Editing Fax (section)

==== JBIG ====

In 1999, ITU-T recommendation T.30 added [[JBIG]] (ITU-T T.82) as another [[lossless]] [[Binary image|bi-level]] compression algorithm, or more precisely a "fax profile" subset of JBIG (ITU-T T.85). JBIG-compressed pages result in 20% to 50% faster transmission than MMR-compressed pages, and up to 30 times faster transmission if the page includes [[halftone]] images.

[[JBIG]] performs [[adaptive compression]], that is, both the encoder and decoder collect statistical information about the transmitted image from the pixels transmitted so far, in order to predict the probability for each next pixel being either black or white. For each new pixel, JBIG looks at ten nearby, previously transmitted pixels. It counts, how often in the past the next pixel has been black or white in the same neighborhood, and estimates from that the probability distribution of the next pixel. This is fed into an [[arithmetic coder]], which adds only a small fraction of a bit to the output sequence if the more probable pixel is then encountered.

The ITU-T T.85 "fax profile" constrains some optional features of the full JBIG standard, such that codecs do not have to keep data about more than the last three pixel rows of an image in memory at any time. This allows the streaming of "endless" images, where the height of the image may not be known until the last row is transmitted.

ITU-T T.30 allows fax machines to negotiate one of two options of the T.85 "fax profile":
* In "basic mode", the JBIG encoder must split the image into horizontal stripes of 128 lines (parameter L0&nbsp;=&nbsp;128) and restart the arithmetic encoder for each stripe.
* In "option mode", there is no such constraint.