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diff --git a/pgm.html b/pgm.html index f02a67a9..c0fb4d8d 100644 --- a/pgm.html +++ b/pgm.html @@ -1,80 +1,80 @@ -<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 3.2//EN"> -<HTML> -<HEAD> -<TITLE>PGM Format Specification</TITLE> -<META NAME="manual_section" CONTENT="5"> -</HEAD> -<BODY> -<H1>pgm</H1> +<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 3.3//EN"> +<html> +<head> +<title>PGM Format Specification</title> +<meta name="manual_section" content="5"> +</head> +<body> +<h1>pgm</h1> Updated: 09 October 2016 -<BR> -<A HREF="#index">Table Of Contents</A> +<br> +<a href="#index">Table Of Contents</a> -<H2>NAME</H2> +<h2>NAME</h2> pgm - Netpbm grayscale image format -<H2 id="description">DESCRIPTION</H2> +<h2 id="description">DESCRIPTION</h2> <p>This program is part of <a href="index.html">Netpbm</a>. -<P>The PGM format is a lowest common denominator grayscale file format. +<p>The PGM format is a lowest common denominator grayscale file format. 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 +<p>A PGM image represents a grayscale graphic image. There are many pseudo-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>The name "PGM" is an acronym derived from "Portable Gray Map." +<p>The name "PGM" is an acronym derived from "Portable Gray Map." -<P>One official variant of PGM is the transparency mask. A transparency +<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. <h2 id="format">THE FORMAT</h2> -<P>The format definition is as follows. You can use the <a +<p>The format definition is as follows. You can use the <a href="libnetpbm.html">libnetpbm</a> C subroutine library to conveniently and accurately read and interpret the format. -<P>A PGM file consists of a sequence of one or more PGM images. There are +<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: +<p>Each PGM image consists of the following: -<OL> +<ol> -<LI>A "magic number" for identifying the file type. +<li>A "magic number" for identifying the file type. A pgm image's magic number is the two characters "P5". -<LI>Whitespace (blanks, TABs, CRs, LFs). +<li>Whitespace (blanks, TABs, CRs, LFs). -<LI>A width, formatted as ASCII characters in decimal. +<li>A width, formatted as ASCII characters in decimal. -<LI>Whitespace. +<li>Whitespace. -<LI>A height, again in ASCII decimal. +<li>A height, again in ASCII decimal. -<LI>Whitespace. +<li>Whitespace. -<LI>The maximum gray value (Maxval), again in ASCII decimal. Must be less +<li>The maximum gray value (Maxval), again in ASCII decimal. Must be less than 65536, and more than zero. -<LI>A single whitespace character (usually a newline). +<li>A single whitespace character (usually a newline). -<LI>A raster of Height rows, in order from top to bottom. Each row +<li>A raster of Height rows, in order from top to bottom. Each row consists of Width gray values, in order from left to right. 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. -<P>A row of an image is horizontal. A column is vertical. The pixels +<p>A row of an image is horizontal. A column is vertical. The pixels in the image are square and contiguous. <p>Each gray value is a number proportional to the intensity of the @@ -89,10 +89,10 @@ might be compared. <p>Note that a common variation from 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 +without the gamma adjustment. <b>pnmgamma</b> takes such a PGM variant as input and produces a true PGM as output. -<P>Another popular variation from PGM is to substitute the newer sRGB transfer +<p>Another popular variation from PGM is to substitute the newer sRGB transfer function for the BT.709 one. You can use <b>pnmgamma</b> to convert between this variation and true PGM. @@ -106,12 +106,12 @@ compute the intensity of a composite pixel of an "under" and (alpha/alpha_maxval). Note that there is no gamma transfer function in the transparency mask. -</OL> +</ol> <p>Strings starting with "#" may be comments, the same as with <a href="pbm.html">PBM</a>. -<P>Note that you can use <B>pamdepth</B> to convert between a the +<p>Note that you can use <b>pamdepth</b> to convert between a the format with 1 byte per gray value and the one with 2 bytes per gray value. @@ -123,13 +123,13 @@ calls white space). <h3 id="plainpgm">Plain PGM</h3> -<P>There is actually another version of the PGM format that is fairly +<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="pbm.html">pbm</A></B> for some commentary on how plain +See <b><a href="pbm.html">pbm</a></b> for some commentary on how plain and raw formats relate to one another and how to use them. -<P>The difference in the plain format is: +<p>The difference in the plain format is: <ul> <li> @@ -149,7 +149,7 @@ No line should be longer than 70 characters. <p>Here is an example of a small image in the plain PGM format. -<PRE> +<pre> P2 # feep.pgm 24 7 @@ -161,9 +161,9 @@ P2 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> +</pre> -<P>There is a newline character at the end of each of these lines. +<p>There is a newline character at the end of each of these lines. <p>Programs that read this format should be as lenient as possible, accepting anything that looks remotely like a PGM. @@ -187,43 +187,43 @@ cases where distinguishing between the particular subformats of PNM is not convenient. -<H2 id="compatibility">COMPATIBILITY</H2> +<h2 id="compatibility">COMPATIBILITY</h2> -<P> +<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> +<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. -<H2 id="seealso">SEE ALSO</H2> +<h2 id="seealso">SEE ALSO</h2> -<A HREF="pnm.html">pnm</A>, -<A HREF="pbm.html">pbm</A>, -<A HREF="ppm.html">ppm</A>, -<A HREF="pam.html">pam</A>, -<A HREF="libnetpbm.html">libnetpbm</A>, -<A HREF="directory.html">programs that process PGM</A>, +<a href="pnm.html">pnm</a>, +<a href="pbm.html">pbm</a>, +<a href="ppm.html">ppm</a>, +<a href="pam.html">pam</a>, +<a href="libnetpbm.html">libnetpbm</a>, +<a href="directory.html">programs that process PGM</a>, -<H2 id="author">AUTHOR</H2> +<h2 id="author">AUTHOR</h2> Copyright (C) 1989, 1991 by Jef Poskanzer. -<HR> -<H2 id="index">Table Of Contents</H2> -<UL> -<LI><A HREF="#description">DESCRIPTION</A> -<LI><A HREF="#description">THE FORMAT</A> +<hr> +<h2 id="index">Table Of Contents</h2> <ul> - <li><A HREF="#plainpgm">Plain PGM</A> +<li><a href="#description">DESCRIPTION</a> +<li><a href="#format">THE FORMAT</a> +<ul> + <li><a href="#plainpgm">Plain PGM</a> </ul> -<li><A HREF="#internetmediatype">INTERNET MEDIA TYPE</A> -<li><A HREF="#filename">FILE NAME</A> -<LI><A HREF="#compatibility">COMPATIBILITY</A> -<LI><A HREF="#seealso">SEE ALSO</A> -<LI><A HREF="#author">AUTHOR</A> -</UL> -</BODY> -</HTML> +<li><a href="#internetmediatype">INTERNET MEDIA TYPE</a> +<li><a href="#filename">FILE NAME</a> +<li><a href="#compatibility">COMPATIBILITY</a> +<li><a href="#seealso">SEE ALSO</a> +<li><a href="#author">AUTHOR</a> +</ul> +</body> +</html> |