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
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 the GIMP learn that
“painting the selection” is the easiest and most
effective way to delicately manipulate it.
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 1.
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.
You can learn more on Quickmask and
Selection masks in the
section dedicated to the channel dialog.