DjVu (// DAY-zhah-VOO, like English "déjà vu") 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 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 creators, manipulators, converters, browser plug-ins, and desktop viewers are available. DjVu is supported by a number of multi-format document viewers and e-book reader software on Linux (Okular, Evince) and Windows (SumatraPDF).
Despite its advantages, DjVu is not widely supported by scanning and viewing software.
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.
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:
|Support status||Version||Release date||Notes|
|Unsupported||1–19||1996–1999||Developmental versions by AT&T labs preceding the sale of the format to LizardTech.|
|Unsupported||Version 20||April 1999||DjVu version 3. DjVu changed from a single-page format to a multipage format.|
|Older, still supported||Version 21||September 1999||Indirect storage format replaced. The searchable text layer was added.|
|Older, still supported||Version 22||April 2001||Page orientation, color JB2|
|Unsupported||Version 23||July 2002||CID chunk|
|Unsupported||Version 24||February 2003||LTAnno chunk|
|Older, still supported||Version 25||May 2003||NAVM chunk. Support for DjVu bookmarks (outlines) was added. Changes made by Versions 23 and 24 were made obsolete.|
|Current||Version 26||April 2005||Text/line annotations|
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
DJVM for a single-page or a multi-page document, respectively.
|Chunk identifier||Contained by||Description|
|FORM:DJVU||FORM:DJVM||Describes a single page. Can either be at the root of a document and be a single-page document or referred to from a
|FORM:DJVM||N/A||Describes a multi-page document. Is the document's root chunk.|
|FORM:DJVI||FORM:DJVM||Contains data shared by multiple pages.|
|INFO||FORM:DJVU||Must be the first chunk. Describes the page width, height, format version, resolution, gamma, and rotation.|
|DIRM||FORM:DJVM||Must be the first chunk. References other
|NAVM||FORM:DJVM||If present, must immediately follow the
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 similarly looking numbers (such as replacing 6 with 8) if the text was scanned at a low resolution prior to lossy compression.
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.
Despite its advantages, DjVu is not widely supported by scanning and viewing software. While viewers can be downloaded, opening DjVu files is not implemented in most operating systems by default.
|Wikimedia Commons has media related to DjVu file format.|