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DjVu file format



The DjVu technology was originally developed by Yann Le Cun, Leon Bottou, Patrick Haffner, and Paul G. Howard at AT&T Laboratories in 1996. DjVu is a free file format. The file format specification is published as well as source code for the reference library. The ownership rights to the commercial development of the encoding software have been transferred to different companies over the years, including AT&T and LizardTech. The original authors maintain a GPLed implementation named "DjVuLibre".

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., 100dpi); the mask image is a high-resolution bilevel image (e.g., 300dpi) 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.

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