DJVU

DJVU is a format originally developed by Lizardtech. It was transfered to Celartem Technology Inc. in 2008. The file extension is DJVU or sometimes abbreviated to DJV.

Overview

DjVu software enables businesses, publishers and creative professionals to easily exchange and efficiently archive complex color documents without compromising image quality or text legibility.

DjVu (pronounced "déjà vu") is a digital image format with advanced compression technology and high performance value. DjVu allows for the distribution on the Internet and on DVD of very high resolution images of scanned documents, digital documents, and photographs. DjVu viewers are available for web browsers, the desktop, and PDA devices. Its main characteristics is that the compress ratio is about 10x better than in PDF format at the same quality.

One of the main technologies behind DjVu is the ability to separate an image into a background layer (i.e., paper texture and pictures) and foreground layer (text and line drawings). Traditional image compression techniques are fine for simple photographs, but they drastically degrade sharp color transitions between adjacent highly contrasted areas - which is why they render type fonts so poorly. By separating the text from the backgrounds, DjVu can keep the text at high resolution (thereby preserving the sharp edges and maximizing legibility), while at the same time compressing the backgrounds and pictures at lower resolution with a wavelet-based compression technique. DjVu is used by many commercial and non-commercial web sites on the Web today.

How it works

DjVu starts by segmenting a page into layers.

• Foreground layer includes text, line art and other thin, low-color elements.
• Background layer includes photos, graphics, tint, and paper texture. Areas of the background that are covered by the foreground are smoothly interpolated to minimize coding costs. Lower resolution is used on this layer.

Then the foreground layer is further divided into black and white mask layer and a color mask layer.

Once everything is separated different compression techniques are used on the different layers. For example the black and white stuff that looks like text or repeated graphics is compressed using pattern matching. Repeats are stored once as individual elements in another layer and then placed on the page by just referencing the location. Using this "dictionary" of shapes permits high compression, typically 100 to 1, with precise reproduction.

The foreground color layer is compressed using a similar technique to JPEG 2000. The background layer is compressed using a technique that typically 3 times better than classic JPEG.

These techniques permit a visually better image than Jpeg with much less storage.

DjVu supports an OCR hidden XML text layer that permits text searching, indexing etc and works even with color text. The OCR is superior to traditional approaches on colored background.

When separate layers are not needed the format is called IW44.

Viewers

• For Linux GNOME Desktop use Evince.
• For a viewer that runs on windows and macOS try: http://sourceforge.net/projects/windjview/
• For Windows there's SumatraPDF. Free and, despite the name, compatible with multiple text formats, including djvu.
• The Hanlin V3 eBook Reader can read these files.
• Bookr on PSP can read these files.
• STDU Viewer - DjVu reader.
• DjVuLibre is an open source (GPL'ed) implementation of DjVu, including viewers, browser plugins, decoders, simple encoders, and utilities.
• DJVU Viewer - free download for Windows.
• DjVuToy is an English version, unfortunately the main site is in Chinese. With it you can split and merge DJVUs, insert bookmarks and manipulate hidden text by exporting it, editing it then re-importing it.

For more information

• see http://djvu.org/
• http://djvu.org/links/ for everything you need to find, creation programs, readers and more.
• Sample books in DJVU format.
• Converter from PDF to DJVU
• STDU Converter - DjVu to PDF converter.