GIMP Quick Mask: Paint Soft Selections with Airbrush

This guide explains how to use GIMP's Quick Mask mode in combination with the Airbrush tool to generate complex, soft-edged selections. By replacing the standard selection tools with a paintable mask, you gain precise control over edge feathering and semi-transparent boundaries. You will learn how to activate Quick Mask, configure the Airbrush for subtle tonal transitions, paint using grayscale values, and convert your painted mask back into a usable selection.

Step 1: Enable Quick Mask Mode

Open your image in GIMP. Activate Quick Mask mode using either of the following methods:

  • Press Shift + Q on your keyboard.
  • Click the small dashed square icon located in the bottom-left corner of the image window (at the intersection of the horizontal and vertical rulers).

Once activated, your image will be covered with a translucent red overlay (rubylith). Any area covered in red represents an unselected area, while clear areas represent selected pixels.

Step 2: Configure the Airbrush Tool

Select the Airbrush Tool from the Toolbox or press A. The Airbrush is ideal for complex soft selections because it continues to build up color as long as the mouse button or stylus is held down, creating smooth gradients.

In the Tool Options panel, set the following parameters:

  • Brush: Choose a standard soft round brush (such as 2. Hardness 050 or 2. Hardness 025).
  • Size: Adjust according to the detail of the edge you are working on.
  • Rate: Set between 10 and 30 to maintain control over the paint flow.
  • Flow: Set between 15 and 40 for gradual buildup.

Step 3: Paint the Selection Boundaries

Quick Mask operates based on grayscale values:

  • White: Removes the red mask and adds to the final selection.
  • Black: Adds the red mask and removes from the final selection.
  • Gray / Low Opacity: Creates a feathered, semi-transparent selection.

Press D to reset your foreground and background colors to black and white.

  1. Set your foreground color to White to paint the area you want to select.
  2. Lightly build up the strokes along the contours of your subject, especially around hair, fur, smoke, or soft light transitions.
  3. Switch your foreground color to Black (press X to swap colors) to paint back over any areas where the mask spilled over.
  4. Lower the tool's opacity or use light pressure with a graphics tablet to create ultra-smooth transitions.

Step 4: Toggle Back to an Active Selection

When you finish painting your mask:

  1. Press Shift + Q or click the Quick Mask icon in the bottom-left corner again to turn off Quick Mask mode.
  2. The red overlay will disappear and be replaced by standard "marching ants."

Note: Marching ants only indicate the threshold where pixels are at least 50% selected. Even if the dashed line looks sharp, the underlying soft gradient and feathered transparency created by your airbrush strokes are fully preserved.

Step 5: Utilize the Soft Selection

With your soft selection active, you can now perform your desired edit:

  • Add a Layer Mask: Click Layer > Mask > Add Layer Mask, choose Selection, and click Add to isolate the subject cleanly.
  • Tonal Adjustments: Apply curves, levels, or color adjustments that seamlessly blend into the surrounding image.
  • Cut or Copy: Extract the subject onto a new layer without harsh, jagged outlines.