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<h2 class="title" style="clear: both"><a id="gimp-histogram-dialog"></a>6. Histogram dialog</h2>
</div>
</div>
</div>
<a id="id3337461" class="indexterm"></a>
<div class="informalfigure">
<div class="mediaobject">
<img src="../images/dialogs/dialogs-histogram-basic.png" />
<div class="caption">
<p>
The Histogram dialog
</p>
</div>
</div>
</div>
<p>
The Histogram dialog shows you information about the statistical
distribution of color values in the image that is currently
active. This information is often useful when you are trying to
<span class="emphasis"><em>color balance</em></span> an image. However, the
Histogram dialog is purely informational: nothing you do with it
will cause any change to the image. If you want to perform a
histogram-based color correction, use the <a href="ch03s05s06.html" title="5.6. Levels tool">Levels</a> tool.
</p>
<p>
This is a dockable dialog; see the section on <a href="ch02s03s04.html" title="3.4. Dialogs and Docking">Dialogs and Docking</a> for help on
manipulating it. It can be activated in two ways:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
From the Toolbox menu:
<span class="guimenu">File</span>-><span class="guisubmenu">Dialogs</span>-><span class="guimenuitem">Histogram</span>
</p>
</li>
<li>
<p>
From the image menu:
<span class="guimenu">Dialogs</span>-><span class="guimenuitem">Histogram</span>
</p>
</li>
</ul>
</div>
<div class="simplesect" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="id3337799"></a>About Histograms</h3>
</div>
</div>
</div>
<p>
In GIMP, each layer of an image can be decomposed into one or
more color channels: for an RGB image, into R, G, and B
channels; for a grayscale image, into a single Value channel.
Layers that support transparency have an additional channel, the
alpha channel. Each channel supports a range of intensity
levels from 0 to 255 (integer valued). Thus, a black pixel is
encoded by 0 on all color channels; a white pixel by 255 on all
color channels. A transparent pixel is encoded by 0 on the
alpha channel; an opaque pixel by 255.
</p>
<p>
For RGB images, it is convenient to define a Value "pseudochannel".
This is not a real color channel: it does not reflect
any information stored directly in the image. Instead, the
Value at a pixel is given by the equation
<tt class="code">
V = (R + G + B)/3
</tt>,
rounded to the nearest integer. Essentially, the Value is what
you would get at that pixel if you converted the image to
Grayscale mode.
</p>
<p>
For more information on channels, please consult the <a href="ch02s04.html" title="4. Working with Images">Working with Images</a> section.
</p>
<div class="variablelist">
<p class="title">
<b>Using the Histogram dialog</b>
</p>
<dl>
<dt>
<span class="term">Channel</span>
</dt>
<dd>
<div class="informalfigure">
<div class="mediaobject">
<img src="../images/dialogs/dialogs-histogram-menu.png" />
<div class="caption">
<p>
Channel options for an RGB layer with alpha channel.
</p>
</div>
</div>
</div>
<p>
This allows you to select which channel to use. The
possibilities depend on the layer type of the active
layer. Here are the entries you might see, and what they
mean:
</p>
<div class="variablelist">
<dl>
<dt>
<span class="term">Value</span>
</dt>
<dd>
<p>
For RGB and Grayscale images, this shows the
distribution of brightness values
across the layer. For a grayscale image, these are
read directly from the image data. For an RGB
image, they are taken from the Value pseudochannel.
</p>
<p>
For an indexed image, the "Value" channel actually
shows the distribution of frequencies for each
colormap index: thus, it is a "pseudocolor"
histogram rather than a true color histogram.
</p>
</dd>
<dt>
<span class="term">Red, Green, Blue</span>
</dt>
<dd>
<p>
These only appear for layers from RGB images. They
show the distribution of intensity levels for the
Red, Green, or Blue channels respectively.
</p>
</dd>
<dt>
<span class="term">Alpha</span>
</dt>
<dd>
<p>
This shows the distribution of opacity levels. If
the layer is completely opaque or completely
transparent, the histogram will consist of a single
bar on the left or right edge.
</p>
</dd>
<dt>
<span class="term">RGB</span>
</dt>
<dd>
<div class="informalfigure">
<div class="mediaobject">
<img src="../images/dialogs/dialogs-histogram-rgb.png" />
<div class="caption">
<p>
Combined histograms of R, G, and B channels.
</p>
</div>
</div>
</div>
<p>
This entry, only available for RGB layers, shows the
R, G, and B histograms superimposed, so that you can
see all of the color distribution information in a
single view.
</p>
</dd>
</dl>
</div>
</dd>
<dt>
<span class="term">Linear/Logarithmic buttons</span>
</dt>
<dd>
<div class="informalfigure">
<div class="mediaobject">
<img src="../images/dialogs/dialogs-histogram-log.png" />
<div class="caption">
<p>
The histogram shown at the top, changed to logarithmic
mode.
</p>
</div>
</div>
</div>
<p>
These buttons determine whether the histogram will be
displayed using a linear or logarithmic Y axis. For
images taken from photographs, the linear mode is most
commonly useful. For images that contain substantial
areas of constant color, though, a linear histogram will
often be dominated by a single bar, and a logarthmic
histogram will often be more useful.
</p>
</dd>
<dt>
<span class="term">Range Setting</span>
</dt>
<dd>
<div class="informalfigure">
<div class="mediaobject">
<img src="../images/dialogs/dialogs-histogram-range.png" />
<div class="caption">
<p>
Dialog aspect after range fixing.
</p>
</div>
</div>
</div>
<p>
You can restrict the analysis, for the statistics shown at
the bottom of the dialog, to a limited range of values if
you wish. You can set the range in one of three ways:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
Click and drag the pointer across the histogram
display area, from the lowest level to the highest
level of the range you want.
</p>
</li>
<li>
<p>
Click and drag the black or white triangles on the
slider below the histogram.
</p>
</li>
<li>
<p>
Use the spinbutton entries below the slider (left
entry: bottom of range; right entry: top of range).
</p>
</li>
</ul>
</div>
</dd>
<dt>
<span class="term">Statistics</span>
</dt>
<dd>
<p>
At the bottom of the dialog are shown some basic
statistics describing the distribution of channel values,
restricted to the selected range. These are the mean,
standard deviation, and median of the selected histogram
portion; the number of pixels in the image; the number
whose values fall within the selected range; and the
percentage whose values fall within the selected range.
</p>
</dd>
</dl>
</div>
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