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.

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 Layer->Colors->Auto in the image menu.

Here they are, with a few words about each:

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 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 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 previous state.

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 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.)

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 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:

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.

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. Activate the Color Picker 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 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.

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.