From:	gert@greenie.muc.de (Gert Doering)

Fax input format:

   raw G3 data (according to CCITT standard T.4, 1-dimensionally
   compressed). Can be created by GhostScript's "dfaxhigh" or "dfaxlow"
   drivers (they will create raw G3 data with a 64 byte header, that
   sendfax + g3cat + g32pbm will recognize and skip) or by the "pbm2g3"
   program.
   Warning: the pbmtog3 program from the "pbmplus" distribution does *not*
   create valid G3 data according to T.4, to be precise, the lines are
   shorter than 1728 pixels and the leading EOL code is missing. To fix
   it, use the patch that is provided in "patches/pbmtog3.p1". Even
   better, use my own utility. (The patch is *NOT* needed for my pbm2g3
   program!).
   Use of a unpatched pbmtog3 will most likely cause +FHNG:50 or +FHNG:54
   error codes.

   Do not use "tiff-G3" data as input. Though the data itself is G3 encoded,
   it's wrapped into a complex layer of TIFF headers that would
   require non-trivial parsing that's simply not needed for sendfax.
   Faxing a TIFF file will result in a warning message from sendfax, and,
   most likely, in a failed transmission.


Fax output format:

   The files that mgetty places into FAX_SPOOL_IN are in the same format
   as the files that sendfax wants to send, raw G3 data as of CCITT T.4.
   To convert them to "pbm", use the g32pbm program. To convert them to
   X-Windows xwd format, use the "contrib/g3toxwd" program.

   If the files are received without transmission errors, it is possibly
   to send received fax files without any additional conversion. Since
   some modems insert filling zero-bits, a run through "g3cat" is
   recommended anyway, this will remove any surplus stuff, and clean up
   garbled lines.