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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 3.3//EN">
<html><head><title>Pamlevels User Manual</title>
</head><body>
<h1>pamlevels</h1>
Updated: 13 June 2018
<br>
<a href="#index">Table Of Contents</a>
<h2>NAME</h2>
pamlevels - effect a "levels" transformation
<h2 id="synopsis">SYNOPSIS</h2>
<b>pamlevels</b>
<b>-from1</b> <i>from1</i>
<b>-to1</b> <i>to1</i>
<b>-from2</b> <i>from2</i>
<b>-to2</b> <i>to2</i>
[<b>-from3</b> <i>from3</i>
<b>-to3</b> <i>to3</i>]
[<b>-linear</b>]
[<b>-fitbrightness</b>]
[<i>pamfile</i>]
<h2 id="description">DESCRIPTION</h2>
<p>This program is part of <a href="index.html">Netpbm</a>.
<p><b>pamlevels</b> reads a Netpbm image from <i>pamfile</i> or standard
input, applies a "levels" transformation, and sends the result to
standard output. It finds a unique transformation that maps color
<i>from1</i> to <i>to1</i>, color <i>from2</i> to <i>to2</i>,
and—if <b>-from3</b> and <b>-to3</b> are supplied—color
<i>from3</i> to <i>to3</i>. Transformations based on two mappings are linear
in light intensity and those based on three mappings quadratic. The
color-mapping options may be shortened to <b>-f1</b>...<b>-f3</b> and
<b>-t1</b>...<b>-t3</b>.
<p>Color values have the following format:
<i>color</i>[:<i>scale</i>], where <i>color</i> is a
<a href="libppm.html#colorname"> Netpbm color specification</a> and
<i>scale</i> an optional coefficient that is applied to the intensity
(i.e. <em>not</em> gamma-adjusted) of each RGB component of <i>color</i>.
<p>When the transformation is linear (i.e. uses two mappings) and preserves
zero (i.e. maps black to black), it corresponds to multiplication of light
intensity by a constant and preserves
<a href="http://www.c-f-systems.com/ColorIntegrity.html">color integrity</a>.
<h2 id="examples">EXAMPLES</h2>
In the examples below, <code>\</code> denotes a line continuation:
<p>To brighten an image by setting a darker white point—
<pre>
pamlevels -f1 black -t1 black \
-f2 white:0.9 -t2 white in.ppm > out.ppm
</pre>
<p>To adjust the white point—
<pre>
pamlevels -f1 black -t1 black \
-f2 rgbi:0.9/0.83/0.80 -t2 white in.ppm > out.ppm
</pre>
<p>To set a lighter black point—
<pre>
pamlevels -f1 white:0.06 -t1 black \
-f2 white -t2 white in.ppm > out.ppm
</pre>
<p>To increase brightness by compression—
<pre>
pamlevels \
-f1 black -t1 black -f2 white -t2 white \
-f3 white:0.5 -t3 white:0.6 in.ppm > out.ppm
</pre>
The latter transformation is similar to gamma-correction.
<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>pamlevels</b> recognizes the following
command line options:
<dl>
<dt><b>-from1</b>
<dt><b>-to1</b>
<dt><b>-from2</b>
<dt><b>-to2</b>
<dt><b>-from3</b>
<dt><b>-to3</b>
<dd>
These options define the mappings of input colors to output colors that anchor
the transformation function. See <a href="#description">DESCRIPTION</a>.
<p>
You must specify at least two of these pairs.
<dt><b>-linear</b>
<dd>This option tells <b>pamlevels</b> to work with the intensity-linear
variation on PPM where the samples are proportional to light intensity, rather
than brightness (gamma-adjusted) as in true PPM. The input must be of this
form and the <b>pamlevels</b> makes the output of this form.
<p>Note that the numbers in a color specification like
<b>rgbi:0.9/0.83/0.80</b> are brightness levels (gamma-adjusted) regardless of
the input and output format.
<p>You cannot use this with <b>-fitbrightness</b> because that function is
not implemented.
<dt><b>-fitbrightness</b>
<dd>This option selects a transformation which is not very useful - it is
linear or quadratic in brightness rather than light intensity of the pixels.
There is no physical basis for doing it this way and the result is normally
undesirable.
<p>Note that many tools other than Netpbm do the transformation this way. One
use for this option is simply to demonstrate the poor result of this method.
<p>One advantage of this transformation is that it is faster, because the
input and output image formats use brightness values. The result is
approximately correct.
<p>You cannot use this with <b>-linear</b> because that function is not
implemented.
</dl>
<h2 id="seealso">SEE ALSO</h2>
<b><a href="pnmnorm.html">pnmnorm</a></b>,
<b><a href="pamrecolor.html">pamrecolor</a></b>,
<b><a href="pnmgamma.html">pnmgamma</a></b>,
<b><a href="pam.html">pnm</a></b>
<h2 id="author">AUTHOR</h2>
<p>This program was first submitted by Anton Shepelev
(<a href="mailto:anton.txt@gmail.com">anton.txt@gmail.com</a>).
<h2 id="history">HISTORY</h2>
<p><b>pamlevels</b> was new in Netpbm 10.83 (June 2018).
<hr>
<h2 id="index">Table Of Contents</h2>
<ul>
<li><a href="#synopsis">SYNOPSIS</a>
<li><a href="#description">DESCRIPTION</a>
<li><a href="#examples">EXAMPLES</a>
<li><a href="#options">OPTIONS</a>
<li><a href="#seealso">SEE ALSO</a>
<li><a href="#author">AUTHOR</a>
<li><a href="#history">HISTORY</a>
</ul>
</body>
</html>
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