How to Use Quick Mask in GIMP

In GIMP, the tool used to convert an active selection into a temporary channel mask for manual painting is the Quick Mask. This feature overlays a translucent colored screen over your image, allowing you to refine, expand, or subtract from your selection using standard painting tools like the Paintbrush, Eraser, and Smudge tools. Once you finish painting the mask, toggling the feature off converts the painted areas back into a standard "marching ants" selection.

Activating Quick Mask

You can toggle the Quick Mask on and off using any of the following methods:

  • Keyboard Shortcut: Press Shift + Q.
  • Menu Bar: Navigate to Select > Toggle Quick Mask.
  • Canvas Shortcut: Click the small, red-outlined square icon located in the extreme bottom-left corner of the image window (at the intersection of the horizontal and vertical scroll bars).

When activated, any area not currently selected is covered by a translucent red overlay (by default), while your selected area remains completely clear and editable.

Editing the Mask with Paint Tools

Once Quick Mask is active, the mask behaves like a grayscale channel:

  • Painting with White: Removes the red overlay, which adds those areas to your active selection.
  • Painting with Black: Adds the red overlay, subtracting those areas from your selection.
  • Painting with Shades of Gray: Creates soft, feathered, or semi-transparent selections, which is ideal for complex edges like hair or soft shadows.
  • Using Filters and Blurs: You can apply filters such as Gaussian Blur (Filters > Blur > Gaussian Blur) directly to the mask to create smooth, feathered transitions along the selection boundaries.

Converting Back to a Selection

After refining the mask with your brushes:

  1. Press Shift + Q or click the bottom-left Quick Mask icon again.
  2. The red overlay will disappear, and your painted modifications will instantly convert into an active selection boundary.

Customizing Quick Mask Options

If your image contains a lot of red and the default mask color is difficult to see, you can adjust the display settings. Right-click the Quick Mask icon in the bottom-left corner and choose Configure Color and Opacity. From this dialog, you can change the overlay color to green, blue, or any contrasting shade, as well as adjust its opacity level.