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). Then, after a few more seconds, the final full-quality version of the page is completed.
In another second or two, the first versions of the pictures and backgrounds will appear. For example, the text of a typical magazine page would appear in just three seconds over a 56Kbps modem connection. Users get an initial version of the page very quickly, and the visual quality of the page progressively improves as more bits arrive. 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. 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. This is impractical for high-resolution document images since they typically go beyond the memory capacity of many PCs, causing excessive disk swapping.
A unique on the fly decompression technology allows images that normally require 25MB of RAM to be decompressed to require only 2MB of RAM.Ĭonventional image viewing software decompresses images in their entirety before displaying them. The DjVu plug-in allows for easy panning and zooming of document images. The DjVu plug-in is available for standard Web browsers on various platforms. In that case, the file sizes are between 15 to 20KB per page at 300 DPI. In addition to scanned documents, DjVu can also be applied to documents produced electronically in formats such as Adobe's PostScript or PDF.
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). For black-and-white pages, DjVu files are typically 10 to 20 times smaller than JPEG and five times smaller than GIF. This puts the size of high-quality scanned pages within the realm of an average HTML page (which is typically around 50KB).įor color document images that contain both text and pictures, DjVu files are typically 5 to 10 times smaller than JPEG at similar quality. Black-and-white pages at 300 DPI typically occupy 5 to 30KB when compressed.
Scanned pages at 300 DPI in full color can be compressed down to 30 to 100KB files from 25MB. The Next Generation Compression TechnologiesĭjVu 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. The file format specification, as well as an open source implementations of the decoder (and part of the encoder) are available. The developing and distribution of DjVu technology is now handled by Cuminas corporation.ĭjVu is an open standard. Companies can distribute internal documents on their intranets. Research institutions, libraries, and government agencies can give access to their archives. Information that was previously trapped in hard copy form can now be made available to wide audience. 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. DjVu allows the distribution on the Internet of very high resolution images of scanned documents, digital documents, and photographs. As a result, Web site content developers have been largely unable to leverage existing printed materials.ĭjVu (pronounced "déjà vu") is a new image compression technology developed since 1996 at AT&T Labs to solve precisely that problem. Conventional web formats such as JPEG, GIF, and PNG produce prohibitively large image files at decent resolution.
Reducing resolution to achieve satisfactory download speed means forfeiting quality and legibility. 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. That's because scanning such documents and getting them onto a Web site has been problematic at best. And almost none of that rich content is on the Internet. Many of those paper documents include color graphics and/or photographs that represent significant invested value. Over 90 percent of the information in the world is still on paper. Download & Resources: Articles and Benchmarks What is DjVu, and what are the secrets behind DjVu's superior performance? It may be worth a thousand words.