Remove Dust Spots on Sky Gradients in GIMP

Removing camera sensor dust rings and spots from clear sky gradients can be challenging because simple cloning often creates visible smudges or banding. This guide demonstrates how to use the Heal Selection feature (powered by the Resynthesizer plugin) in GIMP to analyze surrounding color transitions, calculate gradient continuity, and automatically blend away dust spots seamlessly.

Prerequisites

The "Heal Selection" tool is not included in the default GIMP installation; it requires the third-party Resynthesizer plugin. Ensure you have installed the Resynthesizer package for your operating system so that the option appears under Filters > Enhance > Heal selection... before proceeding.

Step 1: Open Your Image and Locate Dust Spots

  1. Open your image in GIMP via File > Open.
  2. Zoom in to 100% or greater on the sky area using the Zoom Tool (Z).
  3. (Optional) If faint dust rings are hard to spot, duplicate your image layer and apply a temporary high-contrast curve via Colors > Curves to make the dark halos clearly visible. Delete or hide this reference layer when you are ready to edit the original layer.

Step 2: Select the Dust Spot

  1. Activate the Free Select Tool (Lasso) or the Ellipse Select Tool (E).
  2. Draw a tight selection around the sensor dust spot.
  3. Include just a small margin of clean sky around the perimeter of the spot. Do not make the selection excessively large, as this forces the algorithm to synthesize unnecessary area.
  4. Go to Select > Feather... and add a feather radius of 2 to 5 pixels depending on the resolution of your image. This softens the transition boundary along the gradient.

Step 3: Apply the Heal Selection Tool

  1. With the selection active, navigate to Filters > Enhance > Heal selection....
  2. Adjust the settings in the dialog box:
    • Context sampling width (pixels): Set this between 15 and 50 pixels. A larger width gives the algorithm more surrounding gradient data to sample from.
    • Sample from: Select All around. Because skies are continuous two-dimensional gradients, sampling from all sides allows the tool to interpolate luminance and hue correctly.
    • Filling order: Choose Random or Inwards towards center. "Inwards towards center" generally performs best for circular blemishes like dust rings.
  3. Click OK.

Step 4: Verify and Deselect

  1. Clear your selection by pressing Ctrl + Shift + A (or Cmd + Shift + A on macOS).
  2. Inspect the repaired area at 100% zoom. The Heal Selection algorithm synthesizes pixel texture and luminance matched precisely to the surrounding gradient, eliminating hard edges or haloing.
  3. Repeat the selection and healing steps for any remaining sensor artifacts across the sky.