7. Coding

7.1 The need for coding

Group 3 and Group 4 fax machines cut images into lines and pixels.

Analyzed at the typical resolution of 8 dots/mm x 7.7 lines/mm, and without halftones, an A4 page represents 500 kilobytes, or a transmission time of 7 minutes for a standard fax machine, at 9600 bit/s, which is prohibitive.

A classic image is highly redundant (uniform areas, lines, etc.).

The coder's role is to exploit this redundancy to significantly reduce the volume of information to be transmitted. The compression ratio achieved, i.e. the ratio between the size of the original document and that of the coded document, is one of the performance criteria for coding algorithms. A second factor to take into account is the...