In spite of sophisticated exposure-control systems, pictures
taken with digital cameras often come out over- or
under-exposed, or with color casts due to imperfections in
lighting. Gimp gives you a variety of tools to correct colors
in an image, ranging to automated tools that run with a simple
button-click to highly sophisticated tools that give you many
parameters of control. We will start with the simplest first.
</p>
<p>
Gimp gives you five automated color correction tools.
Unfortunately they don't usually give you quite the results you
are looking for, but they only take a moment to try out, and if
nothing else they often give you an idea of some of the
possibilities inherent in the image. Except for "Auto Levels",
you can find them in the Layer menu, by following the menu path
<span class="guimenu">Layer</span>-><span class="guisubmenu">Colors</span>-><span class="guisubmenu">Auto</span> in the image menu.
</p>
<p>
Here they are, with a few words about each:
</p>
<div class="variablelist">
<dl>
<dt>
<span class="term">
<span class="guimenuitem">Normalize</span>
</span>
</dt>
<dd>
<p>
This tool (it is really a plug-in) is useful for
underexposed images: it adjusts the whole image uniformly
until the brightest point is right at the saturation limit,
and the darkest point is black. The downside is that the
amount of brightening is determined entirely by the lightest
and darkest points in the image, so even one single white
pixel and/or one single black pixel will make normalization
ineffective.
</p>
</dd>
<dt>
<span class="term">
<span class="guimenuitem">Equalize</span>
</span>
</dt>
<dd>
<p>
This is a very powerful adjustment that tries to spread the
colors in the image evenly across the range of possible
intensities. In some cases the effect is amazing, bringing
out contrasts that are very difficult to get in any other
way; but more commonly, it just makes the image look weird.
Oh well, it only takes a moment to try.
</p>
</dd>
<dt>
<span class="term">
<span class="guimenuitem">Color Enhance</span>
</span>
</dt>
<dd>
<p>
Help me, what exactly does this do? Obviously it makes some
things more saturated.
</p>
</dd>
<dt>
<span class="term">
<span class="guimenuitem">Stretch Contrast</span>
</span>
</dt>
<dd>
<p>
This is like "Normalize", except that it operates on the
red, green, and blue channels independently. It often has
the useful effect of reducing color casts.
</p>
</dd>
<dt>
<span class="term">
<span class="guimenuitem">Auto Levels</span>
</span>
</dt>
<dd>
<p>
This is done by activating the Levels tool (
<span class="guimenu">Tools</span>-><span class="guisubmenu">Color Tools</span>-><span class="guimenuitem">Levels</span> in the image menu), clicking on the image to
bring up the tool dialog, and then pressing the <span class="guibutton">Auto</span>
button near the center of the dialog. You will see a
preview of the result; you must press
<span class="guibutton">Okay</span>
for it to take
effect. Pressing
<span class="guibutton">Cancel</span> instead will cause your image to
revert to its previous state.
</p>
<p>
If you can find a point in the image that ought to be
perfect white, and a second point that ought to be perfect
black, then you can use the Levels tool to do a
semi-automatic adjustment that will often do a good job of
fixing both brightness and colors throughout the
image. First, bring up the Levels tool as previously
described. Now, look down near the bottom of the Layers
dialog for three buttons with symbols on them that look like
eye-droppers (at least, that is what they are supposed to
look like). The one on the left, if you mouse over it,
shows its function to be
"Pick Black Point". Click on this,
then click on a point in the image that ought to be
blackΓÇôreally truly perfectly black, not just sort of
darkΓÇôand watch the image change. Next, click on the
rightmost of the three buttons ("Pick White Point"), and
then click a point in the image that ought to be white, and
once more watch the image change. If you are happy with the
result, click the <span class="guibutton">Okay</span> button otherwise
<span class="guibutton">Cancel</span>.
</p>
</dd>
</dl>
</div>
<p>
Those are the automated color adjustments: if you find
that none of them quite does the job for you, it is time to try
one of the interactive color tools. All of these, except one,
can be accessed via Tools->Color Tools in the image menu. After
you select a color tool, click on the image (anywhere) to
