JBIG-KIT lossless image compression library ------------------------------------------- by Markus Kuhn The latest release of JBIG-KIT can be downloaded from http://www.cl.cam.ac.uk/~mgk25/jbigkit/ JBIG-KIT implements a highly effective data compression algorithm for bi-level high-resolution images such as fax pages or scanned documents. JBIG-KIT provides two variants of a portable library of compression and decompression functions with a documented interface. You can very easily include into your image or document processing software. In addition, JBIG-KIT provides ready-to-use compression and decompression programs with a simple command line interface (similar to the converters found in Jef Poskanzer's PBM graphics file conversion package). JBIG-KIT implements the specification International Standard ISO/IEC 11544:1993 and ITU-T Recommendation T.82(1993), "Information technology - Coded representation of picture and audio information - progressive bi-level image compression", <http://www.itu.int/rec/T-REC-T.82>, which is commonly referred to as the "JBIG1 standard". JBIG (Joint Bi-level Image experts Group) is the committee which developed this international standard for the lossless compression of images using arithmetic coding. Like the well-known compression algorithms JPEG and MPEG, JBIG has also been developed and published by the International Organization for Standardization (ISO) and the International Telecommunication Union (ITU). See also http://www.jpeg.org/jbig/ http://www.iso.ch/ http://www.itu.int/ The JBIG compression algorithm offers the following features: - Close to state-of-the-art lossless compression ratio for high resolution bi-level images. - About 1.1 to 1.5 times better compression ratio on typical scanned documents compared to G4 fax compression (ITU-T T.6), which has been the best compression algorithm for scanned documents available prior to JBIG. - Up to 30 times better compression of scanned images with dithered images compared to G4 fax compression. - About 2 times better compression on typical 300 dpi documents compared to 'gzip -9' on raw bitmaps. - About 3-4 times better compression than GIF on typical 300 dpi documents. - Even much better competitive compression results on computer generated images which are free of scanning distortions. - JBIG supports hierarchical "progressive" encoding, that means it is possible to encode a low resolution image first, followed by resolution enhancement data. This allows, for instance, a document browser to display already a good 75 dpi low resolution version of an image, while the data necessary to reconstruct the full 300 dpi version for laser printer reproduction is still arriving (say over a slow network link or mass storage medium). - The various resolution layers of a JBIG image in progressive encoding mode together require not much more space than a normal non-progressive mode encoded image (which JBIG also supports). - The progressive encoding mode utilizes a quite sophisticated resolution reduction algorithm which offers high quality low resolution versions that preserve the shape of characters as well as the integrity of thin lines and dithered images. - JBIG supports multiple bit planes and can this way also be used for grayscale and color images, although the main field of application is compression of bi-level images, i.e. images with only two different pixel values. For grayscale images with up to 6 bit per pixel, JBIG performs superior to JPEG's lossless mode. JBIG-KIT can be used as free software under the GNU General Public License. Other license arrangements more suitable for commercial applications are available as well, please contact the author for details. JBIG-KIT provides two portable libraries implemented in ANSI/ISO C for encoding and decoding JBIG data streams, along with documentation. The first library, jbig.c, implements nearly all of the options that the JBIG standard provides, but keeps the entire uncompressed image in memory. The second library, jbig85.c, implements only the ITU-R T.85 subset of the standard that black/white fax machines use (single bit per pixel, no "progressive" encoding), and keeps only three lines of the uncompressed image in memory, making it particularly attractive for low-memory embedded applications. The libraries are not intended for 8-bit or 16-bit machine architectures (e.g., old MS-DOS C compilers). For maximum performance, a 32-bit processor is required (64-bit systems work too, of course). On architectures with 16-bit pointer arithmetic, the full-featured jbig.c library can process only very small images. Special features of the full-featured jbig.c variant: - Fully reentrant multithread-capable design (no global or static variables, isolated malloc()/free() calls, etc.) - Capable of handling incomplete and growing JBIG data streams in order to allow earliest display of low resolution versions - Capable of handling several incoming data streams simultaneously in one single process and thread - Especially designed with applications in mind that want to display incoming data as early as possible (e.g., similar to the way in which Netscape Navigator handles incoming GIF images) - Implements all JBIG features and options including progressive and sequential encoding, multiple bit planes, user specified resolution reduction and deterministic prediction tables, adaptive template changes for optimal performance on half-tone images, deterministic prediction, typical prediction in lowest and differential layers, various stripe orderings, etc; only the SEQ and HITOLO options are currently not supported by the decoder (they are normally never required, but could be added later in case of user requirements) - Suitable for fax applications, satisfies ITU-T T.85 profile - Efficient code, optimized utilization of 32-bit processor registers - Very easy to use documented C library interface - Included Gray code conversion routines for efficient encoding of grayscale images - Ready-to-use pbmtojbg and jbgtopbm converters. Special features of the light-weight jbig85.c variant: - Suitable for low-memory embedded applications - Implements only the JBIG1 subset defined in the ITU-T T.85 profile (single bit plane, no differential layers) - Requires only three pixel rows of the uncompressed image to be kept in memory - Handles all NEWLEN modes of operation required by ITU-T T.85 with just a single pass over the data, automatically performing the necessary lookahead after the last stripe - Codec buffers only a few bytes of arithmetic-codec data and outputs resulting bytes or lines as soon as they are available. I will try to provide free support and maintenance for this software for the foreseeable future, depending on my available time. Happy compressing ... Markus Kuhn -- Markus Kuhn, Computer Laboratory, University of Cambridge http://www.cl.cam.ac.uk/~mgk25/ || CB3 0FD, Great Britain