3. Undoing

Almost anything you do to an image in GIMP can be undone. You can undo the most recent action by choosing Edit → Undo from the main menu, but this is done so frequently that you really should memorize the keyboard shortcut, Ctrl+Z.

Undoing can itself be undone. After having undone an action, you can redo it by choosing Edit → Redo from the main menu, or use the keyboard shortcut, Ctrl+Y. It is often helpful to judge the effect of an action by repeatedly undoing and redoing it. This is usually very quick, and does not consume any extra resources or alter the undo history, so there is never any harm in it.

	Caution
	If you undo one or more actions and then operate on the image in any way except by using Undo or Redo, it will no longer be possible to redo those actions: they are lost forever. The solution to this, if it creates a problem for you, is to duplicate the image and then test on the copy. ( Do Not test the original, because the undo/redo history is not copied when you duplicate an image.)

If you often find yourself undoing and redoing many steps at a time, it may be more convenient to work with the Undo History dialog, a dockable dialog that shows you a small sketch of each point in the Undo History, allowing you to go back or forward to that point by clicking.

	Tip
If you make a small mistake while actively drawing with a mouse like painting a little too far in one direction (over an important line, etc) you can also cancel the action early by pressing both mouse buttons at the same time then releasing the left (primary) button first. Canceling an action in this manner is even faster than accessing the Undo command (it avoids adding the action to the Undo History at all).

Tip

If you make a small mistake while actively drawing with a mouse like painting a little too far in one direction (over an important line, etc) you can also cancel the action early by pressing both mouse buttons at the same time then releasing the left (primary) button first. Canceling an action in this manner is even faster than accessing the Undo command (it avoids adding the action to the Undo History at all).

Undo is performed on an image-specific basis: the "Undo History" is one of the components of an image. GIMP allocates a certain amount of memory to each image for this purpose. You can customize your Preferences to increase or decrease the amount, using the System Resources page of the Preferences dialog. There are two important variables: the minimal number of undo levels, which GIMP will maintain regardless of how much memory they consume, and the maximum undo memory, beyond which GIMP will begin to delete the oldest items from the Undo History.

	Note
	Even though the Undo History is a component of an image, it is not saved when you save the image using GIMP's native XCF format, which preserves every other image property. When the image is reopened, it will have an empty Undo History.

GIMP's implementation of Undo is rather sophisticated. Many operations require very little Undo memory (e.g., changing visibility of a layer), so you can perform long sequences of them before they drop out of the Undo History. Some operations, such as changing layer visibility, are compressed, so that doing them several times in a row produces only a single point in th