DjVu

DjVu
Filename extensions .djvu, .djv
Internet media type image/vnd.djvu, image/x-djvu
Developed by AT&T Labs – Research
Initial release 1998 (1998)
Latest release
Version 26[1]
(June 2006 (2006-06))
Type of format Image file formats
Open format? GNU GPLv2 for DjVu Reference Library and DjVuLibre-3.5;
License grants under the GNU GPL for several patents that cover aspects of the library[2]
Website djvu.org

DjVu (/ˌdeɪʒɑːˈvuː/ DAY-zhah-VOO, like French "déjà vu"[3]) is a computer file format designed primarily to store scanned documents, especially those containing a combination of text, line drawings, indexed color images, and photographs. It uses technologies such as image layer separation of text and background/images, progressive loading, arithmetic coding, and lossy compression for bitonal (monochrome) images. This allows high-quality, readable images to be stored in a minimum of space, so that they can be made available on the web.

DjVu has been promoted as providing smaller files than PDF for most scanned documents.[4] The DjVu developers report that color magazine pages compress to 40–70 kB, black-and-white technical papers compress to 15–40 kB, and ancient manuscripts compress to around 100 kB; a satisfactory JPEG image typically requires 500 kB.[5] Like PDF, DjVu can contain an OCR text layer, making it easy to perform copy and paste and text search operations.

Free creators, manipulators, converters, browser plug-ins, and desktop viewers are available.[6] DjVu is supported by a number of multi-format document viewers and e-book reader software on Linux (Okular, Evince) and Windows (SumatraPDF).

History

The DjVu technology was originally developed[5] by Yann LeCun, Léon Bottou, Patrick Haffner, and Paul G. Howard at AT&T Labs from 1996 to 2001.

Due to its declared higher compression ratio (and thus smaller file size) and the ease of converting large volumes of text into DjVu format, and because it is an open file format, it has been considered superior to PDF. Independent technologist Brewster Kahle in a 2004 talk on IT Conversations discussed the benefits of allowing easier access to DjVu files.[7][8]

The DjVu library distributed as part of the open-source package DjVuLibre has become the reference implementation for the DjVu format. DjVuLibre has been maintained and updated by the original developers of DjVu since 2002.[9]

The DjVu file format specification has gone through a number of revisions, the most recent being from 2005.

Revision history
Support status Version Release date Notes
Unsupported 1–19[1] 1996–1999 Developmental versions by AT&T labs preceding the sale of the format to LizardTech.
Unsupported Version 20[1] April 1999 DjVu version 3. DjVu changed from a single-page format to a multipage format.
Older, still supported Version 21[1] September 1999 Indirect storage format replaced. The searchable text layer was added.
Older, still supported Version 22[1] April 2001 Page orientation, color JB2
Unsupported Version 23[1] July 2002 CID chunk
Unsupported Version 24[1] February 2003 LTAnno chunk
Older, still supported Version 25[1] May 2003 NAVM chunk. Support for DjVu bookmarks (outlines) was added. Changes made by Versions 23 and 24 were made obsolete.
Current Version 26[1] April 2005 Text/line annotations

Role in the software ecosystem

The primary usage of the DjVu format has been the electronic distribution of documents with a quality comparable to that of printed documents. As that niche is also the primary usage for PDF, it was inevitable that the two formats would become competitors. It should however be observed that the two formats approach the problem of delivering high resolution documents in very different ways: PDF primarily encodes graphics and text as vectorised data, whereas DjVu primarily encodes them as pixmap images. This means PDF places the burden of rendering the document on the reader, whereas DjVu places that burden on the creator.

During a number of years, significantly overlapping with the period when DjVu was being developed, there were no PDF viewers for free operating systems — a particular stumbling block was the rendering of vectorised fonts, which are essential for combining small file size with high resolution in PDF. Since displaying DjVu was a simpler problem for which free software was available, there were suggestions that the free software movement should employ DjVu instead of PDF for distributing documentation; rendering for creating DjVu is in principle not much different from rendering for a device-specific printer driver, and DjVu can as a last resort be generated from scans of paper media. However when FreeType 2.0 in 2000 began provide rendering of all major vectorised font formats, that specific advantage of DjVu began to erode.

In the 2000s, with the growth of the world wide web and before widespread adoption of broadband, DjVu was often adopted by digital libraries as their format of choice, thanks to its integration with software like Greenstone[10] and the Internet Archive,[11] browser plugins which allowed advanced online browsing, smaller file size for comparable quality of book scans and other image-heavy documents[12] and support for embedding and searching full text from OCR.[13] Some features such as the thumbnail previews were later integrated in the Internet Archive's BookReader[14] and DjVu browsing was deprecated in its favour as around 2015 some major browsers stopped supporting Java applets and DjVu plugins with them.[15]

Technical overview

File structure

The DjVu file format is based on the Interchange File Format and is composed of hierarchically organized chunks. The IFF structure is preceded by a 4-byte AT&T magic number. Following is a single FORM chunk with a secondary identifier of either DJVU or DJVM for a single-page or a multi-page document, respectively.

Chunk types

Chunk identifierContained byDescription
FORM:DJVUFORM:DJVMDescribes a single page. Can either be at the root of a document and be a single-page document or referred to from a DIRM chunk.
FORM:DJVMN/ADescribes a multi-page document. Is the document's root chunk.
FORM:DJVIFORM:DJVMContains data shared by multiple pages.
FORM:THUMFORM:DJVMContains thumbnails.
INFOFORM:DJVUMust be the first chunk. Describes the page width, height, format version, resolution, gamma, and rotation.
DIRMFORM:DJVMMust be the first chunk. References other FORM chunks. These chunks can either follow this chunk inside the FORM:DJVM chunk or be contained in external files. These types of documents are referred to as bundled or indirect, respectively.
NAVMFORM:DJVMIf present, must immediately follow the DIRM chunk. Contains a BZZ-compressed outline of the document.

Compression

DjVu divides a single image into many different images, then compresses them separately. To create a DjVu file, the initial image is first separated into three images: a background image, a foreground image, and a mask image. The background and foreground images are typically lower-resolution color images (e.g., 100 dpi); the mask image is a high-resolution bilevel image (e.g., 300 dpi) and is typically where the text is stored. The background and foreground images are then compressed using a wavelet-based compression algorithm named IW44.[5] The mask image is compressed using a method called JB2 (similar to JBIG2). The JB2 encoding method identifies nearly identical shapes on the page, such as multiple occurrences of a particular character in a given font, style, and size. It compresses the bitmap of each unique shape separately, and then encodes the locations where each shape appears on the page. Thus, instead of compressing a letter "e" in a given font multiple times, it compresses the letter "e" once (as a compressed bit image) and then records every place on the page it occurs.

Optionally, these shapes may be mapped to UTF-8 codes (either by hand or potentially by a text recognition system) and stored in the DjVu file. If this mapping exists, it is possible to select and copy text.

Since JBIG2 was based on JB2, both compression methods have the same problems when performing lossy compression. Numbers may be substituted with similarly looking numbers (such as replacing 6 with 8) if the text was scanned at a low resolution prior to lossy compression.

Format licensing

DjVu is an open file format with patents.[4] The file format specification is published, as well as source code for the reference library.[4] The original authors distribute an open-source implementation named "DjVuLibre" under the GNU General Public License. The rights to the commercial development of the encoding software have been transferred to different companies over the years, including AT&T Corporation, LizardTech,[16] Celartem[17] and Cuminas.[18]

Celartem acquired LizardTech and Extensis.[19][20][17][21][22]

Support

Despite its advantages, DjVu is not widely supported by scanning and viewing software.[23] While viewers can be downloaded, opening DjVu files is not implemented in most operating systems by default.[24]

In 2002, the DjVu file format was chosen by the Internet Archive as a format in which its Million Book Project provides scanned public-domain books online (along with TIFF and PDF).[25]

Wikimedia Commons, a media repository used by Wikipedia among others, conditionally permits PDF and DjVu media files.[26]

Third party tools using APIs and SDKs can create and manipulate DjVu files.[9][27][28][29][30][31][32][33]

A JavaScript based library can read DjVu files online.[34]

See also

References

  1. 1 2 3 4 5 6 7 8 9 DjVu File Format Version, By Jim Rile, Posted: Fri Feb 23, 2007 1:08 am, PlanetDjVu
  2. ↑ "DjVu Licensing". DjVu Sourceforge page. Sourceforge.net. 2011-08-17. Retrieved 2011-09-21.
  3. ↑ "DjVu.org - the premier menu for djvu resources". djvu.org. Retrieved 2017-07-02.
  4. 1 2 3 "What is DjVu – DjVu.org". DjVu.org. Retrieved 2009-03-05.
  5. 1 2 3 Léon Bottou; Patrick Haffner; Paul G. Howard; Patrice Simard; Yoshua Bengio; Yann Le Cun (1998). "High Quality Document Image Compression with DjVu, 7(3):410–425" (PDF). Journal of Electronic Imaging.
  6. ↑ http://djvu.org djvu.org
  7. ↑ Brewster Kahle (December 16, 2004). "Universal Access to All Knowledge" (Audio; Speech at 1h:31m:20s). Conversations Network.
  8. ↑ "LizardTech To Open Source A DjVu Java Viewer". ECM Connection. 7 December 2004. Retrieved 18 August 2017.
  9. 1 2 "DjVuLibre: Open Source DjVu library and viewer". djvu.sourceforge.net.
  10. ↑ "NZDL projects and Demonstrations".
  11. ↑ Eric Rumsey (2018-09-05). "Google Books vs DjVu in Internet Archive".
  12. ↑ Eric Rumsey (2018-09-10). "DjVu again".
  13. ↑ Jeff Kaplan (2004-12-09). "New book collection: color scans, djvu, some pdf".
  14. ↑ Eric Rumsey (2010-09-10). "Internet Archive's BookReader Thumbnail View".
  15. ↑ Brewster Kahle and Jeff Kaplan (2016-02-26). "DjVu files for new uploads".
  16. ↑ Extensis. "Company - About - LizardTech". www.lizardtech.com.
  17. 1 2 "Celartem, Inc.: Private Company Information - Bloomberg". www.bloomberg.com.
  18. ↑ "会社情報 - Cuminas Corporation". www.cuminas.jp.
  19. ↑ "Company Overview - Celartem Technology, Inc".
  20. ↑ "Celartem Technology Announces Merger of US Holdings - Extensis.com".
  21. ↑ "Celartem Technology Inc.: Private Company Information - Bloomberg". www.bloomberg.com.
  22. ↑ "Celartem Sells Extensis and LizardTech Plugins and XTensions to onOne Software - Big Picture - Wide Format Printing". bigpicture.net.
  23. ↑ Manual for Xerox/Visioneer OneTouch, widely used scanning software for business and home use, showing support for several file formats, but not DjVu.
  24. ↑ A test DjVu file. Click on the image in the page to open the file on a computer with support for the .djvu format.
  25. ↑ "Image file formats – OLPC". Wiki.laptop.org. Retrieved 2008-09-09.
  26. ↑ Wikimedia Commons. Project scope: PDF and DjVu.
  27. ↑ "Jakub Wilk's software". jwilk.net.
  28. ↑ "DjVu tools - Poliqarp for DjVu search engine and others". bitbucket.org.
  29. ↑ "PlanetDjVu • Why won't Google index DjVu files after all this time?". djvu.org.
  30. ↑ SumatraPDF (Windows)
  31. ↑ "Any2DjVu Server". 22 May 2011.
  32. ↑ "Table of Djvu Programmes (Russian)". www.djvu-soft.narod.ru.
  33. ↑ "DjVu to PDF: Convert your DjVus to PDF online instantly". www.djvu-pdf.com.
  34. ↑ "DjVu.js - Open and view DjVu online!". djvu.js.org.
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