The general category of schemes a fax uses to compress a scanned page's data before it's transmitted.
- A coding method is the general category of scheme a fax device or service uses to compress a scanned page's data before sending it, not one specific algorithm.
- The most common fax coding methods, Modified Huffman, Modified READ, and Modified Modified READ, are grouped and explained together as fax compression methods.
- A coding method built for plain black-and-white text doesn't handle a grayscale or photo image well, since the two produce very different data patterns.
- Fax.Plus applies compression automatically as part of transmission; there's no coding method setting for the sender to choose.
What Is a Coding Method?
A coding method is the general category of scheme a fax device or service uses to compress a scanned page's data before transmitting it, reducing a page that could be megabytes of raw image data down to a size that transmits in seconds. It's a category, not one specific algorithm, several different coding methods exist, each suited to a different kind of page content.
Coding Methods Used in Fax
The three coding methods actually used in Group 3 fax transmission, Modified Huffman, Modified READ, and Modified Modified READ, are close variations of the same underlying idea and are documented together as fax compression methods. A newer, more advanced coding method, JBIG, achieves better compression on the same kind of black-and-white page but saw more limited adoption, see JBIG for how it compares.
Coding Method and Image Type
A coding method built for plain black-and-white text assumes long runs of a single color, exactly what typed text produces, and compresses that pattern efficiently. That same method handles an image using many shades of gray far less efficiently, since a continuous-tone image doesn't have the same long, predictable runs, which is part of why fax quality on photos and detailed graphics has always lagged behind plain text. The standards body defining these schemes for fax is the ITU, whose T.4 recommendation specifies them formally.
A Common Case: Sending an Old Blueprint by Fax
An engineering office faxing a decades-old blueprint notices the fine linework compresses and transmits differently than a typed cover letter would, a direct result of the coding method being built around plain text patterns rather than dense technical drawings.
FAQs
Is a coding method the same as a compression method on a fax?
Yes, in fax terminology the two describe the same thing: the scheme used to shrink a scanned page's data before it's sent.
Does the same coding method compress text and photos equally well?
No. A coding method built around plain text handles long runs of a single color efficiently, which fits typed pages well. Continuous-tone images like photos and detailed graphics lack that pattern, so they compress less efficiently.
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