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