Формат файлов DjVu: структура и особенности

Computer file format

DjVu (,/d//eɪ//ʒ//ɑː/'/v//uː/ DAYzhahVOO', like French "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 providing 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, web browser plug ins, and desktop viewers are available. FBReader, EBookDroid, PocketBook).

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. All the chunks can be contained in a single file in the case of the so called bundled documents, or can be contained in several files: one file for every page plus some files with shared chunks.

+Chunk types in DjVu files 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 chunk. FORM:DJVM Describes a multi page document. Is the document's root chunk. FORM:DJVI FORM:DJVM Contains data shared by multiple pages. FORM:THUM FORM:DJVM Contains thumbnails. 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 chunks. These chunks can either follow this chunk inside the chunk or be contained in external files. These types of documents are referred to as bundled or indirect, respectively. NAVM FORM:DJVM If present, must immediately follow the chunk. Contains a BZZ compressed outline of the document. ANTa, ANTz FORM:DJVI or FORM:DJVU Annotations. TXTa, TXTz FORM:DJVU Unicode text and layout information. INCL FORM:DJVU The ID of an included chunk. Sjbz FORM:DJVU BZZ compressed JB2 bitonal data used to store mask. Djbz FORM:DJVI or FORM:DJVU Shared shape table. WMRM ? JB2 data required to remove a watermark. CIDa FORM:DJVU Obsolete chunk with unknown content.

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. both compression methods have the same problems when performing lossy compression. In 2013 it emerged that Xerox photocopiers and scanners had been substituting digits for similar looking ones, for example replacing a 6 with an 8. A DjVu document has been spotted in the wild with character substitutions, such as an n with bleeding serifs turning into a u and an o with a spot inside turning into an e. Whether lossy compression has occurred is not stored in the file.

DjVu is an open file format with patents. Celartem and Cuminas. Celartem acquired LizardTech and Extensis.

The selection of downloadable DjVu viewers is wider on Linux distributions than it is on Windows or Mac OS. Additionally, the format is rarely supported by proprietary scanning software. 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). In February 2016, the Internet Archive announced that DjVu would no longer be used for new uploads, among other reasons citing the format's declining use and the difficulty of maintaining their Java applet based viewer for the format.