DJVU

Last modified by Kashif Iqbal on 2019/02/06 13:39

DjVu, pronounced as “déjà vu”, is a graphics file format intended for scanned documents and books especially those which contain the combination of text, drawings, images and photographs. It was developed by AT&T Labs. It uses multiple techniques like image layer separation of text and background images, progressive loading, arithmetic coding and lossy compression for bitonal images. Since DJVU file can contain compressed yet high-quality colour images, photographs, text, and drawings and can be saved in less space therefore, it's used on web as eBooks, manuals, newspapers, ancient documents, etc.

DjVu can be graded superior alternative to PDF.  File extensions associated to DjVu are .DJVU or .DJV. DjVu can achieve compression ratios about 5 – 10 better than existing methods such as JPEG & GIF for colour documents and 3 – 8 times better than TIFF in black and white documents. Scanned documents at 300 DPI with full colour upto 25 MB can be compressed down to 30 to 100 KB. Similarly Black and white documents can be compressed upto 5 to 30 KB. Average HTML page can be up to 50 KB, therefore, these documents can be uploaded on net without any problem.

Brief History

The DjVu technology was developed in AT&T labs by Yann LeCunLéon Bottou, Patrick Haffner, and Paul G from 1996 to 2001. The DjVu file format has passed through various revisions, the most recent being from 2005.

VersionRelease dateNotes
1–191996–1999These are the developmental versions.
20April 1999Single page was changed to Multipage format.
23July 2002CID chunk
24February 2003LTAnno chunk
21September 1999Indirect storage format replaced. Text search layer was added.
22April 2001Page orientation, color JB2
25May 2003NAVM chunk. Support for DjVu bookmarks was added.
26April 2005Text/line annotations

DJVU File Format Information

DjVu documents are IFF85 files. The structure provides a hierarchy of containers which holds information in a DjVu file. These containers are also called “Chunks”. Chunk type and Chunk ID describes how the chunk is used. There is a 4byte header followed by IFF structure. The first four bytes of a DjVu file are 0x41 0x54 0x26 0x54. This section discusses the various kinds of DjVu documents and the corresponding chunks of which they consist.

Chunk IDUsage
FORMThe composite chunk having first four data bytes of the FORM chunk which are secondary identifier.
FORM:DJVMA multipage DjVu document. Composite chunk that contains the DIRM chunk.
FORM:DJVUSingle page DjVu document. Composite chunk that contains the chunks which make up a page in a djvu document.
FORM:DJVIA “shared” DjVu file which is included via the INCL chunk.  Shared annotations and shape dictionary.
FORM:THUMComposite chunk that contains the TH44 chunks which are the embedded thumbnails.
DIRMPage name information for multi-page documents.
NAVMBookmark information
ANTa, ANTzAnnotations including both initial view settings and overlaid hyperlinks, text boxes, etc.
TXTa, TXTzUnicode Text and layout information.
DjbzShared shape table.
SjbzBZZ compressed JB2 bitonal data used to store mask.
FG44IW44 data used to store foreground
BG44IW44 data used to store background
TH44IW44 data used to store embedded thumbnail images
WMRMJB2 data required to remove a watermark
FGbzColor JB2 data. Provides a color for each (blit or shape?) in the corresponding Sjbz chunk.
INFOInformation about the a DjVu page
INCLThe ID of an included FORM:DJVI chunk.
BGjpJPEG encoded background
FGjpJPEG encoded foreground
SmmrG4 encoded mask

Compression

Single image is divided into many different images, and then every image is compressed separately. For the creation of a DjVu file the image is first separated into three images, a background, foreground and a mask image. Typically the background and foreground images are lower-resolution color images; but the mask image is a higher-resolution image and typically the text is stored there. After the separation the foreground and background images are compressed through a wavelet based compression algorithm IW44, while the mask image is compressed using another method called JB2.

The JB2 encoding method eliminates much of the redundancy in the text image by identifying identical shapes on the page, such as multiple occurrences of a character in a particular font.  JB2 first codes the bitmap of each unique shape by taking advantage of the redundancy between similar shapes. It then codes the locations at which each shape appears on the page. Both JB2 and IW44 rely on a new type of adaptive binary arithmetic coder called the ZP-coder that squeezes out any remaining redundancy within a few percent of the Shannon limit.  The ZP-coder is adaptive, and faster than other approximate binary arithmetic coders.

References

Created by kashifiqbal on 2019/01/06 05:32