Provided by: netpbm_11.10.02-1build1_amd64 
NAME
g3topbm - convert a Group 3 fax file into a PBM image
SYNOPSIS
g3topbm [-reversebits] [-stretch] [-width=pixels | paper_size={A3|A4|A5|A6|B4}] [-stop_error]
[-correctlong] [-kludge] [g3file]
Minimum unique abbreviation of option is acceptable. You may use double hyphens instead of single hyphen
to denote options. You may use white space in place of the equals sign to separate an option name from
its value.
DESCRIPTION
This program is part of Netpbm(1).
g3topbm reads a Group 3 fax file with MH (Modified Huffman) compression as input and produces a PBM image
as output.
g3topbm tolerates various deviations from the standard, so as to recover some of the image if there was a
transmission error. One thing it tolerates is lines of varying length. The standard requires all the
lines to be the same length; g3topbm makes the output image as wide as the longest line in the input and
pads the others on the right. It warns you when it does this.
You can use the stop_error option to make g3topbm insist on valid input.
There is no Netpbm program that understands the other G3 fax compression methods: MR (Modified Read) and
MMR (Modified Modified Read).
Note that the Group 3 fax file format does not include any kind of a signature so that g3topbm might
verify it's actually looking at a G3 file or that the compression method is MH. The program will
interpret any sequence of bytes you give it as if it is G3 and, while typically issuing a lot of error
messages about the file not conforming to the G3 MH format, will produce output (unless you use
-stoperror). In particular, if you feed g3topbm an MR or MMR file, it will not tell you of your mistake.
There are subformats of TIFF that use the Group 3 fax encodings inside. See tifftopnm.
OPTIONS
In addition to the options common to all programs based on libnetpbm (most notably -quiet, see Common
Options ), g3topbm recognizes the following command line options:
-reversebits
Tells g3topbm to interpret bits least-significant first, instead of the default most-significant
first. Apparently some fax modems do it one way and others do it the other way. If you get a
whole bunch of "bad code word" messages, try using this option.
-stretch
This option tells g3topbm to stretch the image vertically by duplicating each row. This is for
the low-quality transmission mode.
-width=pixels
This option tells g3topbm that the image is supposed to be pixels pixels wide. If any line in it
is not that size, g3topbm issues a warning or fails, depending on whether you specify -stop_error.
You cannot specify both -width and -paper_size.
This option was new in Netpbm 10.33 (March 2006).
-paper_size={A3,A4,A5,A6,B4}
This option tells g3topbm for what size paper this image is supposed to be formatted. g3topbm
uses the width of the paper the same way as with the -width option. g3topbm does not use the
height of the paper for anything.
You cannot specify both -width and -paper_size.
This option was new in Netpbm 10.33 (March 2006).
-stop_error
This option tells g3topbm to fail when it finds a problem in the input. "Fail" means it
terminates with a nonzero status code with the contents of the output file undefined.
If you don't specify this option, g3topbm does its best to work around input errors and salvage as
much of the image as possible in the output image. It first tries to resynchronize to a later
line by searching for the next End Of Line marker, skipping any lines or partial lines in between.
It saves the beginning of the line in which it encountered the problem. If the input file ends
prematurely, g3topbm produces output containing the lines up to where it encountered the problem.
g3topbm issues warning messages when it continues in spite of input errors.
This option was new in Netpbm 10.24 (August 2004). Before that, g3topbm always failed when it
encountered premature EOF and never failed when it encountered other problems. -correctlong
This option helps with certain corrupted input files. Faxes often are corrupted because of
communication line errors. A particularly annoying corruption causes a line to be much longer
than it is supposed to be. One way that can happen is where an End of Line marker is missing, so
two consecutive lines turn into one.
Without this option, g3topbm faithfully renders the document as coded, so the output PBM image
simply contains that long line. This is an especially problematic corruption because it makes the
entire output image about twice as wide as it is supposed to be. But with -correctlong, g3topbm
truncates that line so the overall effect of the input corruption is that a line is missing rather
than that the image is twice as wide as it is supposed to be.
Specifically, with -correctlong, the program looks at the lengths of all the lines (which would
all be the same length in an uncorrupted G3 image) and considers the line length that occurs the
most to be the intended image width. It truncates every line that is longer than that.
The program warns you when corruption has caused the input image to have lines of more than one
length, whether you specify -correctlong or not.
Note that there is no point to specifying -correctlong if you also specify -stop_error.
This option was new in Netpbm 11.04 (September 2023).
-kludge
Tells g3topbm to ignore the first few lines of the file; sometimes fax files have some junk at the
beginning.
SEE ALSO
pbmtog3(1), tifftopnm(1), pbm(1), fax formats(1)
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/g3topbm.html
netpbm documentation 02 July 2023 G3topbm User Manual(1)