1Fax Formats(1) General Commands Manual Fax Formats(1)
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6 This page, part of the Netpbm user's guide(1), describes FAX formats in
7 relation to Netpbm facilities.
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11 The ITU (formerly CCITT) publishes standards for operation of fax
12 machines (the idea is to provide a way to be sure that a fax machine is
13 able to receive a fax sent by another). These standards incidentally
14 specify graphics file formats -- a protocol for representing a visual
15 image in sequences of bits.
16 The two relevant standards are called Group 3 (G3) and Group 4 (G4)
18 (Groups 1 and 2 are analog standards no longer in use). Virtually
19 every fax machine in existence conforms at least generally to at least
20 one of these standards.
21 The standard for Group 3 fax is defined in ITU Recommendation T.4. In
23 the U.S., that is implemented by EIA standards EIA-465 and EIA-466.
24 These standards cover more than the file format as well, including how
25 to transmit bits over a telephone line and procedures for handling doc‐
26 ument transmissions.
27 G3 faxes are 204 dots per inch (dpi) horizontally and 98 dpi (196 dpi
29 optionally, in fine-detail mode) vertically.
30 The standards specify three file formats (also called coding methods
32 and compression schemes -- remember the standard doesn't mention com‐
33 puter files; it talks about the format of a stream of bits travelling
34 over a telephone line):
35 MH This compresses in one dimension: it compresses individual
40 raster lines but makes no attempt to compress redundancy between
41 lines.
42 One dimensional compression is traditionally the best a fax
44 machine could handle because G3 neither assumes error free
45 transmission not retransmits when errors occur, and receiving
46 fax machines traditionally could not afford to buffer much of a
47 page. It's important that when there is an error in a raster
48 line, its impact not spread to many lines after it.
49 All Group 3 and Group 4 fax machines must be able to send and
51 receive MH.
52 MH is sometimes called "G3," but that is a poor name because
54 while the Group 3 standard does specify MH, it has always speci‐
55 fied other formats too.
56 MH is sometimes called "T4" based on the name of the document
58 that specifies it, ITU T.4. But this is a poor name because T.4
59 also specifies MR.
60 MR This compresses in two dimensions, horizontally and vertically.
64 MR has always been part of the Group 3 standard, but is optional
66 (a Group 3 fax machine may or may not be able to send and
67 receive it).
68 MMR This is a more advanced format than the others. It is even more
71 two-dimensional than MR. It is optional in the Group 3 stan‐
72 dard, and didn't even exist in earlier versions of it. It was
73 developed specifically for the Group 4 standard, but then added
74 to an extended Group 3 standard as well.
75 MMR is sometimes called Group 4, but that is a poor name because
77 of the fact that it is also part of the current Group 3 stan‐
78 dard.
79 MMR is sometimes called "T6" based on the name of the document
81 that specifies it, ITU T.6.
82 g3topbm converts the MH format to PBM. pbmtog3 converts PBM to MH.
86 There is no Netpbm program to convert to or from other fax formats.
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91 The TIFF format is flexible enough to allow lots of different coding
92 methods, within it. There are TIFF subformats for MH, MR, and MMR,
93 among others. These are particularly useful when you receive a fax as
94 a TIFF file.
95 tifftopnm recognizes and can convert from any of these.
97 pamtotiff can convert to any of these; you use command options to
99 choose which.
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102 This manual page was generated by the Netpbm tool 'makeman' from HTML
103 source. The master documentation is at
104 http://netpbm.sourceforge.net/doc/faxformat.html
106netpbm documentation 03 December 2008 Fax Formats(1)