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
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
in the image menu.
Here they are, with a few words about each:
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.
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.
Help me, what exactly does this do? Obviously it makes some
things more saturated.
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
This is done by activating the Levels tool (
in the image menu), clicking on the image to bring up the tool
dialog, and then pressing the Auto
button near the center of the dialog. You will see a preview of
the result; you must press Okay
for it to take effect. Pressing Cancel
instead will cause your image to revert to its previous state.
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 Okay button otherwise
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.
The simplest tool to use is the Brightness/Contrast
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
Okay and they will take effect. If you can't
get results that you are happy with, press
Cancel and the image will revert to its
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 Levels Tool Help
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.)
A very powerful way of correcting exposure problems is to use the
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
Curves Tool Help
for detailed instructions, but the easiest way to learn how to use it
is by experimenting.
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.
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:
Duplicate the layer (producing a new layer above it).
Desaturate the new layer.
Apply a Gaussian blur to the result, with a large radius (100 or
Set Mode in the Layers dialog to Divide.
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.
When you are happy with the result, you can use
to combine the control layer and the original layer into a single
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.
Adjusting Hue and Saturation
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.)
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.
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.
If your image is washed out---which can easily happen when you take
pictures in bright light---try the
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
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.
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
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.