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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.
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.
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.
Figure 7.3: 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.
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 more).
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 more).
Figure 7.4: Same selection in QuickMask mode after feathering
7.1.1
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.
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.
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 unobtru-
sively with its surroundings.
sively with its surroundings.
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 Select → Feather. 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 Select → Sharpen.
from the image menu, by choosing Select → Feather. 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 Select → Sharpen.