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JBIG2
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〔(Press release from the Joint Bi-level Image experts Group )〕 from the Group, in its lossless mode JBIG2 typically generates files one third to one fifth the size of Fax Group 4 and one half to one quarter the size of JBIG, the previous bi-level compression standard released by the Group. JBIG2 has been published in 2000 as the international standard ITU T.88,〔(【引用サイトリンク】title=ITU-T Recommendation T.88 -- T.88 : Information technology - Coded representation of picture and audio information - Lossy/lossless coding of bi-level images )〕 and in 2001 as ISO/IEC 14492.〔(【引用サイトリンク】title=ISO/IEC 14492:2001 - Information technology -- Lossy/lossless coding of bi-level images )〕
==Functionality==
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 QM 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.〔''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.〕 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.
PDF files versions 1.4 and above may contain JBIG2 compressed data. Open source decoders for JBIG2 are jbig2dec,〔(jbig2dec home page )〕 the java-based jbig2-imageio〔(open source jbig2 plugin for Java's ImageIO )〕 and the decoder found in versions 2.00 and above of xpdf. An open source encoder is jbig2enc.〔(jbig2enc home page )〕
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