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CHAPTER 2. GIMP CONCEPTS
2.3. WORKING WITH IMAGES
2.3.4.2. Making a selection partially transparent
You can set layer opacity, but you cannot do that directly for a selection. It is quite useful to make the
image of a glass transparent. You can achieve this by using these methods:
• For simple selections, use the Eraser tool with the wanted opacity.
• For complex selections: use the command Selection → Floating to create a floating selection.
This creates a new layer called “Floating Selection”. Activate it and use the opacity slider to get
the wanted opacity. Then anchor the selection: outside the selection, the mouse pointer comes
with an anchor icon. When you click, the floating selection disappears from the Layer Dialog and
the selection is at the right place and partially transparent (anchoring works this way only if a
selection tool is activated : you can also use the command in the context menu that you get by
right clicking on the selected layer in the layer dialog).
And, if you use this function frequently: Ctrl–C to copy the selection, Ctrl–V to paste it, creating
so a floating selection, adapt the opacity then make Layer/New Layer that pastes the floating
selection into the new layer. You can also create a shortcut for the New Layer command to use
keys only.
• Another way: Layer → Mask → Layer mask to add a layer mask to the layer with the selection, initializing it with the selection. Then use a brush with the wanted opacity to paint the
selection with black, i-e paint it with transparency. Then Layer/Mask/Apply Layer Mask. See
Section 9.2.1.3 on page 196.
2.3.5. 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 image 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 image 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.
C AUTION
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 operate on the copy. ( Not 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.
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 Environment 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.
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