4.2. QuickMask

The selection tools sometimes show their limits when they have to be used for creating a complex selection. In these cases, using the QuickMask can make things much easier. Simply put, the QuickMask allows you to paint a selection instead of just tracing its outline.

Overview

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 256 (fully selected). The marching ants are drawn along a contour of half-selected pixels. Thus, what the marching ants show you as either-or–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 Select->Toggle QuickMask, 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 the Gimp learn that "painting the selection" is the easiest and most effective way to delicately manipulate it.

[Tip] Tip

To save in a channel the selection done with the Quickmask select in the image menu Select/Save to Channel

[Tip] Tip

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.

Properties

There are two QuickMask properties you can change by right-clicking on the QuickMask button.

  • Normally the QuickMask shows unselected areas "fogged over" and selected areas "in clear", but you can reverse this by choosing "Mask Selected Areas" instead of the default "Mask Unselected Areas".

  • By choosing "Configure Color and Opacity", you can bring up a dialog that allows you to set these to values other than the defaults, which are red at 50% opacity.

Usage

  1. Open an image or begin a new document;

  2. Activate the Quickmask using the left-bottom button in the image window. If a selection is present the mask is initialized with the content of the selection;

  3. Choose any drawing tool and use it with greyscale colors on the QuickMask;

  4. Toggle off the Quickmask using the left-bottom button in the image window;