High Frequency Separation

GIMP

1. Import Image – Open your 16-bit TIFF exported from your RAW developer software file in GIMP: File → Open.

   1. 3rd Party: CaptureOne, Lightroom, DxO PhotoLab, etc.
   2. Camera Brand: Imaging Edge Desktop (Sony), NX Studio (Nikon), Digital Photo Professional (Canon)
   3. Optional: Skip RAW developer software and direct import
      1. Install the GIMP "Darktable" or "RawTherapee" plug-in for direct import (like Adobe Camera Raw)

2. Save as XCF – Immediately save the file as .xcf to preserve layer and bit depth: File → Save As.

3. Rename Base Layer – In the Layers panel, rename the imported image layer to "Original".

4. Duplicate Layer – Right-click the Original layer → Duplicate Layer, and rename the copy "Low Frequency" or "Colour and Tone".

5. Gaussian Blur – With the "Low Frequency" layer selected, go to Filters → Blur → Gaussian Blur.

   1. Experiment with Size X and Y values to remove texture while preserving shape and color.
   2. A starting point is roughly 1/1000th of the image width (e.g., 3000 px image → blur radius ~3).
   3. Use the Magnifying Glass Tool to select a relevant zoom area.
   4. Optionally zoom in to 1:1 (100%) to examine texture detail.
   5. Increase the blur until fine texture just begins to fade — don’t overblur.
   6. Apply the blur.

6. Fit Image in Window – Use View → Zoom → Fit Image in Window or press Shift + Ctrl + J.

7. Set Layer Mode to Grain Extract – In the Layers panel, change the mode of the "Low Frequency" layer to Grain Extract.

   1. The result should resemble a gray, high-contrast etching — this is your texture separation.

8. New Layer from Visible – Go to Layer → New from Visible to merge the current view into a new layer.

9. Rename New Layer – Rename this new layer "High Frequency" or "Texture".

10. Reset Layer Modes – Set the "Low Frequency" layer mode back to Normal. The image should now visually resemble the original layer again.

11. Edit the Low Frequency Layer – Select the "Low Frequency" (Colour and Tone) layer for tone, color, and shape corrections.

    1. Use the Smudge Tool (S) for quick blending, especially effective on large patches; use low strength to preserve realism.

    2. Use Select → Feather, then Gaussian Blur within the selection to smooth blotchy zones.

       1. Use the Free Select Tool (F) to draw around the skin area, close the selection, and press Enter.

       2. Change to Subtract from Selection mode, and subtract non-skin features like eyes and lips.

       3. Feather the selection for soft edges, then apply Gaussian Blur for natural tonal blending.

          Note: Effective but easy to overdo — avoid flattening natural gradations or sculpting features unnaturally.

12. Edit the High Frequency Layer – Select the "High Frequency" (Texture) layer for fine detail corrections like blemishes, rough texture, or isolated imperfections.

    1. Use the Heal Tool (H) or Clone Tool (C) to target specific problem areas, such as:
       1. Pimples
       2. Fine lines or stray hairs
       3. Flaky or rough skin patches
       4. Use a small brush size and soft edges to avoid creating visible edges or duplicating patterns.
    2. Avoid smudging or blurring this layer — doing so can introduce ghosting or damage critical texture fidelity.

13. Optional Final Layer – When retouching is complete, go to Layer → New from Visible to create a final composite layer for export or further adjustment.

14. Going Further – Use Wavelet Decompose to create 2 or more frequency layers and automate the layering process. Go to Filters → Enhance → Wavelet Decompose (if installed, else add the plug-in).

Photoshop

1. Import Image – Open your 16-bit TIFF or RAW file in Photoshop using Adobe Camera Raw (ACR), or launch Photoshop directly from Lightroom (Photo → Edit In → Adobe Photoshop) or Capture One (Right-click image → Edit With → Adobe Photoshop) with the image opened as a rendered TIFF or PSD.

2. Save as PSD – Immediately save the file as .psd to preserve layers and full bit depth: File → Save As.

3. Rename Base Layer – In the Layers panel, rename the background layer to "Original".

4. Duplicate Layer – Right-click the Original layer → Duplicate Layer, rename it "Low Frequency" or "Colour and Tone"`.

5. Gaussian Blur – With the "Low Frequency" layer selected, go to Filter → Blur → Gaussian Blur.

   1. Start with a blur radius around 1/1000th of the image width (e.g., 3000 px wide → blur ~3).
   2. Zoom to 100% (1:1) to judge texture clarity.
   3. Increase the blur until fine texture is removed but shapes remain clear.
   4. Apply the blur.

6. Duplicate Original Again – Select the "Original" layer, duplicate it, and move this copy above the "Low Frequency" layer. Rename it "High Frequency" or "Texture".

7. Apply Image – With the "High Frequency" layer selected, go to Image → Apply Image:

   • Layer: Low Frequency
   • Blending: Subtract
   • Scale: 2
   • Offset: 128
   • Mode: RGB
   • Click OK.
   • The result will look like a gray, high-contrast texture layer.

8. Set Blend Mode – Set the "High Frequency" layer's blend mode to Linear Light. The image will now resemble the original.

9. Group and Organize – Optionally group the two layers into a folder for cleaner organization: Ctrl + G.

10. Edit the Low Frequency Layer – Select the "Low Frequency" layer to correct tone, color, and shape.

    1. Use the Lasso Tool (L) to select blotchy skin areas.
    2. Feather the selection: Select → Modify → Feather (~10–30 px depending on image size).
    3. Use Gaussian Blur to smooth transitions inside the selection.
    4. For precise blending, use the Smudge Tool (low strength) to gently blend edges.

    Avoid overly aggressive blur—maintain natural transitions in tone and light.

11. Edit the High Frequency Layer – Select the "High Frequency" layer for texture cleanup.

    1. Use the Healing Brush Tool (J) or Clone Stamp Tool (S) to fix blemishes and isolated imperfections:
       • Pimples
       • Fine lines or wrinkles
       • Stray hairs
       • Flaky skin
    2. Use a small brush with soft edges. Avoid cloning patterns or smearing texture.
    3. Do not use blur or smudge tools on this layer — doing so may destroy essential texture.

12. Optional Final Merge – Once edits are complete, create a new merged layer for export or final adjustments: Shift + Ctrl + Alt + E (Mac: Shift + Cmd + Option + E).