Group 4 Compression – ITU-T T.6

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

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.

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