What is Group 4 Compression (ITU-T T.6)?

Last Update: August 9th, 2021

G4, or Group 4 Compression, is a method of image compression in G4 fax machines.

G4 is defined in the ITU-T T.6 fax standard for transmitting black and white images.

Also used in the TIFF and PDF document formats.

Group 4 Compression works through a combination of two methods:
– Using vertical features to allow further compression
– Encoding each scan line in the bitmap with a series of variable length codes to represent the sequence of changes between white and black that can be found in the scan line. In this sense, it is similar to Huffman compression.

More Fax Technology Descriptions:

• G4 Compression – Compression format for fax signals
• Radiofax HF Fax and Weatherfax – Fax Transmissions over Radiowaves
• T.38 -ITU recommendation for faxing over IP
• Fax Demodulator – Unit for Intercepting Fax Communication