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Manipulating Graphics with The GIMP 211
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14.5.4 Adjusting Color Levels
Images often need a little adjusting to get ideal print or display results. In many programs designed
for inexperienced users, the brightness and contrast levels are modified. This can work and is also
available in The GIMP, but better results can be obtained by adjusting the color levels.
To do this, select Layer > Colors > Levels . A dialog opens for controlling the levels in the image.
Good results can usually be obtained by clicking Auto . To make manual adjustments to all channels,
use the dropper tools in All Channels to pick areas in the image that should be black, neutral gray,
and white.
To modify a channel individually, select the desired channel in Channel . Then drag the black, white,
and middle markers in the slider in Input Levels . Alternatively, use the dropper tools to select points
in the image that should serve as the white, black, and gray points for that channel.
If Preview is checked, the image window shows a preview of how the image would look with the
modifications applied. When the desired result is achieved, press OK to apply the changes. With
Reset , restore the original settings. Cancel aborts level adjustment.
14.5.5 Undoing Mistakes
Most modifications made in The GIMP can be undone. To view a history of modifications, use the
undo dialog included in the default window layout or open one from the toolbox menu with
File >
Dialogs > Undo History .
The dialog shows a base image and a series of editing changes that can be undone. Use the buttons
to undo and redo changes. In this way, you can work back to the base image. If you undo a
modification then make a new one, the undone modification cannot be redone.
Changes can also be undone and redone with the Edit menu. Alternatively, use the shortcuts Ctrl+Z
and Ctrl+Y.
14.5.6 Layers
Layers are a very important aspect of The GIMP. By drawing parts of your image on separate layers,
change, move, or delete those parts without damaging the rest of the image. To understand how
layers work, imagine an image created from a stack of transparent sheets. Different parts of the
image are drawn on different sheets. The stack can be rearranged, changing which parts are on top.
Individual layers or groups of layers can shift position, moving sections of the image to other
locations. New sheets can be added and others set aside.
Use the Layers dialog to view the available layers of an image. The text tool automatically creates
special text layers when used. The active layer is highlighted. The buttons at the bottom of the dialog
offer a number of functions. More are available in the menu opened when a layer is right-clicked in
the dialog. The two icon spaces before the image name are used for toggling image visibility (eye
icon when visible) and for linking layers. Linked layers are marked with the chain icon and moved
as a group.
Only layers with transparency (an alpha channel) can be placed above other layers in a stack. To add
this to a layer, right-click and select it from the menu.