200 lines
8.7 KiB
HTML
200 lines
8.7 KiB
HTML
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<HTML><HEAD><TITLE>Man page of ppm</TITLE>
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</HEAD><BODY>
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<H1>ppm</H1>
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Section: File Formats (5)<BR>Updated: 08 April 2000<BR><A HREF="#index">Index</A>
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<A HREF="/cgi-bin/man/man2html">Return to Main Contents</A><HR>
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<A NAME="lbAB"> </A>
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<H2>NAME</H2>
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ppm - portable pixmap file format
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<A NAME="lbAC"> </A>
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<H2>DESCRIPTION</H2>
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The portable pixmap format is a lowest common denominator color image
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file format.
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<A NAME="ixAAB"></A>
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<P>
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It should be noted that this format is egregiously inefficient. It is
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highly redundant, while containing a lot of information that the human
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eye can't even discern. Furthermore, the format allows very little
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information about the image besides basic color, which means you may
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have to couple a file in this format with other independent
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information to get any decent use out of it. However, it is very easy
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to write and analyze programs to process this format, and that is the
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point.
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<P>
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It should also be noted that files often conform to this format in
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every respect except the precise semantics of the sample values.
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These files are useful because of the way PPM is used as an
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intermediary format. They are informally called PPM files, but to be
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absolutely precise, you should indicate the variation from true PPM.
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For example, "PPM using the red, green, and blue colors that the
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scanner in question uses."
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<P>
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The format definition is as follows.
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<P>
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A PPM file consists of a sequence of one or more PPM images. There are
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no data, delimiters, or padding before, after, or between images.
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<P>
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Each PPM image consists of the following:
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<DL COMPACT>
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<DT id="1">-<DD>
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A "magic number" for identifying the file type.
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A ppm image's magic number is the two characters "P6".
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<A NAME="ixAAC"></A>
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<DT id="2">-<DD>
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Whitespace (blanks, TABs, CRs, LFs).
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<DT id="3">-<DD>
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A width, formatted as ASCII characters in decimal.
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<DT id="4">-<DD>
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Whitespace.
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<DT id="5">-<DD>
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A height, again in ASCII decimal.
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<DT id="6">-<DD>
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Whitespace.
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<DT id="7">-<DD>
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The maximum color value (Maxval), again in ASCII decimal. Must be less
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than 65536.
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<DT id="8">-<DD>
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Newline or other single whitespace character.
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<DT id="9">-<DD>
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A raster of Width * Height pixels, proceeding through the image in
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normal English reading order. Each pixel is a triplet of red, green,
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and blue samples, in that order. Each sample is represented in pure
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binary by either 1 or 2 bytes. If the Maxval is less than 256, it is
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1 byte. Otherwise, it is 2 bytes. The most significant byte is
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first.
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<DT id="10">-<DD>
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In the raster, the sample values are "nonlinear." They are
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proportional to the intensity of the CIE Rec. 709 red, green, and blue
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in the pixel, adjusted by the CIE Rec. 709 gamma transfer function.
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(That transfer function specifies a gamma number of 2.2 and has a
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linear section for small intensities). A value of Maxval for all
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three samples represents CIE D65 white and the most intense color in
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the color universe of which the image is part (the color universe is
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all the colors in all images to which this image might be compared).
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<DT id="11">-<DD>
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Note that a common variation on the PPM format is to have the sample
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values be "linear," i.e. as specified above except without the gamma
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adjustment.
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<B>pnmgamma</B>
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takes such a PPM variant as input and produces a true PPM as output.
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<P>
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<DT id="12">-<DD>
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Characters from a "#" to the next end-of-line, before the maxval line,
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are comments and are ignored.
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</DL>
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<P>
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Note that you can use
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<B>pnmdepth</B>
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to convert between a the format with 1 byte per sample and the one
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with 2 bytes per sample.
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<P>
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There is actually another version of the PPM format that is fairly rare:
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"plain" PPM format. The format above, which generally considered the
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normal one, is known as the "raw" PPM format. See
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<B><A HREF="/cgi-bin/man/man2html?5+pbm">pbm</A></B>(5)
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for some commentary on how plain and raw formats relate to one another.
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<P>
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The difference in the plain format is:
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<DL COMPACT>
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<DT id="13">-<DD>
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There is exactly one image in a file.
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<DT id="14">-<DD>
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The magic number is P3 instead of P6.
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<DT id="15">-<DD>
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Each sample in the raster is represented as an ASCII decimal number
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(of arbitrary size).
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<DT id="16">-<DD>
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Each sample in the raster has white space before and after it. There must
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be at least one character of white space between any two samples, but there
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is no maximum. There is no particular separation of one pixel from another --
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just the required separation between the blue sample of one pixel from the
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red sample of the next pixel.
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<DT id="17">-<DD>
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No line should be longer than 70 characters.
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</DL>
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<P>
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Here is an example of a small pixmap in this format:
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<PRE>
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P3
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# feep.ppm
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4 4
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15
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0 0 0 0 0 0 0 0 0 15 0 15
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0 0 0 0 15 7 0 0 0 0 0 0
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0 0 0 0 0 0 0 15 7 0 0 0
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15 0 15 0 0 0 0 0 0 0 0 0
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</PRE>
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<P>
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Programs that read this format should be as lenient as possible,
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accepting anything that looks remotely like a pixmap.
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<P>
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<A NAME="lbAD"> </A>
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<H2>COMPATIBILITY</H2>
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<P>
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Before April 2000, a raw format PPM file could not have a maxval greater
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than 255. Hence, it could not have more than one byte per sample. Old
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programs may depend on this.
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<P>
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Before July 2000, there could be at most one image in a PPM file. As
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a result, most tools to process PPM files ignore (and don't read) any
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data after the first image.
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<P>
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<A NAME="lbAE"> </A>
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<H2>SEE ALSO</H2>
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<A HREF="/cgi-bin/man/man2html?1+giftopnm">giftopnm</A>(1), <A HREF="/cgi-bin/man/man2html?1+gouldtoppm">gouldtoppm</A>(1), <A HREF="/cgi-bin/man/man2html?1+ilbmtoppm">ilbmtoppm</A>(1), <A HREF="/cgi-bin/man/man2html?1+imgtoppm">imgtoppm</A>(1), <A HREF="/cgi-bin/man/man2html?1+mtvtoppm">mtvtoppm</A>(1),
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<A HREF="/cgi-bin/man/man2html?1+pcxtoppm">pcxtoppm</A>(1), <A HREF="/cgi-bin/man/man2html?1+pgmtoppm">pgmtoppm</A>(1), <A HREF="/cgi-bin/man/man2html?1+pi1toppm">pi1toppm</A>(1), <A HREF="/cgi-bin/man/man2html?1+picttoppm">picttoppm</A>(1), <A HREF="/cgi-bin/man/man2html?1+pjtoppm">pjtoppm</A>(1), <A HREF="/cgi-bin/man/man2html?1+qrttoppm">qrttoppm</A>(1),
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<A HREF="/cgi-bin/man/man2html?1+rawtoppm">rawtoppm</A>(1), <A HREF="/cgi-bin/man/man2html?1+rgb3toppm">rgb3toppm</A>(1), <A HREF="/cgi-bin/man/man2html?1+sldtoppm">sldtoppm</A>(1), <A HREF="/cgi-bin/man/man2html?1+spctoppm">spctoppm</A>(1), <A HREF="/cgi-bin/man/man2html?1+sputoppm">sputoppm</A>(1), <A HREF="/cgi-bin/man/man2html?1+tgatoppm">tgatoppm</A>(1),
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<A HREF="/cgi-bin/man/man2html?1+ximtoppm">ximtoppm</A>(1), <A HREF="/cgi-bin/man/man2html?1+xpmtoppm">xpmtoppm</A>(1), <A HREF="/cgi-bin/man/man2html?1+yuvtoppm">yuvtoppm</A>(1),
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<A HREF="/cgi-bin/man/man2html?1+ppmtoacad">ppmtoacad</A>(1), <A HREF="/cgi-bin/man/man2html?1+ppmtogif">ppmtogif</A>(1), <A HREF="/cgi-bin/man/man2html?1+ppmtoicr">ppmtoicr</A>(1), <A HREF="/cgi-bin/man/man2html?1+ppmtoilbm">ppmtoilbm</A>(1), <A HREF="/cgi-bin/man/man2html?1+ppmtopcx">ppmtopcx</A>(1), <A HREF="/cgi-bin/man/man2html?1+ppmtopgm">ppmtopgm</A>(1),
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<A HREF="/cgi-bin/man/man2html?1+ppmtopi1">ppmtopi1</A>(1), <A HREF="/cgi-bin/man/man2html?1+ppmtopict">ppmtopict</A>(1), <A HREF="/cgi-bin/man/man2html?1+ppmtopj">ppmtopj</A>(1), <A HREF="/cgi-bin/man/man2html?1+ppmtopuzz">ppmtopuzz</A>(1), <A HREF="/cgi-bin/man/man2html?1+ppmtorgb3">ppmtorgb3</A>(1),
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<A HREF="/cgi-bin/man/man2html?1+ppmtosixel">ppmtosixel</A>(1), <A HREF="/cgi-bin/man/man2html?1+ppmtotga">ppmtotga</A>(1), <A HREF="/cgi-bin/man/man2html?1+ppmtouil">ppmtouil</A>(1), <A HREF="/cgi-bin/man/man2html?1+ppmtoxpm">ppmtoxpm</A>(1), <A HREF="/cgi-bin/man/man2html?1+ppmtoyuv">ppmtoyuv</A>(1),
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<A HREF="/cgi-bin/man/man2html?1+ppmdither">ppmdither</A>(1), <A HREF="/cgi-bin/man/man2html?1+ppmforge">ppmforge</A>(1), <A HREF="/cgi-bin/man/man2html?1+ppmhist">ppmhist</A>(1), <A HREF="/cgi-bin/man/man2html?1+ppmmake">ppmmake</A>(1), <A HREF="/cgi-bin/man/man2html?1+ppmpat">ppmpat</A>(1), <A HREF="/cgi-bin/man/man2html?1+ppmquant">ppmquant</A>(1),
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<A HREF="/cgi-bin/man/man2html?1+ppmquantall">ppmquantall</A>(1), <A HREF="/cgi-bin/man/man2html?1+ppmrelief">ppmrelief</A>(1),
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<A HREF="/cgi-bin/man/man2html?5+pnm">pnm</A>(5), <A HREF="/cgi-bin/man/man2html?5+pgm">pgm</A>(5), <A HREF="/cgi-bin/man/man2html?5+pbm">pbm</A>(5)
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<A NAME="lbAF"> </A>
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<H2>AUTHOR</H2>
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Copyright (C) 1989, 1991 by Jef Poskanzer.
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<P>
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<HR>
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<A NAME="index"> </A><H2>Index</H2>
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<DL>
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<DT id="18"><A HREF="#lbAB">NAME</A><DD>
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<DT id="19"><A HREF="#lbAC">DESCRIPTION</A><DD>
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<DT id="20"><A HREF="#lbAD">COMPATIBILITY</A><DD>
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<DT id="21"><A HREF="#lbAE">SEE ALSO</A><DD>
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<DT id="22"><A HREF="#lbAF">AUTHOR</A><DD>
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</DL>
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<HR>
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This document was created by
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<A HREF="/cgi-bin/man/man2html">man2html</A>,
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using the manual pages.<BR>
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Time: 00:06:04 GMT, March 31, 2021
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</BODY>
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</HTML>
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