Djvu

What is DjVu, and what are the secrets behind DjVu's superior performance? It may be worth a thousand words... but it's not worth 1,000 Kb

Over 90 percent of the information in the world is still on paper. Many of those paper documents include color graphics and/or photographs that represent significant invested value. And almost none of that rich content is on the Internet.

That's because scanning such documents and getting them onto a Web site has been problematic at best. At the high resolution necessary to ensure the readability of the text and to preserve the quality of the images, file sizes become far too bulky for acceptable download speed. Reducing resolution to achieve satisfactory download speed means forfeiting quality and legibility. Conventional web formats such as JPEG, GIF, and PNG produce prohibitively large image files at decent resolution. As a result, Web site content developers have been largely unable to leverage existing printed materials.

DjVu (pronounced "déjà vu") is a new image compression technology developed since 1996 at AT&T Labs to solve precisely that problem. DjVu allows the distribution on the Internet of very high resolution images of scanned documents, digital documents, and photographs. DjVu allows content developers to scan high-resolution color pages of books, magazines, catalogs, manuals, newspapers,historical or ancient documents, and make them available on the Web.

Information that was previously trapped in hard copy form can now be made available to wide audience.

Benefits... and Opportunities

Research institutions, libraries, and government agencies can give access to their archives. Companies can distribute internal documents on their intranets.

The commercialization of DjVu is handled by Seattle-based LizardTech Inc. in partnership with AT&T Labs. DjVu is an open standard. The file format specification, as well as an open source implementations of the decoder (and part of the encoder) are available.

The Next Generation Compression Technologies

DjVu typically achieves compression ratios about 5 to 10 times better than existing methods such as JPEG and GIF for color documents, and 3 to 8 times than TIFF for black and white documents.

Scanned pages at 300 DPI in full color can be compressed down to 30 to 100KB files from 25MB.. Black-and-white pages at 300 DPI typically occupy 5 to 30KB when compressed.

This puts the size of high-quality scanned pages within the realm of an average HTML page (which is typically around 50KB).

For color document images that contain both text and pictures, DjVu files are typically 5 to 10 times smaller than JPEG at similar quality. For black-and-white pages, DjVu files are typically 10 to 20 times smaller than JPEG and five times smaller than GIF. DjVu files are also about 3 to 8 times smaller than black and white PDF files produced from scanned documents (scanned documents in color are impractical in PDF).

In addition to scanned documents, DjVu can also be applied to documents produced electronically in formats such as Adobe's PostScript or PDF. In that case, the file sizes are between 15 to 20KB per page at 300 DPI.

The DjVu plug-in is available for standard Web browsers on various platforms. The DjVu plug-in allows for easy panning and zooming of document images. A unique on the fly decompression technology allows images that normally require 25MB of RAM to be decompressed to require only 2MB of RAM.

Conventional image viewing software decompresses images in their entirety before displaying them. This is impractical for high-resolution document images since they typically go beyond the memory capacity of many PCs, causing excessive disk swapping. DjVu, on the other hand, never decompresses the entire image, but instead keeps the image in memory in a compact form, and decompresses the piece displayed on the screen in real time as the user views the image. Images as large as 2,500 pixels by 3,300 pixels (a standard page image at 300 DPI) can be downloaded and displayed on very low-end PCs.

The DjVu format is progressive. Users get an initial version of the page very quickly, and the visual quality of the page progressively improves as more bits arrive. For example, the text of a typical magazine page would appear in just three seconds over a 56Kbps modem connection. In another second or two, the first versions of the pictures and backgrounds will appear. Then, after a few more seconds, the final full-quality version of the page is completed.

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 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