Fax Formats(1) Fax Formats(1)
This page, part of the Netpbmuser'sguide(1), describes FAX formats in
relation to Netpbm facilities.
The ITU (formerly CCITT) publishes standards for operation of fax
machines (the idea is to provide a way to be sure that a fax machine is
able to receive a fax sent by another). These standards incidentally
specify graphics file formats -- a protocol for representing a visual
image in sequences of bits.
The two relevant standards are called Group 3 (G3) and Group 4 (G4)
(Groups 1 and 2 are analog standards no longer in use). Virtually
every fax machine in existence conforms at least generally to at least
one of these standards.
The standard for Group 3 fax is defined in ITU Recommendation T.4. In
the U.S., that is implemented by EIA standards EIA-465 and EIA-466.
These standards cover more than the file format as well, including how
to transmit bits over a telephone line and procedures for handling doc‐
ument transmissions.
G3 faxes are 204 dots per inch (dpi) horizontally and 98 dpi (196 dpi
optionally, in fine-detail mode) vertically.
The standards specify three file formats (also called coding methods
and compression schemes -- remember the standard doesn't mention com‐
puter files; it talks about the format of a stream of bits travelling
over a telephone line):
MH This compresses in one dimension: it compresses individual
raster lines but makes no attempt to compress redundancy between
lines.
One dimensional compression is traditionally the best a fax
machine could handle because G3 neither assumes error free
transmission not retransmits when errors occur, and receiving
fax machines traditionally could not afford to buffer much of a
page. It's important that when there is an error in a raster
line, its impact not spread to many lines after it.
All Group 3 and Group 4 fax machines must be able to send and
receive MH.
MH is sometimes called 'G3,' but that is a poor name because
while the Group 3 standard does specify MH, it has always speci‐
fied other formats too.
MH is sometimes called 'T4' based on the name of the document
that specifies it, ITU T.4. But this is a poor name because T.4
also specifies MR.
MR This compresses in two dimensions, horizontally and vertically.
MR has always been part of the Group 3 standard, but is optional
(a Group 3 fax machine may or may not be able to send and
receive it).
MMR This is a more advanced format than the others. It is even more
two-dimensional than MR. It is optional in the Group 3 stan‐
dard, and didn't even exist in earlier versions of it. It was
developed specifically for the Group 4 standard, but then added
to an extended Group 3 standard as well.
MMR is sometimes called Group 4, but that is a poor name because
of the fact that it is also part of the current Group 3 stan‐
dard.
MMR is sometimes called 'T6' based on the name of the document
that specifies it, ITU T.6.
g3topbm converts the MH format to PBM. pbmtog3 converts PBM to MH.
There is no Netpbm program to convert to or from other fax formats.
TIFF
The TIFF format is flexible enough to allow lots of different coding
methods, within it. There are TIFF subformats for MH, MR, and MMR,
among others. These are particularly useful when you receive a fax as
a TIFF file.
tifftopnm recognizes and can convert from any of these.
pamtotiff can convert to any of these; you use command options to
choose which.
netpbm documentation 03 December 2008 Fax Formats(1)
