Expert photo processing with GIMP
Touch up your digital images with the GIMP image processing tool.
Today more people than ever can take great digital photographs inexpensively, and that means more people than ever have a need for high-quality image processing software. The GNU Image Manipulation Program (GIMP)  is a great free tool you can use to process your digital photos. GIMP can easily handle even the most demanding photo retouching tasks.
When you open GIMP, the first thing you'll notice is a collection of separate floating windows scattered across your screen. To make your first steps a little easier, it might help to take advantage of the new "single-window" mode option, which places all the components in a single window. From the Windows menu, choose Single-Window Mode. This option remains enabled through subsequent restarts of GIMP.
The GIMP main window (in Single-Window mode) is shown in Figure 1. Notice the following important components:
- The canvas – The display area where your image appears and where you will be doing all your image editing.
- The toolbox – The box on the left side (outlined in red in Figure 1) with all the available tools for adjusting your image (as well as others). The toolbox is your primary interface to the majority of the tools.
- The dockable dialog areas (on the right side) are where you will see many of the different dialogs available in GIMP. Some of the most common dialogs manage tool options, layers, and histograms.
Every tool included with GIMP has associated options. A Tool Options dialog on the left displays the available options, which differ depending on the active tool.
One common task is to create a new image that contains a smaller portion of the original image. Often you will notice an undesirable element in the original, or you might want to "reframe" the image for some visual effect. Cutting out undesirable parts of the image is known as "cropping."
To crop an image in GIMP, simply define the area you want to keep and tell GIMP to discard everything else. Use the Rectangle Select tool to make a rectangular selection of the area you want to keep, then crop the image to fit that rectangle.
The Rectangle Select tool is the first item in the toolbox. (When you click on it, your cursor in the image canvas will change to indicate the tool you are using.) You can also access the Rectangle Select tool through the menus by choosing Tools | Selection Tools | Rectangle Select.
To select a region on your canvas, left-click at one corner of your selection area and drag the cursor to the opposite corner. You will see a visual outline of the rectangular selection you are making (Figure 2). Don't worry at this point about choosing the perfect point – it is quite easy to modify the selection boundaries afterward. Once you've reached the opposite corner, release the left mouse button, and your selection will have a marching dashed line surrounding it.
To fine-tune or modify the selection boundary, mouse over one of the selection corners. You will notice the corner "box" turns yellow. Left-click and drag the corner to reposition it. As you drag the corner, notice that the opposite corner keeps its position. You can also hover close to any of the four edges of the selection, and the edge behaves similarly.
If you hover your mouse anywhere else inside of the selection, you should notice that the cursor takes on the appearance of a multidirectional arrow. You can now left-click and drag to move the selection around the image canvas.
As mentioned previously, it's important to pay attention to the available options for each tool, and the Rectangle Select tool does have some helpful ones. See the box titled "Select Tool Options."
Select Tool Options
To help you visualize what the selection will look like isolated from the rest of the image, enable the Highlight option. Highlighting will darken all of the image outside of your current selection (Figure 3).
GIMP can also overlay guides on the selection area to help you compose. Some common guides include Center lines, Rule of thirds, and Golden sections, as well as some less common guides, such as Diagonal lines and Rule of fifths.
You can also specify an aspect ratio for your image that will then remain constant regardless of how you resize the selection. Simply enable Fixed beside the Aspect ratio drop-down and change the value in the input box to your desired ratio. For instance, if you want a square crop, you could simply enter 1:1 in the input box. If you want something to fit your widescreen monitor, 16:10 might be what you're looking for. Perhaps you'd like to emulate the aspect ratio of widescreen cinema movies? In that case, you could try 2.40:1.
Once you have a selection for your new crop, choose Image | Crop to Selection to crop the image.
Rotating an Image
Unless you're shooting on a tripod (or have a fancy digital level built into your camera), some of your images might turn out just slightly out of level. Or, you might want to rotate your image for purely artistic purposes. Whatever the reason, you can easily rotate an image in GIMP.
Select Tools | Transform Tools | Rotate to activate the Rotate tool. After it's selected, simply click anywhere on your image. You should see that your image is now overlaid with a grid (Figure 4), and a dialog box appears to give you further control over the rotation (Figure 5). If you happen to know the specific value you would like to rotate your image, simply enter the value directly in the input box.
You might not know the exact value most of the time and might prefer to rotate the image manually. To do so, simply click and drag anywhere on your image canvas. As you drag your mouse, the image rotates on the canvas. Once the image is rotated to the position in which you would like it to be, simply press the Rotate button in the dialog.
In some cases, it might be difficult to align features in your image to be precisely horizontal or vertical. Luckily, it's easy to drag guides onto the canvas to give you a visual reference. Simply left-click in any of the rulers along the top or left of the canvas. You will see a guide that you can use to position by simply dragging into your canvas. Use the guide as a reference while rotating your image.
Once the rotate operation finishes, you might notice corners with gray checkerboard patterns that have no image data. The checkerboard represents transparent areas. To clean up the results after rotating, simply use the methods discussed in the previous section to crop the image down to fit the rotation.
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