Running Linux, 5th Edition - Matthias Kalle Dalheimer [169]
The first three selection tools are, except for the shape of the selection made, quite similar. While dragging out a rectangular or elliptical selection, it is possible to keep a constant aspect ratio by holding down the Shift key. In the option window for each selection tool, it is possible to choose a selection mode to add to an existing selection, subtract from one, replace the current selection, and intersect with one.
All selection tools have a feather parameter that will control how soft the edges of the selection are. See Figure 9-15 for an example.
The magic wand allows you to click on a pixel in the image and thereby select a contiguous area around the pixel with similar color. Use the threshold slider to control how similar the colors must be. Selection by color works like the magic wand, but it selects all pixels with similar value — contiguous or not. Finally, selection by shapes allows you to place points in the image and try to connect the points with curves that follow edges in the image. When you have selected enough points to contain an area, click in the middle of that area to convert the traced curve to a selection.
Figure 9-14. GIMP toolbox
Painting and erasing tools. To paint in an image, the Pencil, Paintbrush, Airbrush and Ink tools can be used. They differ in the way the shapes you draw look: Pencil paints with hard edges, and Paintbrush with soft edges, Airbrush paints semitransparently and Ink thickens the line when you paint slowly and thins the line when you paint quickly.
To fill in an area, make a selection and use the paintbucket or gradient fill tool to fill it with color. Selecting the pen style, color, and/or gradient can be done by clicking the controls in the middle of the toolbox window.
Some people have trouble drawing a straight line in GIMP, but since you have this clever book in your hands, you will know the secret: select one of the drawing tools, place the cursor where you want the line to start, press and hold Shift, and then move the mouse to where the line should end and click once with the left mouse button. Now either do the same again to draw another line segment or release the Shift key and enjoy your straight line.
Figure 9-15. GIMP selections
If you make a mistake, use the most often used keyboard shortcut in GIMP: Ctrl-Z to undo. Multiple levels of undo are available. There is also an eraser tool that allows you to selectively erase pixels.
Everything you do with the painting tools will be confined to the currently selected area if there is a selection.
Photo retouching tools . The tools in this section are mostly for modifying digital photos in subtle (and not so subtle) ways. The Clone tool is very useful to remove blemishes from a photo. It works by first Ctrl-clicking in an image to set the source point, and then painting somewhere in an image. You will now paint with "copies" of the source area. Figure 9-16 shows the upper-right corner of a landscape photo that got a bit of the roof from a house into the frame. The left image is the original, and the right one has the undesired feature removed by using the clone tool with some other part of the clouds as the source area.
The last tool in the toolbox is the Dodge and Burn tool. It is used to lighten (dodge) and darken (burn) parts of an image by drawing on it. This tool can be used to finetune areas with shadows or highlights.
Color adjustment. During postprocessing of digital photos, it can be very useful to adjust the overall appearance of the light, color, and contrast of a photo. GIMP supports quite a number of tools for this. They are available in the Layer/Colors context menu.
Figure 9-16. GIMP clone tool
One of the more useful tools is the Levels tool. It allows you to adjust the black and white points of an image. Figure