Now that the newer bi-level image compression standard JBIG2 has been released, JBIG is also known as JBIG1.
In most situations JBIG offers between a 20% and 50% increase in compression efficiency over Fax Group 4 compression, and in some situations, it offers a 30-fold improvement.
JBIG is based on a form of arithmetic coding developed by IBM (known as the Q-coder) that also uses a relatively minor refinement developed by Mitsubishi, resulting in what became known as the QM-coder.
It bases the probability estimates for each encoded bit on the values of the previous bits and the values in previous lines of the picture.
JBIG also supports progressive transmission, which generally incurs a small overhead in bit rate (around 5%).