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.\" Copyright (c) 2001-2003 Leon Bottou, Yann Le Cun, Patrick Haffner,
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.\" Copyright (c) 2001 AT&T Corp., and Lizardtech, Inc.
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.\"
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.\" This is free documentation; you can redistribute it and/or
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.\" modify it under the terms of the GNU General Public License as
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.\" published by the Free Software Foundation; either version 2 of
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.\" the License, or (at your option) any later version.
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.\"
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.\" The GNU General Public License's references to "object code"
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.\" and "executables" are to be interpreted as the output of any
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.\" document formatting or typesetting system, including
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.\" intermediate and printed output.
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.\"
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.\" This manual is distributed in the hope that it will be useful,
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.\" but WITHOUT ANY WARRANTY; without even the implied warranty of
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.\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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.\" GNU General Public License for more details.
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.\"
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.\" You should have received a copy of the GNU General Public
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.\" License along with this manual. Otherwise check the web site
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.\" of the Free Software Foundation at http://www.fsf.org.
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.TH DJVU 1 "10/11/2001" "DjVuLibre-3.5" "DjVuLibre-3.5"
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.de SS
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.SH \\0\\0\\0\\$*
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..
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.SH NAME
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DjVu \- DjVu and DjVuLibre.
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.SH INTRODUCTION
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Although the Internet has given us a worldwide infrastructure on which to
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build the universal library, much of the world knowledge, history, and
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literature is still trapped on paper in the basements of the world's
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traditional libraries. Many libraries and content owners are in the process of
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digitizing their collections. While many such efforts involve the painstaking
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process of converting paper documents to computer-friendly form, such as
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.SM SGML
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based formats, the high cost of such conversions limits their
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extent. Scanning documents, and distributing the resulting images
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electronically is not only considerably cheaper, but also more faithful to the
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original document because it preserves its visual aspect.
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.PP
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Despite the quickly improving speed of network connections and computers, the
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number of scanned document images accessible on the Web today is relatively
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small. There are several reasons for this.
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.PP
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The first reason is the relatively high cost of scanning anything else but
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unbound sheets in black and white. This problem is slowly going away with the
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appearance of fast and low-cost color scanners with sheet feeders.
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.PP
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The second reason is that long-established image compression standards and
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file formats have proved inadequate for distributing scanned documents at high
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resolution, particularly color documents. Not only are the file sizes and
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download times impractical, the decoding and rendering times are also
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prohibitive. A typical magazine page scanned in color at 100 dpi in
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.SM JPEG
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would typically occupy 100
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.SM KB
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to 200
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.SM KB
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, but the text would be hardly readable: insufficient for screen viewing and
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totally unacceptable for printing. The same page at 300 dpi would have
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sufficient quality for viewing and printing, but the file size would be 300
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.SM KB
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to 1000
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.SM KB
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at best, which is impractical for remote access. Another major problem is that
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a fully decoded 300 dpi color images of a letter-size page occupies 24
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.SM MB
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of memory and easily causes disk swapping.
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.PP
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The third reason is that digital documents are more than just a collection of
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individual page images. Pages in a scanned documents have a natural serial
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order. Special provision must be made to ensure that flipping pages be
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instantaneous and effortless so as to maintain a good user experience. Even
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more important, most existing document formats force users to download the
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entire document first before displaying a chosen page. However, users often
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want to jump to individual pages of the document without waiting for the
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entire document to download. Efficient browsing requires efficient random
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page access, fast sequential page flipping, and quick rendering. This can be
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achieved with a combination of advanced compression, pre-fetching,
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pre-decoding, caching, and progressive rendering. DjVu decomposes each page
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into multiple components (text, backgrounds, images, libraries of common
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shapes...) that may be shared by several pages and downloaded on demand. All
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these requirements call for a very sophisticated but parsimonious control
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mechanism to handle on-demand downloading, pre-fetching, decoding, caching,
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and progressive rendering of the page images. What is being considered here
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is not just a document image compression technique, but a whole platform for
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document delivery.
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.PP
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DjVu is an image compression technique, a document format, and a software
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platform for delivering documents images over the Internet that fulfills the
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above requirements.
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.SH DJVU IMAGE COMPRESSION
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The DjVu image compression is based on three technologies:
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.SS DjVuPhoto
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DjVuPhoto, also known as
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.SM IW44,
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is a wavelet-based continuous-tone image
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compression technique with progressive decoding/rendering. It is best used
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for encoding photographic images in colors or in shades of gray. Images are
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typically half the size as
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.SM JPEG
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for the same distortion.
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.SS DjVuBitonal
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DjVuBitonal, also known as
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.SM JB2,
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is a bitonal image compression that takes
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advantage of repetitions of nearly identical shapes on the page (such as
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characters) to efficiently compress text images. It is best used to compress
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black and white images representing text and simple drawings. A typical
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300 dpi page in DjVuBitonal occupies 5 to 25
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.SM KB
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(3 to 8 times better than
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.SM TIFF-G4
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or
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.SM PDF
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).
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.SS DjVuDocument
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DjVuDocument is a compression technique specifically designed for color
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digital documents images containing both pictures and text, such as a page of
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a magazine. DjVuDocument represents images into separately compressed layers.
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The foreground layer is usually compressed with DjVu Bitonal and contains the
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text and drawings. The background layer is usually compressed with DjVuPhoto
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and contains the background texture and the pictures at lower resolution.
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.SH DJVU DOCUMENT DELIVERY PLATFORM
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The DjVu technology is designed from the ground up to support the efficient
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delivery of digital documents over the Internet. It provides various ways to
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deal with multi-page documents, and various ways to enrich the content with
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hyper-links, meta-data, searchable text, etc.
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.SS MIME types
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The DjVu format has an official MIME type of
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.BR image/vnd.djvu ,
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which is the preferred content-type to be given by http servers for
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DjVu files. Unofficial mime types used historically are
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.B image/x.djvu
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and
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.BR image/x-djvu ,
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which may still be encountered. Ideally, clients should be configured
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to handle all three. (For web server configuration help, see
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.BR http://www.djvuzone.org/support/tutorial/chapter-authoring1.html .)
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.SS Bundled multi-page documents
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Bundled multi-page DjVu document uses a single file to represent the entire
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document. This single file contains all the pages as well as ancillary
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information (e.g. the page directory, data shared by several pages,
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thumbnails, etc.). Using a single file format is very convenient for storing
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documents or for sending email attachments.
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.PP
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When you type the
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.SM URL
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of a multi-page document, the DjVu browser plugin starts
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downloading the whole file, but displays the first page as soon as it is
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available. You can immediately navigate to other pages using the DjVu
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toolbar. Suppose however that the document is stored on a remote web server.
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You can easily access the first page and see that this is not the document you
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wanted. Although you will never display the other pages the browser is
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transferring data for these pages and is wasting the bandwidth of your server
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(and the bandwidth of the Internet too). You could also see the summary of the
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document on the first page and jump to page 100. But page 100 cannot be
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displayed until data for pages 1 to 99 has been received. You may have to
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wait for the transmission of unnecessary page data. This second problem (the
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unnecessary wait) can be solved using the ``byte serving'' options of the
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.SM HTTP/1.1
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protocol. This option has to be supported by the web server, the
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proxies, the caches and the browser. Byte serving however does not solve the
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first problem (the waste of bandwidth).
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.SS Indirect multi-page documents
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Indirect multi-page DjVu documents solve both problems. An indirect
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multi-page DjVu document is composed of several files. The main file is named
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the index file. You can browse a document using the
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.SM URL
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of the index file, just like you do with a bundled multi-page document. The
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index file however is very small. It simply contains the document directory
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and the
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.SM URLs
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of secondary files containing the page data. When you browse an indirect
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multi-page document, the browser only accesses data for the pages you are
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viewing. This can be done at a reasonable speed because the browser maintains
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a cache of pages and sometimes pre-fetches a few pages ahead of the current
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page. This model uses the web serving bandwidth much more effectively. It
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also eliminates unnecessary delays when jumping ahead to pages located
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anywhere in a long document.
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.SS Annotations
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Every DjVu image optionally includes so-called annotation chunks. The
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annotation chunk is often used to define hyper-links to other document pages or
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to arbitrary web pages. Annotation chunks can also be used for other purposes
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such as setting the initial viewing mode of a page, defining highlighted zones, or
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storing arbitrary meta-data about the page or the document.
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.SS Hidden text
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Every DjVu image optionally includes a hidden text layer that associated
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graphical features with the corresponding text. The hidden text layer is
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usually generated by running an Optical Character Recognition software. This
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textual information provides for indexing DjVu documents and copying/pasting
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text from DjVu page images.
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.SS Thumbnails
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DjVu documents sometimes contain pre-computed page thumbnails.
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.SS Outline
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DjVu documents sometimes contain a navigation chunk
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containing an outline, that is, a hierarchical
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table of contents with pointers to the corresponding
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document pages.
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.SH DJVUZONE AND DJVULIBRE
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The DjVu technology was initially created by a few researchers in AT&T Labs
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between 1995 and 1999. Lizardtech, Inc. (
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.B http://www.lizardtech.com
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) then obtained a commercial license from AT&T and continued
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the development. They have now a variety of solutions for producing
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and distributing documents using the DjVu technology.
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.PP
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The DjVuZone web site (
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.B http://www.djvuzone.org
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) is managed by the few AT&T Labs researchers who created the
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DjVu technology in the first place. We promote the DjVu technology
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by providing an independent source of information about DjVu.
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.PP
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Understanding how little room there is for a proprietary document format,
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Lizardtech released the DjVu Reference Library under the
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.SM GNU
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Public License in December 2000. This library entirely defines the
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compression format and the elementary codecs. Six month later, Lizardtech
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released an updated DjVu Reference Library as well as the source code of the
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Unix viewer.
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.PP
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These two releases form the basis of our initial DjVuLibre software. We
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modified the build system to comply with the expectations of the open source
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community. Various bugs and portability issues have been fixed. We also
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tried to make it simpler to use and install, while preserving the essential
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structure of the Lizardtech releases.
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.PP
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The DjVuLibre software contains the following components:
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.TP
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.BR bzz (1)
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A general purpose compression command line program. Many internal DjVu data
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structures are compressed using this technique.
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.TP
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.BR c44 (1)
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A DjVuPhoto command line encoder. This state-of-the-art wavelet compressor
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produces DjVuPhoto images from PPM or JPEG images.
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.TP
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.BR cjb2 (1)
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A DjVuBitonal command line encoder. This soft-pattern-matching compressor
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produces DjVuBitonal images from PBM images. It can encode images without loss,
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or introduce small changes in order to improve the compression ratio. The
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lossless encoding mode is competitive with that of the Lizardtech commercial
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encoders.
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.TP
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.BR cpaldjvu (1)
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A DjVuDocument command line encoder for images with few colors. This encoder
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is well suited to compressing images with a small number of distinct colors
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(e.g. screen-shots). The dominant color is encoded by the background layer.
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The other colors are encoded by the foreground layer.
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.TP
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.BR csepdjvu (1)
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A DjVuDocument command line encoder for separated images. This encoder takes
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a file containing pre-segmented foreground and background images and produces
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a DjVuDocument image.
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.TP
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.BR ddjvu (1)
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A command line decoder for DjVu images. This program produces a
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.SM PNM
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image representing any segment of any page of a DjVu document at any
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resolution.
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.TP
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.BR djview (1)
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A stand-alone viewer for DjVu images. This sophisticated viewer displays DjVu
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documents. It implements document navigation as well as fast zooming and
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panning.
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.TP
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.BR nsdejavu (1)
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A web browser plugin for viewing DjVu images. This small plugin allows for
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viewing DjVu documents from web browsers. It internally uses djview to
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perform the actual work.
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.TP
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.BR djvups (1)
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A command line tool for converting DjVu documents into
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PostScript .
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.TP
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.BR djvm (1)
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A command line tool for manipulating bundled multi-page DjVu documents. This
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program is often used to collect individual pages and produce a bundled
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document.
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.TP
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.BR djvmcvt (1)
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A command line tool for converting bundled documents to indirect documents and
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conversely.
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.TP
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.BR djvused (1)
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A powerful command line tool for manipulating multi-page documents, creating
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or editing annotation chunks, creating or editing hidden text layers,
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pre-computing thumbnail images, and more...
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.TP
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.BR djvutxt (1)
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A command line tool to extract the hidden text from DjVu documents.
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.TP
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.BR djvudump (1)
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A command line tool for inspecting DjVu files and displaying their internal
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structure.
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.TP
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.BR djvuextract (1)
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A command line tool for dis-assembling DjVu image files.
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.TP
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.BR djvumake (1)
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A command line tool for assembling DjVu image files.
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.TP
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.BR djvuserve (1)
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A
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.SM CGI
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program for generating indirect multi-page DjVu documents
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on the fly.
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.TP
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.BR djvutoxml "(1), " djvuxmlparser (1)
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Command line tools to edit DjVu metadata as XML files.
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.SH DJVU ENCODERS AND ANY2DJVU
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DjVuLibre comes with a variety of specialized encoders,
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.BR c44 (1)
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for photographic images,
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.BR cjb2 (1)
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for bitonal images, and
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.BR cpaldjvu (1)
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for images with few distinct colors.
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Although these encoders perform well in their specialized domain,
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they cannot handle complex tasks involving segmentation and
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multipage encoding.
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The Lizardtech commercial products
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.BR "" "(see " "http://www.lizardtech.com/solutions/document" )
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can perform these complex encoding tasks
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Another solution is provided by the compression server at
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.BR "" ( "http://any2djvu.djvuzone.org" ).
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This machine uses pre-lizardtech prototype encoders from AT&T Labs and
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performs almost as well as the commercial Lizardtech encoders. Please note
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that the Any2DjVu compression server comes with no guarantee, that
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nothing is done to ensure that your documents will remain confidential, and
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that there is only one computer working for the whole planet.
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.SH CREDITS
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Numerous people have contributed to the DjVu source code during the
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last five years. Please submit a sourceforge bug report to update the
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following list.
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.IP "" 3
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Yoshua Bengio,
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L\('eon Bottou,
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Chakradhar Chandaluri,
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Regis M\. Chaplin,
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Ming Chen,
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Parag Deshmukh,
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Royce Edwards,
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Andrew Erofeev,
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Praveen Guduru,
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Patrick Haffner,
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Paul G\. Howard,
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Orlando Keise,
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Yann Le Cun,
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Artem Mikheev,
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Florin Nicsa,
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Joseph M\. Orost,
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Steven Pigeon,
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Bill Riemers,
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Patrice Simard,
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Jeffery Triggs,
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Luc Vincent,
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Pascal Vincent.
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