Many facsimile and document imaging file formats support a form of lossless
data compression often described as CCITT encoding. The CCITT (International
Telegraph and Telephone Consultative Committee) is a standards organization
that has developed a series of communications protocols for the facsimile
transmission of black-and-white images over telephone lines and data networks.
These protocols are known officially as the CCITT T.4 and T.6 standards but
are more commonly referred to as CCITT Group 3 and Group 4 compression,
respectively.
The CCITT actually defines three algorithms for the encoding of bi-level image data:
Group 3 One-Dimensional (G31D)
Group 3 Two-Dimensional (G32D) - not implemented
Group 4 Two-Dimensional (G42D)