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painting, the source point, marked by a cross, moves as well. If the Alignment is set to
Non Aligned (the default setting), the source resets to the original when you release the
left mouse button.
18.5.4 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 ad-
justing 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 adjust-
ment.
18.5.5 Undoing Mistakes
Most modifications made in The GIMP can be undone. To view a history of modifica-
tions, 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.
Manipulating Graphics with The GIMP
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Troubleshooting
