It is easy, when taking a picture, to hold the camera not quite perfectly vertical, resulting in a picture where things are tilted at an angle. In GIMP, the way to fix this is to use the Rotate tool. Activate this by clicking its icon in the Toolbox, or by pressing the Shift+R while inside the image. Make sure the Tool Options are visible, and at the top, make sure for “Transform:” that the left button (“Transform Layer”) is selected. If you then click the mouse inside the image and drag it, you will see a grid appear that rotates as you drag. When the grid looks right, click Rotate or press Enter, and the image will be rotated.

Now as a matter of fact, it isn't so easy to get things right by this method: you often find that things are better but not quite perfect. One solution is to rotate a bit more, but there is a disadvantage to that approach. Each time you rotate an image, because the rotated pixels don't line up precisely with the original pixels, the image inevitably gets blurred a little bit. For a single rotation, the amount of blurring is quite small, but two rotations cause twice as much blurring as one, and there is no reason to blur things more than you have to. A better alternative is to undo the rotation and then do another, adjusting the angle.

Fortunately, GIMP provides another way of doing it that is considerably easier to use: in the Rotate Tool Options, for the Transform Direction you can select "Backward (Corrective)". When you do this, instead of rotating the grid to compensate for the error, you can rotate it to line up with the error. If this seems confusing, try it and you will see that it is quite straightforward.

After you have rotated an image, there will be unpleasant triangular "holes" at the corners. One way to fix them is to create a background that fills the holes with some unobtrusive or neutral color, but usually a better solution is to crop the image. The greater the rotation, the more cropping is required, so it is best to get the camera aligned as well as possible when you take the picture in the first place.

When you take a picture with a digital camera, you have some control over what gets included in the image but often not as much as you would like: the result is images that could benefit from trimming. Beyond this, it is often possible to enhance the impact of an image by trimming it so that the most important elements are placed at key points. A rule of thumb, not always to be followed but good to keep in mind, is the “rule of thirds”, which says that maximum impact is obtained by placing the center of interest one-third of the way across the image, both widthwise and heightwise.

To crop an image, activate the Crop tool in the Toolbox, or by pressing Shift+C while inside the image. With the tool active, clicking and dragging in the image will sweep out a crop rectangle. When everything is perfect, hit Enter. Note: if Delete cropped pixels in Crop Tool Options is disabled, the cropped part will not be removed from the image, only the visible image area will be adjusted.

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 several 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 these tools by following the menu path Colors → Auto in the main 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 Colors → Auto in the main menu.

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.

If the focus on the camera is not set perfectly, or the camera is moving when the picture is taken, the result is a blurred image. If there is a lot of blurring, you probably won't be able to do much about it with any technique, but if there is only a moderate amount, you should be able to improve the image.

The most generally useful technique for sharpening a fuzzy image is called the Sharpen (Unsharp Mask. In spite of the rather confusing name, which derives from its origins as a technique used by film developers, its result is to make the image sharper, not “unsharp”. It is a plug-in, and you can access it via Filters → Enhance → Sharpen (Unsharp Mask)… in the main menu. There are two parameters, “Radius” and “Amount”. The default values often work pretty well, so you should try them first. Increasing either the radius or the amount increases the strength of the effect. Don't get carried away, though: if you make the unsharp mask too strong, it will amplify noise in the image and also give rise to visible artifacts where there are sharp edges.

In some situations, you may be able to get useful results by selectively sharpening specific parts of an image using the Blur/Sharpen tool from the Toolbox, in "Sharpen" mode. This allows you to increase the sharpness in areas by painting over them with any paintbrush. You should be restrained about this, though, or the results will not look very natural: sharpening increases the apparent sharpness of edges in the image, but also amplifies noise.

When you take pictures in low-light conditions or with a very fast exposure time, the camera does not get enough data to make good estimates of the true color at each pixel, and consequently the resulting image looks grainy. You can “smooth out” the graininess by blurring the image, but then you will also lose sharpness. There are a couple of approaches that may give better results. Probably the best, if the graininess is not too bad, is to use the filter called Selective Blur, setting the blurring radius to 1 or 2 pixels. The other approach is to use the Despeckle filter. This has a nice preview, so you can play with the settings and try to find some that give good results. When graininess is really bad, though, it is often very difficult to fix by anything except heroic measures (i.e., retouching with paint tools).

Every so often you have the opposite problem: an image is too crisp. The solution is to blur it a bit: fortunately blurring an image is much easier than sharpening it. Since you probably don't want to blur it very much, the simplest method is to use one of the “Blur” filters, accessed via Filters → Blur from the main menu. This will soften the focus of the image a little bit. If you want more softening, just repeat until you get the result you desire.

A good tool for removing dust and other types of lens grunge is the Despeckle filter, accessed via Filters → Enhance → Despeckle… from the main menu. Very important: to use this filter effectively, you must begin by making a small selection containing the artifact and a small area around it. The selection must be small enough so that the artifact pixels are statistically distinguishable from the other pixels inside the selection. If you try to run despeckle on the whole image, you will hardly ever get anything useful. Once you have created a reasonable selection, activate Despeckle, and watch the preview as you adjust the parameters. If you are lucky, you will be able to find a setting that removes the junk while minimally affecting the area around it. The more the junk stands out from the area around it, the better your results are likely to be. If it isn't working for you, it might be worthwhile to cancel the filter, create a different selection, and then try again.

If you have more than one artifact in the image, it is necessary to use Despeckle on each individually.

The most useful method for removing unwanted “clutter” from an image is the Clone tool, which allows you to paint over one part of an image using pixel data taken from another part (or even from a different image). The trick to using the clone tool effectively is to be able to find a different part of the image that can be used to “copy over” the unwanted part: if the area surrounding the unwanted object is very different from the rest of the image, you won't have much luck. For example, if you have a lovely beach scene, with a nasty human walking across the beach who you would like to teleport away, you will probably be able to find an empty part of the beach that looks similar to the part he is walking across, and use it to clone over him. It is quite astonishing how natural the results can look when this technique works well.

Consult the Clone Tool Help for more detailed instructions. Cloning is as much an art as a science, and the more you practice at it, the better you will get. At first it may seem impossible to produce anything except ugly blotches, but persistence will pay off.

Another tool looking very much as the clone tool, but smarter, is the healing tool which also takes the area around the destination into account when cloning. A typical usage is removal of wrinkles and other minor errors in images.

In some cases you may be able to get good results by simply cutting out the offending object from the image, and then using a plug-in called “Resynthesizer” to fill in the void. This plug-in is not included with the main GIMP distribution, but it can be obtained from the author's web site [PLUGIN-RESYNTH]. As with many things, your mileage may vary.

When you take a flash picture of somebody who is looking directly toward the camera, the iris of the eye can bounce the light of the flash back toward the camera in such a way as to make the eye appear bright red: this effect is called “red eye”, and looks very bizarre. Many modern cameras have special flash modes that minimize red-eye, but they only work if you use them, and even then they don't always work perfectly. Interestingly, the same effect occurs with animals, but the eyes may show up as other colors, such as green.

GIMP provides a special remove red eye filter. Make a selection with one of the selection tools of the red part of the eye and then choose the “Remove Red Eye” filter. You may have to fiddle around a bit with the threshold slider to get the right color.

What file format should you use to save the results of your work, and should you resize it? The answers depend on what you intend to use the image for.

See the File Formats section for more information.

You print photos from the main menu through File → Print…. However it is very useful to keep in mind some elementary concepts to prevent some unpleasant surprises when looking at the result, or to fix them if they occur. You must always remember:

Consequently, before any printing, go to Image → Print Size… and choose a convenient output size in the “Print Size” box adjusting either sizes or resolution. The symbol shows that the both values are linked. You can dissociate x and y resolution by clicking on that symbol, but it is risky. Only some printers support different X vs. Y resolutions.

Last recommendation: think of checking your margins as well as centering. It would be a pity if a too much large margin cuts off some part of your image or if an inappropriate centering damages your work especially if you use a special photo paper.

Modern digital cameras, when you take a picture, add information to the data file about the camera settings and the circumstances under which the picture was taken. This data is included in JPEG or TIFF files in a structured format called EXIF. For JPEG files, GIMP is capable of maintaining EXIF data, if it is built appropriately: it depends on a library called “libexif”, which may not be available on all systems. If GIMP is built with EXIF support enabled, then loading a JPEG file with EXIF data, and resaving the resulting image in JPEG format, will cause the EXIF data to be preserved unchanged. This is not, strictly speaking, the right way for an image editor to handle EXIF data, but it is better than simply removing it, which is what earlier versions of GIMP did.

If you would like to see the contents of the EXIF data, you can use the metadata-viewer plug-in. You can access it via Image → Metadata → View Metadata from the main menu.