How to Feather a Layer Mask in GIMP

Feathering a layer mask in GIMP allows you to soften harsh edges and blend images smoothly without needing to feather your selection beforehand. Instead of committing to soft edges during the initial selection phase, you can apply a feathering effect directly to an existing mask at any time using built-in blur filters. This guide outlines the exact steps to quickly soften existing layer mask boundaries with complete control over the blur radius.

Step 1: Select the Layer Mask

Before applying any adjustments, ensure GIMP is targeting the mask rather than the image layer itself:

  1. Open the Layers dialog (Ctrl + L).
  2. Locate the layer containing the mask you wish to feather.
  3. Click directly on the black-and-white layer mask thumbnail located next to the layer's main image preview.
  4. Confirm the mask is active by checking for a white or green outline around the mask thumbnail.

Step 2: Apply the Gaussian Blur Filter

Since layer masks in GIMP are grayscale images (where white reveals, black conceals, and gray creates transparency), applying a blur directly to the mask feathers its edges.

  1. Go to the top menu and select Filters > Blur > Gaussian Blur...
  2. In the dialog window that appears, make sure the Preview checkbox is checked so you can observe the changes in real time on your canvas.
  3. Adjust the Size X and Size Y sliders. Increasing these values expands the blur radius, creating a wider, softer feather. Lower values yield a tighter, more subtle edge transition.
  4. Ensure the chain link icon next to the Size values is linked to maintain an even horizontal and vertical blur.
  5. Click OK to commit the feathering.

Step 3: Fine-Tune the Feathered Edge (Optional)

If the blurred mask reveals too much or too little of the layer underneath, you can adjust the contrast of the mask without redoing the blur:

  1. With the layer mask still selected, go to Colors > Levels...
  2. Move the black, gray, and white input sliders to shift where the opacity transition occurs along the softened edge.
  3. Click OK once the transition matches your project requirements.