Frequency Separation in GIMP: Step-by-Step Guide

Frequency separation is a portrait retouching technique that separates an image into two distinct layers: low frequency (colors, tones, and shadows) and high frequency (fine details, pores, and textures). This method allows you to smooth skin tones, fix discoloration, and remove blemishes independently without creating an artificial, overly airbrushed look. This guide explains the exact steps to manually set up frequency separation in GIMP and how to retouch portraits effectively.

Step 1: Prepare Your Layers

  1. Open your portrait image in GIMP.
  2. In the Layers panel, right-click the base layer and select Duplicate Layer twice.
  3. Rename the middle layer to Low Frequency.
  4. Rename the top layer to High Frequency.

Step 2: Set Up the Low Frequency Layer

The low-frequency layer isolates the colors, lighting, and general skin tones by removing the fine texture.

  1. Click the eye icon next to the High Frequency layer to temporarily hide it.
  2. Click on the Low Frequency layer to make it active.
  3. Navigate to Filters > Blur > Gaussian Blur.
  4. Adjust the Size X and Size Y radius values. Increase the blur just until the fine skin textures and pores disappear, but the main features (edges of eyes, lips, and facial contours) remain recognizable. A value between 5.0 and 10.0 is typical, depending on the image resolution.
  5. Click OK to apply the blur.

Step 3: Set Up the High Frequency Layer

The high-frequency layer extracts only the sharp texture details, such as pores, hairs, and wrinkles.

  1. Turn the visibility of the High Frequency layer back on and select it.
  2. Go to Image > Apply Image (or use the grain subtraction method):
    • Set the Source layer to Low Frequency.
    • Set the Mode (or Blending) to Subtract.
    • Set Scale to 2 and Offset to 128.
    • Click OK.
  3. With the High Frequency layer still selected, change its Layer Mode dropdown at the top of the Layers panel from Normal to Grain Merge (or Linear Light).

Your image should now look identical to the original unedited photo, confirming that the high and low frequencies have been split accurately.

(Alternative Method: You can also use GIMP's built-in tool by selecting your original image and going to Filters > Enhance > Wavelet Decompose, setting the scales to 2, and checking "Create a layer group".)

Step 4: Retouching the Portrait

Fixing Skin Tones (Low Frequency)

  1. Select the Low Frequency layer.
  2. Choose the Free Select Tool (Lasso) and set a feather radius (around 15 to 30 px) in the tool options.
  3. Select an area of uneven skin tone (such as the forehead or cheek), avoiding the eyes, lips, and sharp edges.
  4. Go to Filters > Blur > Gaussian Blur and increase the blur radius until the tone transitions blend smoothly.
  5. Alternatively, use a soft Paintbrush (with low opacity) or the Healing Tool (set to Current Layer) to blend color patches directly.

Removing Blemishes and Texture Flaws (High Frequency)

  1. Select the High Frequency layer.
  2. Choose the Healing Tool or Clone Tool from the toolbox.
  3. In the Tool Options, ensure the Sample mode is set to Current Layer (not "Image" or "Sample Merged").
  4. Hold Ctrl and click on a clean area of skin texture nearby to set the source.
  5. Paint over pimples, stray hairs, flakes, or harsh wrinkles to replace the flawed texture with clean texture while leaving underlying lighting and skin tones untouched.