1Fax Formats(1)              General Commands Manual             Fax Formats(1)
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SYNOPSIS

6       This page, part of the Netpbm user's guide(1), describes FAX formats in
7       relation to Netpbm facilities.
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DESCRIPTION

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.
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17       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.
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22       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.
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28       G3  faxes  are 204 dots per inch (dpi) horizontally and 98 dpi (196 dpi
29       optionally, in fine-detail mode) vertically.
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31       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):
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39       MH     This  compresses  in  one  dimension:  it  compresses individual
40              raster lines but makes no attempt to compress redundancy between
41              lines.
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43              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.
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50              All Group 3 and Group 4 fax machines must be able  to  send  and
51              receive MH.
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53              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.
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57              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.
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63       MR     This compresses in two dimensions, horizontally and vertically.
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65              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).
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70       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.
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76              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.
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80              MMR is sometimes called "T6" based on the name of the document
81               that specifies it, ITU T.6.
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85       g3topbm converts the MH format to PBM.  pbmtog3 converts PBM to MH.
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87       There is no Netpbm program to convert to or from other fax formats.
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TIFF

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.
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96       tifftopnm recognizes and can convert from any of these.
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98       pamtotiff can convert to any of  these;  you  use  command  options  to
99       choose which.
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DOCUMENT SOURCE

102       This  manual  page was generated by the Netpbm tool 'makeman' from HTML
103       source.  The master documentation is at
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105              http://netpbm.sourceforge.net/doc/faxformat.html
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107netpbm documentation           03 December 2008                 Fax Formats(1)
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