Manual: faxformat

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

       This page, part of the Netpbm user's guide(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 document 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 computer 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	 re‐
	      dundancy 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 specified 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 standard, 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	cur‐
	      rent Group 3 standard.

	      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.

DOCUMENT SOURCE
       This manual page was generated by the Netpbm tool 'makeman' from HTML source.  The master documentation is at

	      http://netpbm.sourceforge.net/doc/faxformat.html

netpbm documentation				      03 December 2008					      Fax Formats(1)