value") ... ) Each entry is identified by a symbol representing the nature of the metadata entry. The string <"value"> represents the value associated with the corresponding key. Several sets of keys are noteworthy. * Keys borrowed from the BibTex bibliography system. These key names are always expressed in lowercase, such as 'year', 'booktitle', 'editor', 'author', etc. * Keys borrowed from the PDF DocInfo. These key names start with an uppercase letter: 'Title', 'Author', 'Subject', 'Keywords', 'Creator', 'Producer', 'Trapped', 'CreationDate', and 'ModDate'. The values associated with the last two keys should be dates expressed according to RFC 3339. 4.2- XMP METADATA The XMP specification describes a general purpose RDF/XML format for metadata. Just like DjVuLibre metadata, XMP metadata is embedded in an annotation chunk at the page or document level using the following annotation directive (xmp "") The sole argument of the xmp directive is the serialized XMP data without the "xpacket" wrapper. The "x:xmpmeta" element may also be dropped. Only elements from "rdf:RDF" inwards are needed. Since the XMP data is represented as a string, doublequotes and backslashes must be escaped. Other characters may be escaped as well (see section 2 above). The full XMP specification is available from Adobe: http://www.adobe.com/devnet/xmp/ To maximize interoperability with current viewers, it is recommended that XMP manipulation programs keep the DjVuLibre metadata in sync. This is facilitated by synchronizing the PDF DocInfo keys with XMP properties as follows: DocInfo key XMP property ------------ --------------- Title dc:title Author dc:creator Subject dc:description Keywords pdf:Keywords Producer pdf:Producer Trapped pdf:Trapped Creator xmp:CreatorTool CreationDate xmp:CreateDate ModDate xmp:ModifyDate 4.3- DOCUMENT ANNOTATIONS AND METADATA The above schemes provide ways to specify metadata for each page. But it is often useful to provide metadata that applies to the whole document. Document wide metadata are represented using one or several metadata directives in the shared annotations chunk. This scheme has a potential drawback. Since the shared annotations is included by all pages, the document wide metadata also appears as page metadata for all pages. This might not be adequate for some uses. As a workaround, the djview4 viewer only displays page metadata that differ from the document metadata. A more definitive answer would be the definition of a document annotation chunk located after the DIRM chunk and before any component file. This space is already used by the NAVM chunk. This is being considered. 5- CGI STYLE OPTIONS IN MAPAREA AND OUTLINE LINKS Outline and maparea annotation are UTF-8 encoded strings that can be interpreted as page specification (see section 3.1) or as percent-encoded URLs (see section 1.3.) In addition, strings starting with a question mark '?' are interpreted as CGI style options separated by the ampersand character '&'. These options are ignored when the maparea link target is another window. Otherwise these options are passed verbatim to the viewer. This can cause portability problems because different djvu viewers support different sets of CGI style options. 6- THE "SECURE DJVU" FORMAT. Recently Lizardtech introduced an incompatible "Secure DJVU" format. This format encrypt djvu data in the hope of controlling whether users can use, copy or print djvu documents. A recent specification describes the container format but does not provide enough information to decode the content. Providing such an information would obviously provide a means to avoid the usage restrictions. In fact there is no durable way to enforce such constraints besides "security through obscurity". The current djvulibre library simply emits an error message when encountering such files. Some observations regarding these files: - They use the same IFF85 structure as djvu files. Chunk "SINF" contains a scrambled version of the decryption key and describes which actions are authorized or denied. Chunks "CELX" encapsulate the regular DjVu chunks. - Each CELX chunk starts with four bytes for the original chunk name, and four bytes for the original chunk length. This is followed encrypted data, composed of enough blocks of 8 bytes to covert the initial chunk length. - Lizardtech claims encryption is 32 bit blowfish. It appears to be in fact composed of 64 bits block as you would expect with blowfish.