JBIG2 is an image compression standard for bi-level images, developed by the Joint Bi-level Image Experts Group. It is suitable for both lossless and lossy compression. According to a press release^[1] from the Group, in its lossless mode JBIG2 typically generates files from 3 to 5 times smaller than Fax Group 4 and from 2 to 4 times smaller than JBIG, the previous bi-level compression standard released by the Group. JBIG2 has been published in 2000 as the international standard ITU T.88, and in 2001 as ISO/IEC 14492. Image compression is the application of data compression on digital images. ... A binary image is a digital image that has only two possible values for each pixel. ... The Joint Bi-level Image Experts Group (JBIG) is a group of experts nominated by national standards bodies and major companies to work to produce standards for bi-level image coding. ... Lossless data compression is a class of data compression algorithms that allows the original data to be reconstructed exactly from the compressed data. ... A lossy data compression method is one where compressing data and then decompressing it retrieves data that may well be different to the original, but is close enough to be useful in some way. ... JBIG is a lossless image compression standard from the Joint Bi-level Image Experts Group, standardized as ISO/IEC standard 11544 and as ITU-T recommendation T.82. ...

Ideally, a JBIG2 encoder will segment the input page into regions of text, regions of halftone images, and regions of other data. Regions which are neither text nor halftones are typically compressed using a context-dependent arithmetic coding algorithm called the MQ coder. Textual regions are compressed as follows: the foreground pixels in the regions are grouped into symbols. A dictionary of symbols is then created and encoded, typically also using context-dependent arithmetic coding, and the regions are encoded by describing which symbols appear where. Typically, a symbol will correspond to a character of text, but this is not required by the compression method. For lossy compression the difference between similar symbols (e.g., slightly different impressions of the same letter) can be neglected; for lossless compression, this difference is taken into account by compressing one similar symbol using another as a template. Halftone images may be compressed by reconstructing the grayscale image used to generate the halftone and then sending this image together with a dictionary of halftone patterns. An overview of JBIG2 may be found in ^[2]. Overall, the algorithm used by JBIG2 to compress text is very similar to the JB2 compression scheme used in the DjVu file format for coding binary images. Halftoning is a method of printing shades using a single color ink. ... Arithmetic coding is a method for lossless data compression. ... DjVu (pronounced deja-vu) is a computer file format designed primarily to store scanned images. ...

PDF files versions 1.4 and above may contain JBIG2 compressed data. Open source decoders for JBIG2 are jbig2dec^[3] and the decoder found in versions 2.00 and above of xpdf. Portable Document Format (PDF) is a file format developed by Adobe Systems for representing documents in a manner that is independent of the original application software, hardware, and operating system used to create those documents. ... Open source software refers to computer software and the availability of its source code as open source under an open source license to study, change, and improve its design. ... In computing, Xpdf is an open source PDF viewer for the X Window System, running under Unix and Unix-like systems such as Linux, OpenVMS, and OS/2. ...

References and exterrnal links

1. ^ : Press release from the Joint Bi-level Image experts Group
2. ^ : JBIG2-the ultimate bi-level image coding standard, by F. Ono, W. Rucklidge, R. Arps, and C. Constantinescu, in pp. 140–143, Proceedings, 2000 International Conference on Image Processing, (Vancouver, BC, Canada), vol. 1.
3. ^ : jbig2dec home page