Normally when you create a selection in GIMP, you see it represented by
the "marching ants" that trace along its outline. But really there may
be a lot more to a selection than the marching ants show you: in GIMP a
selection is actually a full-fledged grayscale channel, covering the
image, with pixel values ranging from 0 (unselected) to 255 (fully
selected). The marching ants are drawn along a contour of half-selected
pixels. Thus, what the marching ants show you as either inside or
outside the boundary is really just a slice through a continuum.
The QuickMask is GIMP's way of showing you the full structure of the
selection. Activating it also gives you the ability to interact with the
selection in new, and substantially more powerful, ways. To activate the
QuickMask, click on the small red-outlined button at the lower left of
the image window. The button is a toggle, so clicking it again will
return you to normal marching-ant mode. You can also activate the
QuickMask by selecting in the image window menu
→ , or by using the
Shift+Q
shortcut.
Activating the QuickMask shows you the selection as though it were a
translucent screen overlying the image, whose transparency at each pixel
indicates the degree to which that pixel is selected. By default the
mask is shown in red, but you can change this if another mask color
would be more convenient. The less a pixel is selected, the more it is
obscured by the mask. Fully selected pixels are shown completely clear.
When you are in QuickMask mode, many image manipulations act on the
selection channel rather than the image itself. This includes, in
particular, paint tools. Painting with white causes the painted
pixels to be selected; painting with black causes them to be
unselected. You can use any of the paint tools, as well as the bucket
fill and gradient fill tools, in this way. Advanced users of GIMP
learn that “painting the selection” is the easiest and
most effective way to delicately manipulate it.
![[Tip]](images/tip.png)
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Tip |
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To save the selection done by the Quickmask to a new channel; Make
sure that there is a selection and that Quickmask is not active in
the image window. Select in the image menu
→ .
This will create a new channel in the channel dialog called
“SelectionMask copy” (if you repeat this command you
will create a “..copy#1”, “...copy#2” and
so on...).
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Tip |
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When QuickMask is active, Cut and Paste act on the selection rather
than the image. You can sometimes make use of this as the most
convenient way of transferring a selection from one image to another.
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You can learn more on
Selection masks in the
section dedicated to the channel dialog.
