|Revision $Revision: 1.23 $
Often when you operate on an image, you only want part of it to be
affected. In GIMP, you make this happen by selecting
that part. Each image has a selection
associated with it. Most, but not all, GIMP operations act only on the
selected portions of the image.
How would you isolate the tree?
There are many, many situations where creating just the right selection is
the key to getting the result you want, and often it is not very easy to
do. For example, in the above image, suppose I want to cut the tree out
from its background, and paste it into a different image. In order to do
this, I need to create a selection that contains the tree and nothing but
the tree. It is difficult because the tree has a very complex shape, and
in several spots is hard to distinguish from the objects behind it.
Selection shown as usual with dashed line
Now here is a very important point, and it is crucial to understand this.
Ordinarily when you create a selection, you see it as a dashed line
enclosing a portion of the image. The idea you could get from this is that
the selection is a sort of container, with the selected parts of the image
inside, and the unselected parts outside. This concept of the selection is
okay for many purposes, but it is not really correct.
Actually the selection is implemented as a channel.
In terms of its internal structure, it is identical to the red, green,
blue, and alpha channels of an image. Thus, the selection has a value
defined at each pixel of the image, ranging between 0 (unselected) and 255
(fully selected). The advantage of this approach is that it allows some
pixels to be partially selected,
by giving them intermediate values between 0 and 255. As you will see,
there are many situations where it is desirable to have smooth transitions
between selected and unselected regions.
What, then, is the dashed line that appears when you create a selection?
It is a contour line, dividing areas that are more
than half selected from areas that are less than half selected.
Same selection in QuickMask mode
You should always bear in mind, when looking at the dashed line that
represents the selection, that it only tells you part of the story. If you
want to see the selection in complete detail, the easiest way is to click
the QuickMask button in the lower left corner of the image window. This
causes the selection to be shown as a translucent overlay atop the image.
Selected areas are unaffected; unselected areas are reddened. The more
completely selected an area is, the less red it appears.
QuickMask mode, and its uses, are described in detail below. Meanwhile, if
you are following this discussion by trying things out in GIMP, you should
know that many operations work differently in QuickMask mode, so go ahead
and toggle it off again for now (by clicking the QuickMask button once
Same selection in QuickMask mode after feathering
With the default settings, the basic selection tools, such as the
Rectangle Select tool, create sharp selections. Pixels inside the dashed
line are fully selected, and pixels outside completely unselected. You
can verify this by toggling QuickMask: you see a clear rectangle with
sharp edges, surrounded by uniform red. In the Tool Options, however, is
a checkbox called “Feather edges”. If you enable this, the
tool will instead create graduated selections. The feather radius, which
you can adjust, determines the distance over which the transition occurs.
If you are following along, try this out with the Rectangle Select tool,
and then toggle QuickMask. You will now see that the clear rectangle has
a fuzzy edge.
Feathering is particularly useful when you are cutting and pasting, in
helping the pasted object to blend smoothly and unobtrusively with its
Actually, it is possible to feather a selection at any time, even if it
was originally created as a sharp selection. You can do this from the
image menu, by choosing
This brings up a dialog that allows you to set the feather radius. You
can do the opposite--sharpen a graduated selection into an
all-or-nothing selection--by choosing
For technically oriented readers: feathering works by applying a
Gaussian blur to the selection channel, with the specified blurring
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
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).
to create a floating selection. This creates a new layer called
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.
Section 2.1.3, “
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