Specs for Group 3 and 4 image coding (group 3 is very similar to group 4)
are in CCITT (1988) volume VII fascicle VII.3. They are recommendations
T.4 and T.6 respectively. There is also an updated spec contained in 1992
recommendations T.1 to T.6.

CCITT (now ITU-T) specs are available by anonymous ftp (see above answer on
V.42bis).  The T.4 and T.6 specs are in
ftp://sunsite.doc.ic.ac.uk/computing/ccitt/ccitt-standards/1988/7_3_01.ps.gz
ftp://sunsite.doc.ic.ac.uk/computing/ccitt/ccitt-standards/1988/7_3_02.ps.gz

The following paper covers T.4, T.6 and JBIG:

  "Review of standards for electronic imaging for facsimile systems"
  in Journal of Electronic Imaging, Vol. 1, No. 1, pp. 5-21, January 1992.


Source code can be obtained as part of a TIFF toolkit - TIFF image
compression techniques for binary images include CCITT T.4 and T.6:

    ftp://ftp.sgi.com/graphics/tiff/tiff-v3.4beta035-tar.gz
    Contact: sam@engr.sgi.com

There is also a companion compressed tar file (v3.0pics.tar.Z) that
has sample TIFF image files. A draft of TIFF 6.0 is in TIFF6.ps.Z.
Concerning JPEG compression in TIFF 6.0, Tom Lane <tgl@sss.pgh.pa.us> adds:

  TIFF 6.0's scheme for incorporating JPEG compression (spec section 22) has
  a bunch of serious deficiencies.  Don't use it.  A revised design is given
  by TIFF Technical Note #2, ftp://ftp.sgi.com/graphics/tiff/TTN2.draft.txt
  The revised design will replace section 22 in TIFF 7.0, and is implemented
  in Sam Leffler's libtiff.  See also item 75 of this FAQ for more JPEG info.

Software for reading and writing CCITT Group 3 and 4 images is
also available in directory ftp://merry.cs.monash.edu.au/pub/alanf/TIFF_FAX
Contact: Alan Finlay <alanf@bruce.cs.monash.edu.au>.


See also question 54 below.

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