CCITT Group 3

CCITT Group 3 is a lossless compressed data format for bi-level images. It is also known as T4, T.4, Fax3, Group 3, G3, or ITU-T Fax Group 3. It is one of the formats used by fax machines. It is usually embedded in another image file format such as TIFF, but can be stored in a file by itself.

It comes in two main varieties: so-called 1-dimensional and 2-dimensional (though both are used on 2-dimensional images). The 1-dimensional variety uses Modified Huffman (MH) compression, and the 2-dimensional variety uses Modified READ (MR) compression.

Related formats
Many of the formats listed in the Fax formats article use CCITT Group 3 compression.

In a TIFF file, compression codes 2, 3, and 32771 are varieties of CCITT Group 3 compression.

Other formats that can use CCITT Group 3 compression include PDF and SPIFF.

Specifications

 * ITU-T Recommendation T.4
 * Adobe TIFF standards → TIFF6.pdf, Section 10: Modified Huffman Compression - Defines a slightly simplified form of the 1-d format

Software

 * Konvertor
 * Netpbm: g3topbm, pbmtog3
 * ImageMagick (format named "FAX")
 * Farbfeld Utilities
 * abydos

Sample files

 * https://telparia.com/fileFormatSamples/image/faxGroup3/

Links

 * RFC 1494: Equivalences between 1988 X.400 and RFC-822 Message Bodies
 * RFC 2159: A MIME Body Part for FAX