Filename extension .djvu, .djv Developed by AT&T Labs – Research | Type code DJVU | |
Internet media type image/vnd.djvu, image/x-djvu Initial release 1998; 19 years ago (1998) Latest release Version 26
(June 2006; 10 years ago (2006-06)) | ||
DjVu (/ˌdeɪʒɑːˈvuː/ DAY-zhah-VOO, like French: déjà vu [deʒavy]) 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.
Contents
DjVu has been promoted as an alternative to PDF, promising smaller files than PDF for most scanned documents. 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. Like PDF, DjVu can contain an OCR text layer, making it easy to perform copy and paste and text search operations.
Free browser plug-ins and desktop viewers from different developers are available from the djvu.org website. 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 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, some independent technologists (such as Brewster Kahle) have historically considered it superior to PDF.
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.
The DjVu file format specification has gone through a number of revisions:
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.
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. 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 similar looking numbers (such as replacing 6 with 8) if the text was scanned at a low DPI prior to lossy compression.
Format licensing
DjVu is an open file format with patents. The file format specification is published, as well as source code for the reference library. 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, Celartem and Cuminas.
Support
SumatraPDF (Windows) among others can manipulate DjVu files.
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).
Wikimedia Commons, a media repository used by Wikipedia among others, conditionally permits PDF and DjVu media files.