jbig2dec is a decoder implementation of the JBIG2 image compression format.

JBIG2 is designed for lossy or lossless encoding of ‘bilevel’ (1-bit monochrome) images at moderately high resolution, and in particular scanned paper documents. In this domain it can be very efficient, offering compression ratios on the order of 100:1.

This is a decoder only implementation, and its primary use is in Ghostscript and MuPDF for decoding JBIG2 streams in PDF files. Thus its primary focus is the set of JBIG2 features supported in PDF.

The specification has been published as ISO IEC 14492 and ITU T.88. These documents completely describe the format and they’ll be required if you want to understand the code and contribute. We prefer the ISO version but have not discovered significant differences between the two.

JBIG2 also includes the CCITT T.4 and T.6 (group 3 and group 4) fax encodings, which are documented separately. Their specifications and T.88 can be obtained from the ITU. The ISO version of JBIG2 can be obtained through the ISO website.

Unfortunately, none of these documents are free, though the ITU sometimes offers free download specials. More information about JBIG2 and a copy of the draft spec are available from the JPEG website.