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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 3.3//EN">
<html><head><title>G3topbm User Manual</title></head>
<body>
<h1>g3topbm</h1>
Updated: 02 July 2023
<br>
<a href="#index">Table Of Contents</a>
<h2>NAME</h2>
g3topbm - convert a Group 3 fax file into a PBM image
<h2 id="synopsis">SYNOPSIS</h2>
<b>g3topbm</b>
[<b>-reversebits</b>]
[<b>-stretch</b>]
[<b>-width=</b><i>pixels</i> | paper_size={A3|A4|A5|A6|B4}]
[<b>-stop_error</b>]
[<b>-correctlong</b>]
[<b>-kludge</b>]
[<i>g3file</i>]
<p>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.
<h2 id="description">DESCRIPTION</h2>
<p>This program is part of <a href="index.html">Netpbm</a>.
<p><b>g3topbm</b> reads a Group 3 fax file with MH (Modified Huffman)
compression as input and produces a PBM image as output.
<p><b>g3topbm</b> 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; <b>g3topbm</b> 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.
<p>You can use the <b>stop_error</b> option to make <b>g3topbm</b>
insist on valid input.
<p>There is no Netpbm program that understands the other G3 fax
compression methods: MR (Modified Read) and MMR (Modified Modified Read).
<p>Note that the Group 3 fax file format does not include any kind of a
signature so that <b>g3topbm</b> 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
<b>-stoperror</b>). In particular, if you feed <b>g3topbm</b> an MR or MMR
file, it will not tell you of your mistake.
<p>There are subformats of TIFF that use the Group 3 fax encodings
inside. See <b>tifftopnm</b>.
<h2 id="options">OPTIONS</h2>
<p>In addition to the options common to all programs based on libnetpbm
(most notably <b>-quiet</b>, see <a href="index.html#commonoptions">
Common Options</a>), <b>g3topbm</b> recognizes the following
command line options:
<dl compact>
<dt><b>-reversebits</b>
<dd>Tells <b>g3topbm</b> 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.
<dt><b>-stretch</b>
<dd>This option tells <b>g3topbm</b> to stretch the image vertically by
duplicating each row. This is for the low-quality transmission mode.
<dt><b>-width=</b><i>pixels</i>
<dd>This option tells <b>g3topbm</b> that the image is supposed to be
<i>pixels</i> pixels wide. If any line in it is not that size, <b>g3topbm</b>
issues a warning or fails, depending on whether you specify
<b>-stop_error</b>.
<p>You cannot specify both <b>-width</b> and <b>-paper_size</b>.
<p>This option was new in Netpbm 10.33 (March 2006).
<dt><b>-paper_size=</b>{<b>A3</b>,<b>A4</b>,<b>A5</b>,<b>A6</b>,<b>B4</b>}
<dd>This option tells <b>g3topbm</b> for what size paper this image is
supposed to be formatted. <b>g3topbm</b> uses the width of the paper
the same way as with the <b>-width</b> option. <b>g3topbm</b>
does not use the height of the paper for anything.
<p>You cannot specify both <b>-width</b> and <b>-paper_size</b>.
<p>This option was new in Netpbm 10.33 (March 2006).
<dt><b>-stop_error</b>
<dd>This option tells <b>g3topbm</b> 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.
<p>If you don't specify this option, <b>g3topbm</b> 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,
<b>g3topbm</b> produces output containing the lines up to where it
encountered the problem.
<p><b>g3topbm</b> issues warning messages when it continues in spite of
input errors.
<p>This option was new in Netpbm 10.24 (August 2004). Before that,
<b>g3topbm</b> always failed when it encountered premature EOF and
never failed when it encountered other problems.
<dd><b>-correctlong</b>
<p>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.
<p>Without this option, <b>g3topbm</b> 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 <b>-correctlong</b>,
<b>g3topbm</b> 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.
<p>Specifically, with <b>-correctlong</b>, 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.
<p>The program warns you when corruption has caused the input image to have
lines of more than one length, whether you specify <b>-correctlong</b> or not.
<p>Note that there is no point to specifying <b>-correctlong</b> if you also
specify <b>-stop_error</b>.
<P>This option was new in Netpbm 11.04 (September 2023).
<dt><b>-kludge</b>
<dd>Tells <b>g3topbm</b> to ignore the first few lines of the file; sometimes
fax files have some junk at the beginning.
</dl>
<h2 id="seealso">SEE ALSO</h2>
<a href="pbmtog3.html">pbmtog3</a>,
<a href="tifftopnm.html">tifftopnm</a>,
<a href="pbm.html">pbm</a>,
<a href="faxformat.html">fax formats</a>
<hr>
<h2 id="index">Table Of Contents</h2>
<ul>
<li><a href="#synopsis">SYNOPSIS</a>
<li><a href="#description">DESCRIPTION</a>
<li><a href="#options">OPTIONS</a>
<li><a href="#seealso">SEE ALSO</a>
</ul>
</body>
</html>
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