202 lines
7.8 KiB
HTML
202 lines
7.8 KiB
HTML
|
|
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
|
|
<HTML><HEAD><TITLE>Man page of pgm</TITLE>
|
|
</HEAD><BODY>
|
|
<H1>pgm</H1>
|
|
Section: File Formats (5)<BR>Updated: 12 November 1991<BR><A HREF="#index">Index</A>
|
|
<A HREF="/cgi-bin/man/man2html">Return to Main Contents</A><HR>
|
|
|
|
<A NAME="lbAB"> </A>
|
|
<H2>NAME</H2>
|
|
|
|
pgm - portable graymap file format
|
|
<P>
|
|
<A NAME="lbAC"> </A>
|
|
<H2>DESCRIPTION</H2>
|
|
|
|
The PGM format is a lowest common denominator grayscale file format.
|
|
<A NAME="ixAAB"></A>
|
|
It is designed to be extremely easy to learn and write programs for.
|
|
(It's so simple that most people will simply reverse engineer it
|
|
because it's easier than reading this specification).
|
|
<P>
|
|
A PGM image represents a grayscale graphic image. There are many
|
|
psueudo-PGM formats in use where everything is as specified herein except
|
|
for the meaning of individual pixel values. For most purposes, a PGM
|
|
image can just be thought of an array of arbitrary integers, and all the
|
|
programs in the world that think they're processing a grayscale image
|
|
can easily be tricked into processing something else.
|
|
<P>
|
|
One official variant of PGM is the transparency mask. A transparency
|
|
mask in Netpbm is represented by a PGM image, except that in place of
|
|
pixel intensities, there are opaqueness values. See below.
|
|
<P>
|
|
<P>
|
|
|
|
The format definition is as follows.
|
|
<P>
|
|
|
|
A PGM file consists of a sequence of one or more PGM images. There are
|
|
no data, delimiters, or padding before, after, or between images.
|
|
<P>
|
|
|
|
Each PGM image consists of the following:
|
|
<DL COMPACT>
|
|
<DT id="1">-<DD>
|
|
A "magic number" for identifying the file type.
|
|
A pgm image's magic number is the two characters "P5".
|
|
<A NAME="ixAAC"></A>
|
|
<DT id="2">-<DD>
|
|
Whitespace (blanks, TABs, CRs, LFs).
|
|
<DT id="3">-<DD>
|
|
A width, formatted as ASCII characters in decimal.
|
|
<DT id="4">-<DD>
|
|
Whitespace.
|
|
<DT id="5">-<DD>
|
|
A height, again in ASCII decimal.
|
|
<DT id="6">-<DD>
|
|
Whitespace.
|
|
<DT id="7">-<DD>
|
|
The maximum gray value (Maxval), again in ASCII decimal. Must be less
|
|
than 65536.
|
|
<DT id="8">-<DD>
|
|
Newline or other single whitespace character.
|
|
<DT id="9">-<DD>
|
|
A raster of Width * Height gray values, proceeding through the image
|
|
in normal English reading order. Each gray value is a number from 0
|
|
through Maxval, with 0 being black and Maxval being white. Each gray
|
|
value is represented in pure binary by either 1 or 2 bytes. If the
|
|
Maxval is less than 256, it is 1 byte. Otherwise, it is 2 bytes. The
|
|
most significant byte is first.
|
|
<DT id="10">-<DD>
|
|
Each gray value is a number proportional to the intensity of the
|
|
pixel, adjusted by the CIE Rec. 709 gamma transfer function. (That
|
|
transfer function specifies a gamma number of 2.2 and has a linear
|
|
section for small intensities). A value of zero is therefore black.
|
|
A value of Maxval represents CIE D65 white and the most intense value
|
|
in the image and any other image to which the image might be compared.
|
|
<DT id="11">-<DD>
|
|
Note that a common variation on the PGM format is to have the gray
|
|
value be "linear," i.e. as specified above except without the gamma
|
|
adjustment.
|
|
<B>pnmgamma</B>
|
|
|
|
takes such a PGM variant as input and produces a true PGM as output.
|
|
<DT id="12">-<DD>
|
|
In the transparency mask variation on PGM, the value represents
|
|
opaqueness. It is proportional to the fraction of intensity of a
|
|
pixel that would show in place of an underlying pixel, with the same
|
|
gamma transfer function mentioned above applied. So what normally
|
|
means white represents total opaqueness and what normally means black
|
|
represents total transparency. In between, you would compute the intensity
|
|
of a composite pixel of an "under" and "over" pixel as
|
|
under * (1-(alpha/alpha_maxval)) + over * (alpha/alpha_maxval).<
|
|
<DT id="13">-<DD>
|
|
Characters from a "#" to the next end-of-line, before the maxval line,
|
|
are comments and are ignored.
|
|
</DL>
|
|
<P>
|
|
|
|
Note that you can use
|
|
<B>pnmdepth</B>
|
|
|
|
To convert between a the format with 1 byte per gray value and the one
|
|
with 2 bytes per gray value.
|
|
<P>
|
|
|
|
There is actually another version of the PGM format that is fairly rare:
|
|
"plain" PGM format. The format above, which generally considered the
|
|
normal one, is known as the "raw" PGM format. See
|
|
<B><A HREF="/cgi-bin/man/man2html?5+pbm">pbm</A></B>(5)
|
|
|
|
for some commentary on how plain and raw formats relate to one another.
|
|
<P>
|
|
|
|
The difference in the plain format is:
|
|
<DL COMPACT>
|
|
<DT id="14">-<DD>
|
|
There is exactly one image in a file.
|
|
<DT id="15">-<DD>
|
|
The magic number is P2 instead of P5.
|
|
<DT id="16">-<DD>
|
|
Each pixel in the raster is represented as an ASCII decimal number
|
|
(of arbitrary size).
|
|
<DT id="17">-<DD>
|
|
Each pixel in the raster has white space before and after it. There must
|
|
be at least one character of white space between any two pixels, but there
|
|
is no maximum.
|
|
<DT id="18">-<DD>
|
|
No line should be longer than 70 characters.
|
|
</DL>
|
|
<P>
|
|
|
|
Here is an example of a small graymap in this format:
|
|
<PRE>
|
|
P2
|
|
# feep.pgm
|
|
24 7
|
|
15
|
|
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
|
|
0 3 3 3 3 0 0 7 7 7 7 0 0 11 11 11 11 0 0 15 15 15 15 0
|
|
0 3 0 0 0 0 0 7 0 0 0 0 0 11 0 0 0 0 0 15 0 0 15 0
|
|
0 3 3 3 0 0 0 7 7 7 0 0 0 11 11 11 0 0 0 15 15 15 15 0
|
|
0 3 0 0 0 0 0 7 0 0 0 0 0 11 0 0 0 0 0 15 0 0 0 0
|
|
0 3 0 0 0 0 0 7 7 7 7 0 0 11 11 11 11 0 0 15 0 0 0 0
|
|
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
|
|
</PRE>
|
|
|
|
<P>
|
|
|
|
Programs that read this format should be as lenient as possible,
|
|
accepting anything that looks remotely like a graymap.
|
|
<P>
|
|
<A NAME="lbAD"> </A>
|
|
<H2>COMPATIBILITY</H2>
|
|
|
|
<P>
|
|
Before April 2000, a raw format PGM file could not have a maxval greater
|
|
than 255. Hence, it could not have more than one byte per sample. Old
|
|
programs may depend on this.
|
|
<P>
|
|
Before July 2000, there could be at most one image in a PGM file. As
|
|
a result, most tools to process PGM files ignore (and don't read) any
|
|
data after the first image.
|
|
<P>
|
|
<A NAME="lbAE"> </A>
|
|
<H2>SEE ALSO</H2>
|
|
|
|
<A HREF="/cgi-bin/man/man2html?1+fitstopgm">fitstopgm</A>(1), <A HREF="/cgi-bin/man/man2html?1+fstopgm">fstopgm</A>(1), <A HREF="/cgi-bin/man/man2html?1+hipstopgm">hipstopgm</A>(1), <A HREF="/cgi-bin/man/man2html?1+lispmtopgm">lispmtopgm</A>(1), <A HREF="/cgi-bin/man/man2html?1+psidtopgm">psidtopgm</A>(1),
|
|
<A HREF="/cgi-bin/man/man2html?1+rawtopgm">rawtopgm</A>(1),
|
|
<A HREF="/cgi-bin/man/man2html?1+pgmbentley">pgmbentley</A>(1), <A HREF="/cgi-bin/man/man2html?1+pgmcrater">pgmcrater</A>(1), <A HREF="/cgi-bin/man/man2html?1+pgmedge">pgmedge</A>(1), <A HREF="/cgi-bin/man/man2html?1+pgmenhance">pgmenhance</A>(1), <A HREF="/cgi-bin/man/man2html?1+pgmhist">pgmhist</A>(1), <A HREF="/cgi-bin/man/man2html?1+pgmnorm">pgmnorm</A>(1),
|
|
<A HREF="/cgi-bin/man/man2html?1+pgmoil">pgmoil</A>(1), <A HREF="/cgi-bin/man/man2html?1+pgmramp">pgmramp</A>(1), <A HREF="/cgi-bin/man/man2html?1+pgmtexture">pgmtexture</A>(1),
|
|
<A HREF="/cgi-bin/man/man2html?1+pgmtofits">pgmtofits</A>(1), <A HREF="/cgi-bin/man/man2html?1+pgmtofs">pgmtofs</A>(1), <A HREF="/cgi-bin/man/man2html?1+pgmtolispm">pgmtolispm</A>(1), <A HREF="/cgi-bin/man/man2html?1+pgmtopbm">pgmtopbm</A>(1),
|
|
<A HREF="/cgi-bin/man/man2html?5+pnm">pnm</A>(5), <A HREF="/cgi-bin/man/man2html?5+pbm">pbm</A>(5), <A HREF="/cgi-bin/man/man2html?5+ppm">ppm</A>(5)
|
|
<A NAME="lbAF"> </A>
|
|
<H2>AUTHOR</H2>
|
|
|
|
Copyright (C) 1989, 1991 by Jef Poskanzer.
|
|
|
|
|
|
|
|
|
|
|
|
|
|
<P>
|
|
|
|
<HR>
|
|
<A NAME="index"> </A><H2>Index</H2>
|
|
<DL>
|
|
<DT id="19"><A HREF="#lbAB">NAME</A><DD>
|
|
<DT id="20"><A HREF="#lbAC">DESCRIPTION</A><DD>
|
|
<DT id="21"><A HREF="#lbAD">COMPATIBILITY</A><DD>
|
|
<DT id="22"><A HREF="#lbAE">SEE ALSO</A><DD>
|
|
<DT id="23"><A HREF="#lbAF">AUTHOR</A><DD>
|
|
</DL>
|
|
<HR>
|
|
This document was created by
|
|
<A HREF="/cgi-bin/man/man2html">man2html</A>,
|
|
using the manual pages.<BR>
|
|
Time: 00:06:04 GMT, March 31, 2021
|
|
</BODY>
|
|
</HTML>
|