Fax Compression Methods: MH, MR & MMR

What Are Fax Compression Methods?

Fax compression methods are the specific coding schemes a Group 3 fax device or service applies to shrink a scanned page's data before transmitting it. They're the concrete implementations of the broader coding method concept, and every standard Group 3 fax supports at least the first of the three below.

MH, MR, and MMR

• Modified Huffman (MH): compresses each scanned line on its own, replacing common patterns of black and white pixels with shorter codes. It's the baseline method every Group 3 fax device supports.
• Modified READ (MR): compresses a line by referencing the line immediately above it, encoding only the differences between the two, which shrinks a typical text page further than MH alone.
• Modified Modified READ (MMR): extends the same line-referencing approach with fewer fallback cases, generally achieving the tightest compression of the three, at the cost of needing more processing power to encode and decode.

Compression, TIFF, and Fax.Plus

A fax page compressed with one of these methods was historically often stored or exchanged as a TIFF file, the container format, not the compression itself. These schemes are formally defined within the ITU-T T.4 recommendation, part of the wider ITU-T fax standards. A newer standard, JBIG compression, improves on all three for certain image types, though it saw far less real-world adoption. Fax.Plus applies compression automatically on every transmission; there's no method for a sender to select or configure.