What is GroupĀ 4 Compression and how does it work?

Group 4 Compression standard (or G4 compression) is a method of compressing black and white images used in modern fax machines that achieves great levels of compression (a compression ratio of 20:1 is achieved), a standard also used in TIFF type images and in PDF documents.

The principle behind this method is vertical description of the image, where the info sent is not if every dot on the paper has black or white point, but info on every line on the paper with the description from where to where there are white and black lines.
Naturally, this method works the best when the image is not too detailed and doesn't have too many transitions between white and black bits. In this regard, the method is pretty similar to Huffman Compression, and it achieves further reduction in data transfer by omitting the “end of line” strings.


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