FAX Compression Scheme?

[Wayne Folta]

What kind of data compression does a Group III FAX use, if any? I seem to
remember reading that they do RLE compression of white but not black.

Also, when an add claims that a FAX machine will do a page in 15 seconds,
what does this mean? Fine mode? A blank sheet of paper?

Wayne
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Wayne Folta (fo...@cs.umd.edu 128.8.128.8)

[tni...@hayes.uucp]

In article <33...@mimsy.umd.edu>, fo...@tove.cs.umd.edu (Wayne Folta) writes:

> What kind of data compression does a Group III FAX use, if any? I seem to
> remember reading that they do RLE compression of white but not black.

T.4 has a couple of different encoding schemes. The mandatory one
(one dimensional) does a Huffman-encoded RLE compression of both
black and white pels. The optional scheme is two-dimensional,
meaning that each line specifies the differences from the line
above -- but it also is basically a Huffman-encoded RLE of both
black and white pels.

> Also, when an add claims that a FAX machine will do a page in 15 seconds,
> what does this mean? Fine mode? A blank sheet of paper?

Generally, this means they can send the CCITT standard Group 3 fax
test page in the time stated (actually, there should be "fine print"
at the bottom of the ad clarifying this). No, you won't see that
time in fine mode, and blank page will transmit a lot faster. You
also have to be careful and read the fine print -- the machine might
only acheive that transmission time when sending to another of its
own kind, so they can use proprietary modulation and compression
schemes (group 3 was designed to be able to send a standard page of
text in 1 minute, as opposed to 3 minutes for Group 2 and 6 minutes
for Group 1).

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