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<?xml version="1.0" encoding="UTF-8" standalone="no"?>
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<th colspan="3" align="center" id="chaptername">6.3. Improving Colors</th>
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<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
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<div>
<h3 class="title"><a id="id3313203"></a>6.3. Improving Colors</h3>
</div>
</div>
</div>
<div class="simplesect" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="id3314229"></a>Automated Tools</h4>
</div>
</div>
</div>
<p>
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>
Automatické úpravy úrovní dosáhnete vyvoláním nástroje Úrovně (
<span class="guimenu">Nástroje</span>-><span class="guisubmenu">Nástroje barev</span>-><span class="guimenuitem">Úrovně</span> v nabídce obrázku) a kliknutím na tlačítko
<span class="guibutton">Automaticky</span> v jeho dialogu. V okně obrázku
uvidíte náhled, pokud ho chcete zachovat, stiskněte tlačítko
<span class="guibutton">Budiž</span>. Tlačítkem <span class="guibutton">Zrušit</span>
úpravu zrušíte.
</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
black–really truly perfectly black, not just sort of
dark–and 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
activate it and bring up its dialog.
</p>
</div>
<div class="simplesect" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="id3314794"></a>Exposure Problems</h4>
</div>
</div>
</div>
<p>
The simplest tool to use is the <a href="ch03s05s04.html" title="5.4. Brightness-Contrast tool">Brightness/Contrast</a>
tool. It
is also the least powerful, but in many cases it does
everything you need. This tool is often useful for images that
are overexposed or underexposed; it is not useful for
correcting color casts. The tool gives you two sliders to
adjust, for "Brightness" and "Contrast". If you have the
option "Preview" checked (and almost certainly you should),
you will see any adjustments you make reflected in the
image. When you are happy with the results, press
<span class="guibutton">Okay</span> and they will take effect. If you can't
get results that you are happy with, press
<span class="guibutton">Cancel</span> and the image will revert to its
previous state.
</p>
<p>
A more sophisticated, and only slightly more difficult, way of
correcting exposure problems is to use the Levels tool. The
dialog for this tool looks very complicated, but for the basic
usage we have in mind here, the only part you need to deal
with is the "Input Levels" area, specifically the three
triangular sliders that appear below the histogram. We refer
you to the <a href="ch03s05s06.html" title="5.6. Levels tool">Levels Tool Help</a>
for instructions;
but actually the easiest way to learn how to use it is to
experiment by moving the three sliders around, and watching
how the image is affected. (Make sure that "Preview" is
checked at the bottom of the dialog.)
</p>
<p>
A very powerful way of correcting exposure problems is to use
the Curves tool. This tool allows you to click and drag
control points on a curve, in order to create a function
mapping input brightness levels to output brightness levels.
The Curves tool can replicate any effect you can achieve with
Brightness/Contrast or the Levels tool, so it is more powerful
than either of them. Once again, we refer you to the
<a href="ch03s05s07.html" title="5.7. Curves Tool">Curves Tool Help</a>for
detailed instructions, but the
easiest way to learn how to use it is by experimenting.
</p>
<p>
The most powerful approach to adjusting brightness and
contrast across an image, for more expert Gimp users, is to
create a new layer above the one you are working on, and then
in the Layers dialog set the Mode for the upper layer to
"Multiply". The new layer then serves as a "gain control"
layer for the layer below it, with white yielding maximum gain
and black yielding a gain of zero. Thus, by painting on the
new layer, you can selectively adjust the gain for each area
of the image, giving you very fine control. You should try to
paint only with smooth gradients, because sudden changes in
gain will give rise to spurious edges in the result. Paint
only using shades of gray, not colors, unless you want to
produce color shifts in the image.
</p>
<p>
Actually, "Multiply" is not the only mode that is useful for
gain control. In fact, "Multiply" mode can only darken parts
of an image, never lighten them, so it is only useful where
some parts of an image are overexposed. Using "Divide" mode
has the opposite effect: it can brighten areas of an image
but not darken them. Here is a trick that is often useful for
bringing out the maximum amount of detail across all areas of
an image:
</p>
<div class="procedure">
<ol type="1">
<li>
<p>
Duplicate the layer (producing a new layer above it).
</p>
</li>
<li>
<p>
Desaturate the new layer.
</p>
</li>
<li>
<p>
Apply a Gaussian blur to the result, with a large radius
(100 or more).
</p>
</li>
<li>
<p>
Set Mode in the Layers dialog to Divide.
</p>
</li>
<li>
<p>
Control the amount of correction by adjusting opacity in the
Layers dialog, or by using Brightness/Contrast, Levels, or
Curves tools on the new layer.
</p>
</li>
<li>
<p>
When you are happy with the result, you can use <span class="guibutton">Merge Down</span>
to combine the control layer and the original layer into a
single layer.
</p>
</li>
</ol>
</div>
<p>
In addition to "Multiply" and "Divide", you may every so often
get useful effects with other layer combination modes, such as
"Dodge", "Burn", or "Soft Light". It is all too easy, though,
once you start playing with these things, to look away from
the computer for a moment and suddenly find that you have just
spent an hour twiddling parameters. Be warned: the more
options you have, the harder it is to make a decision.
</p>
</div>
<div class="simplesect" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="id3314491"></a>Adjusting Hue and Saturation</h4>
</div>
</div>
</div>
<p>
In our experience, if your image has a color cast---too much
red, too much blue, etc---the easiest way to correct it is to
use the Levels tool, adjusting levels individually on the red,
green, and blue channels. If this doesn't work for you, it
might be worth your while to try the Color Balance tool or the
Curves tool, but these are much more difficult to use
effectively. (They are very good for creating certain types
of special effects, though.)
</p>
<p>
Sometimes it is hard to tell whether you have adjusted colors
adequately. A good, objective technique is to find a point in
the image that you know should be either white or a shade of
gray. Activate the <a href="ch03s06s04.html" title="6.4. Color Picker Tool">Color Picker</a> tool (the
eyedropper symbol in the Toolbox), and click on the aforesaid
point: this brings up the Color Picker dialog. If the colors
are correctly adjusted, then the red, green, and blue
components of the reported color should all be equal; if not,
then you should see what sort of adjustment you need to make.
This technique, when well used, allows even color-blind people
to color-correct an image.
</p>
<p>
If your image is washed out---which can easily happen when you
take pictures in bright light---try the Hue/Saturation tool,
which gives you three sliders to manipulate, for Hue,
Lightness, and Saturation. Raising the saturation will
probably make the image look better. In same cases it is
useful to adjust the lightness at the same time. ("Lightness"
here is similar to "Brightness" in the Brightness/Contrast
tool, except that they are formed from different combinations
of the red, green, and blue channels.) The Hue/Saturation
tool gives you the option of adjusting restricted subranges of
colors (using the buttons at the top of the dialog), but if
you want to get natural-looking colors, in most cases you
should avoid doing this.
</p>
<div class="tip" style="margin-left: 0.5in; margin-right: 0.5in;">
<table border="0" summary="Tip">
<tr>
<td rowspan="2" align="center" valign="top" width="25">
<img alt="[Tip]" src="../images/tip.png" />
</td>
<th align="left">Tip</th>
</tr>
<tr>
<td colspan="2" align="left" valign="top">
<p>
Even if an image does not seemed washed out, often you can
increase its impact by pushing up the saturation a bit.
Veterans of the film era sometimes call this trick
"Fujifying", after Fujichrome film, which is notorious for
producing highly saturated prints.
</p>
</td>
</tr>
</table>
</div>
<p>
When you take pictures in low light conditions, in some cases
you have the opposite problem: too much saturation. In this
case too the Hue/Saturation tool is a good one to use, only by
reducing the saturation instead of increasing it.
</p>
</div>
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