{"course":{"slug":"image-editing","title":"Editing an Image","url":"https://addaly.com/learn/image-editing","level":"beginner","track":"create"},"lesson":{"slug":"frequency-separation","title":"Splitting texture from tone","url":"https://addaly.com/learn/image-editing/frequency-separation","position":41,"of":78,"minutes":10,"answer":"Frequency separation splits an image into a blurred tone layer and a neutral-grey detail layer that recombine exactly.","keyIdea":"Frequency separation splits an image into a blurred tone layer and a neutral-grey detail layer that recombine exactly, so blemishes are healed on the detail layer where there is no tone to get wrong and colour is evened on the tone layer where there is no texture to destroy."},"body":{"format":"markdown","text":"## The problem it solves\n\nSkin has two kinds of imperfection and they need opposite treatments.\n\n**Tonal unevenness** (a red patch on a cheek, a shadow under the eye, blotchy colour across a forehead) is large and soft. You fix it by smoothing.\n\n**Texture faults** (a spot, a hair, a fleck of mascara) are small and sharp. You fix them by replacing.\n\nSmooth the whole image and you destroy the pores, which is what makes retouched skin look like plastic. Heal every blotch and you get a lumpy patchwork, because the healing brush matches texture and averages tone across an area that legitimately varies.\n\nFrequency separation splits the image into those two layers so each can be treated with its proper tool and neither affects the other.\n\n## Building it\n\nThe **low frequency** layer is a blurred copy: colour and tone with no detail. The **high frequency** layer is what the blur removed: detail with no colour or tone, held around a neutral mid-grey. Recombined, they reconstruct the original exactly.\n\n> Figure: One portrait, split so each fault meets its proper tool. Layers, top to bottom: high: set to Linear Light (Detail with no tone, held around neutral grey. Heal and clone here: there is no tone to get wrong, so a spot comes out without the pale disc.); low: Gaussian blurred, 5 to 10 pixels on a 20-megapixel portrait (Colour and tone with no detail. Lasso generously and blur, or paint at 10 per cent opacity. Even it out; flatten it and the face goes mask-like.); background: the original, untouched (The two layers above recombine to this exactly. The arithmetic is exact rather than approximate, which is what makes the split safe to work in.). Choose the blur radius by looking at the low layer on its own: if the spots are still visible it is too small, and if the nose has disappeared it is too large. Never work on the low layer across a hard edge: the blur has mixed the jaw and the background there, and the result is a pale halo tracing the face.\n\nIn Photoshop or Photopea:\n\n1. Duplicate the background layer twice. Name the lower copy `low`, the upper `high`.\n2. Hide `high`. On `low`, apply **Gaussian Blur**. Choose the radius by eye: large enough that the blemishes disappear, small enough that the face's structure (the nose, the eye sockets) is still recognisable. On a 20-megapixel portrait that is usually 5 to 10 pixels.\n3. Show `high` and select it. **Image > Apply Image**. Set Layer to `low`, Blending to **Subtract**, Scale **2**, Offset **128**. (In a 16-bit document, use Blending **Add**, tick **Invert**, Scale **2**, Offset **0**.)\n4. Set `high` to blend mode **Linear Light**.\n\nThe image now looks identical to the original, and it is: the arithmetic is exact, not approximate.\n\nIn GIMP the classic route needs no dialogue arithmetic. Duplicate twice, blur the lower copy, and set the upper copy's mode to **Grain extract** with the blurred one beneath it; merge those two and set the result to **Grain merge** over the blurred layer. Same split, same reversibility. GIMP also ships **Wavelet Decompose**, which splits into several frequency bands at once, and the **G'MIC** plugin has *Split Details*, which is the most flexible of all.\n\n## Working on each layer\n\n**On `high`, use the Healing Brush and Clone Stamp.** Spots, stray hairs, flecks. Because this layer holds no tone, healing here cannot leave a pale disc: there is no tone to get wrong. This is the single biggest practical gain of the technique.\n\n**On `low`, use the Lasso and blur, or a soft low-opacity brush.** Select the blotchy region with a lasso feathered generously (20 to 40 pixels), then apply a Gaussian blur of roughly the same radius. The colour evens out and the texture above is untouched. Or sample a good colour nearby with the eyedropper and paint at 10 per cent opacity with a very soft brush.\n\nThe discipline that keeps it looking real: on `low` you are *evening* rather than *flattening*. A face has legitimate tonal structure: the shadow beside the nose, the warmth on the cheeks. Blur that away and the face goes flat and mask-like, which is the other way this technique fails.\n\n## Where it breaks, and why\n\n**Hard edges.** At the jawline, where skin meets a dark background, the blur on `low` mixes the two. The `high` layer then holds a strong edge signature in compensation. Work on `low` near that edge and you get a visible halo: a pale or dark glow tracing the jaw.\n\nThe rule that follows: **never work on the low layer across a hard edge.** Stop the lasso short of the boundary, or mask the low-layer work away from it. If you must correct right up to an edge, do it on the original rather than in the separated stack.\n\n**Never clone across an edge on the high layer either.** Cloning skin texture over the boundary between face and background copies the edge signature with it, and you get a ghost of the jaw in the middle of the cheek.\n\n**Wrong radius.** Too small and the blemishes survive into `low`, so you are back to fighting them. Too large and the face's structure lands in `high`, where healing it produces strange results. The test: look at `low` on its own. If you can still see the spots, the radius is too small. If the nose has disappeared, it is too large.\n\n**It does not fix shape.** Frequency separation cannot move anything. A crooked nose, an awkward fold of fabric, a bulge in the wrong place is a warp problem, not a frequency problem.\n\n## Where it is actually used\n\nBeyond skin: product packaging with printed texture, fabric with a weave, wood grain, scanned paper, painted walls, and anything where you need to correct colour without losing surface. A dented cardboard box responds very well: the dent's shading is on `low`, the print on `high`, and you can flatten the shading while keeping the printing legible.\n\nIt is also the basis of **texture transplantation**: copy a patch of good texture from the high layer of one area and paste it over a destroyed area, then correct the tone separately on low. This is how a badly damaged region of a photograph is rebuilt.\n\n## The honest caution\n\nFrequency separation is powerful enough to make bad retouching invisible to the retoucher. The high-frequency layer preserves pores, so the result *reads* as skin even after the low layer has been smoothed into something no face does.\n\nTwo checks. Toggle the whole retouch off and on at 100 per cent: if the person has become someone else's idea of a person, reduce it. And view at the final output size, because at 400 per cent every pore is a decision and at 100 per cent almost none of them matter. Most over-retouching happens at zoom levels nobody will ever see the picture at."},"check":{"question":"A retoucher evens out a red patch on a cheek by lassoing it and blurring the low-frequency layer, but the lasso extends slightly past the jawline onto the dark background. A pale glow now traces the jaw. What caused it?","options":[{"text":"The lasso feather was too large, so the correction bled onto the background and lightened it.","correct":false,"tempting":"Identifies a contributing setting rather than the mechanism: even a hard-edged selection crossing the boundary produces the halo, because the two layers hold complementary edge information."},{"text":"Blurring across a boundary mixes skin and background in the low layer, while the high layer still carries the sharp edge signature, so the recombination shows a glow.","correct":true},{"text":"Linear Light blend mode amplifies any change on the low layer near high-contrast areas, which is why the effect appears only at the jaw.","correct":false,"tempting":"Names the correct blend mode and invents a contrast dependency: Linear Light recombines the two layers uniformly and does not amplify by local contrast."},{"text":"The Apply Image offset was set to 128 in a 16-bit document, which shifts values near the extremes and shows most at edges.","correct":false,"tempting":"Points at a real settings mismatch between 8-bit and 16-bit workflows, which would produce a global tonal shift rather than a halo confined to one boundary."}]},"prev":{"slug":"retouching-honestly","title":"Three tools that all remove a spot, and they are not interchangeable","url":"https://addaly.com/learn/image-editing/retouching-honestly"},"next":{"slug":"portrait-retouching-with-restraint","title":"Keep what is permanent, remove what is passing","url":"https://addaly.com/learn/image-editing/portrait-retouching-with-restraint"},"authorship":"Founder and series editor: Dr Mustafa Mun. Written by AI models to a written standard. Exam keys checked blind by a second model: 3,665 of 3,666 agreed. Academic review invited.","review":{"status":"Draft, open for academic review"},"cite":"Addaly (2026) 'Splitting texture from tone', in Editing an Image. Edited by M. Mun. Addaly. Available at: https://addaly.com/learn/image-editing/frequency-separation (Accessed: 13 October 2026).","licence":{"name":"CC BY-NC-SA 4.0","url":"https://creativecommons.org/licenses/by-nc-sa/4.0/"},"reader":{"schema":1,"title":"Splitting texture from tone","courseTitle":"Editing an Image","module":{"index":5,"slug":"repair-and-retouching","title":"Repair and retouching"},"answer":null,"keyIdea":"Frequency separation splits an image into a blurred tone layer and a neutral-grey detail layer that recombine exactly, so blemishes are healed on the detail layer where there is no tone to get wrong and colour is evened on the tone layer where there is no texture to destroy.","hero":{"kind":"figure","figure":{"t":"figure","label":"Figure","figure":{"type":"stack","title":"One portrait, split so each fault meets its proper tool","layers":[{"label":"high: set to Linear Light","detail":"Detail with no tone, held around neutral grey. Heal and clone here: there is no tone to get wrong, so a spot comes out without the pale disc."},{"label":"low: Gaussian blurred, 5 to 10 pixels on a 20-megapixel portrait","detail":"Colour and tone with no detail. Lasso generously and blur, or paint at 10 per cent opacity. Even it out; flatten it and the face goes mask-like."},{"label":"background: the original, untouched","detail":"The two layers above recombine to this exactly. The arithmetic is exact rather than approximate, which is what makes the split safe to work in."}],"caption":"Choose the blur radius by looking at the low layer on its own: if the spots are still visible it is too small, and if the nose has disappeared it is too large. Never work on the low layer across a hard edge: the blur has mixed the jaw and the background there, and the result is a pale halo tracing the face."},"alt":"Figure: One portrait, split so each fault meets its proper tool. Layers, top to bottom: high: set to Linear Light (Detail with no tone, held around neutral grey. Heal and clone here: there is no tone to get wrong, so a spot comes out without the pale disc.); low: Gaussian blurred, 5 to 10 pixels on a 20-megapixel portrait (Colour and tone with no detail. Lasso generously and blur, or paint at 10 per cent opacity. Even it out; flatten it and the face goes mask-like.); background: the original, untouched (The two layers above recombine to this exactly. The arithmetic is exact rather than approximate, which is what makes the split safe to work in.). Choose the blur radius by looking at the low layer on its own: if the spots are still visible it is too small, and if the nose has disappeared it is too large. Never work on the low layer across a hard edge: the blur has mixed the jaw and the background there, and the result is a pale halo tracing the face."}},"blocks":[{"t":"h2","id":"the-problem-it-solves","runs":[{"text":"The problem it solves"}]},{"t":"p","runs":[{"text":"Skin has two kinds of imperfection and they need opposite treatments."}]},{"t":"p","runs":[{"text":"Tonal unevenness","b":true},{"text":" (a red patch on a cheek, a shadow under the eye, blotchy colour across a forehead) is large and soft. You fix it by smoothing."}]},{"t":"p","runs":[{"text":"Texture faults","b":true},{"text":" (a spot, a hair, a fleck of mascara) are small and sharp. You fix them by replacing."}]},{"t":"p","runs":[{"text":"Smooth the whole image and you destroy the pores, which is what makes retouched skin look like plastic. Heal every blotch and you get a lumpy patchwork, because the healing brush matches texture and averages tone across an area that legitimately varies."}]},{"t":"p","runs":[{"text":"Frequency separation splits the image into those two layers so each can be treated with its proper tool and neither affects the other."}]},{"t":"h2","id":"building-it","runs":[{"text":"Building it"}]},{"t":"p","runs":[{"text":"The "},{"text":"low frequency","b":true},{"text":" layer is a blurred copy: colour and tone with no detail. The "},{"text":"high frequency","b":true},{"text":" layer is what the blur removed: detail with no colour or tone, held around a neutral mid-grey. Recombined, they reconstruct the original exactly."}]},{"t":"p","runs":[{"text":"In Photoshop or Photopea:"}]},{"t":"ol","start":1,"items":[[{"text":"Duplicate the background layer twice. Name the lower copy "},{"text":"low","code":true},{"text":", the upper "},{"text":"high","code":true},{"text":"."}],[{"text":"Hide "},{"text":"high","code":true},{"text":". On "},{"text":"low","code":true},{"text":", apply "},{"text":"Gaussian Blur","b":true},{"text":". Choose the radius by eye: large enough that the blemishes disappear, small enough that the face’s structure (the nose, the eye sockets) is still recognisable. On a 20-megapixel portrait that is usually 5 to 10 pixels."}],[{"text":"Show "},{"text":"high","code":true},{"text":" and select it. "},{"text":"Image > Apply Image","b":true},{"text":". Set Layer to "},{"text":"low","code":true},{"text":", Blending to "},{"text":"Subtract","b":true},{"text":", Scale "},{"text":"2","b":true},{"text":", Offset "},{"text":"128","b":true},{"text":". (In a 16-bit document, use Blending "},{"text":"Add","b":true},{"text":", tick "},{"text":"Invert","b":true},{"text":", Scale "},{"text":"2","b":true},{"text":", Offset "},{"text":"0","b":true},{"text":".)"}],[{"text":"Set "},{"text":"high","code":true},{"text":" to blend mode "},{"text":"Linear Light","b":true},{"text":"."}]]},{"t":"p","runs":[{"text":"The image now looks identical to the original, and it is: the arithmetic is exact, not approximate."}]},{"t":"p","runs":[{"text":"In GIMP the classic route needs no dialogue arithmetic. Duplicate twice, blur the lower copy, and set the upper copy’s mode to "},{"text":"Grain extract","b":true},{"text":" with the blurred one beneath it; merge those two and set the result to "},{"text":"Grain merge","b":true},{"text":" over the blurred layer. Same split, same reversibility. GIMP also ships "},{"text":"Wavelet Decompose","b":true},{"text":", which splits into several frequency bands at once, and the "},{"text":"G’MIC","b":true},{"text":" plugin has "},{"text":"Split Details","i":true},{"text":", which is the most flexible of all."}]},{"t":"h2","id":"working-on-each-layer","runs":[{"text":"Working on each layer"}]},{"t":"p","runs":[{"text":"On ","b":true},{"text":"high","b":true,"code":true},{"text":", use the Healing Brush and Clone Stamp.","b":true},{"text":" Spots, stray hairs, flecks. Because this layer holds no tone, healing here cannot leave a pale disc: there is no tone to get wrong. This is the single biggest practical gain of the technique."}]},{"t":"p","runs":[{"text":"On ","b":true},{"text":"low","b":true,"code":true},{"text":", use the Lasso and blur, or a soft low-opacity brush.","b":true},{"text":" Select the blotchy region with a lasso feathered generously (20 to 40 pixels), then apply a Gaussian blur of roughly the same radius. The colour evens out and the texture above is untouched. Or sample a good colour nearby with the eyedropper and paint at 10 per cent opacity with a very soft brush."}]},{"t":"p","runs":[{"text":"The discipline that keeps it looking real: on "},{"text":"low","code":true},{"text":" you are "},{"text":"evening","i":true},{"text":" rather than "},{"text":"flattening","i":true},{"text":". A face has legitimate tonal structure: the shadow beside the nose, the warmth on the cheeks. Blur that away and the face goes flat and mask-like, which is the other way this technique fails."}]},{"t":"h2","id":"where-it-breaks-and-why","runs":[{"text":"Where it breaks, and why"}]},{"t":"p","runs":[{"text":"Hard edges.","b":true},{"text":" At the jawline, where skin meets a dark background, the blur on "},{"text":"low","code":true},{"text":" mixes the two. The "},{"text":"high","code":true},{"text":" layer then holds a strong edge signature in compensation. Work on "},{"text":"low","code":true},{"text":" near that edge and you get a visible halo: a pale or dark glow tracing the jaw."}]},{"t":"p","runs":[{"text":"The rule that follows: "},{"text":"never work on the low layer across a hard edge.","b":true},{"text":" Stop the lasso short of the boundary, or mask the low-layer work away from it. If you must correct right up to an edge, do it on the original rather than in the separated stack."}]},{"t":"p","runs":[{"text":"Never clone across an edge on the high layer either.","b":true},{"text":" Cloning skin texture over the boundary between face and background copies the edge signature with it, and you get a ghost of the jaw in the middle of the cheek."}]},{"t":"p","runs":[{"text":"Wrong radius.","b":true},{"text":" Too small and the blemishes survive into "},{"text":"low","code":true},{"text":", so you are back to fighting them. Too large and the face’s structure lands in "},{"text":"high","code":true},{"text":", where healing it produces strange results. The test: look at "},{"text":"low","code":true},{"text":" on its own. If you can still see the spots, the radius is too small. If the nose has disappeared, it is too large."}]},{"t":"p","runs":[{"text":"It does not fix shape.","b":true},{"text":" Frequency separation cannot move anything. A crooked nose, an awkward fold of fabric, a bulge in the wrong place is a warp problem, not a frequency problem."}]},{"t":"h2","id":"where-it-is-actually-used","runs":[{"text":"Where it is actually used"}]},{"t":"p","runs":[{"text":"Beyond skin: product packaging with printed texture, fabric with a weave, wood grain, scanned paper, painted walls, and anything where you need to correct colour without losing surface. A dented cardboard box responds very well: the dent’s shading is on "},{"text":"low","code":true},{"text":", the print on "},{"text":"high","code":true},{"text":", and you can flatten the shading while keeping the printing legible."}]},{"t":"p","runs":[{"text":"It is also the basis of "},{"text":"texture transplantation","b":true},{"text":": copy a patch of good texture from the high layer of one area and paste it over a destroyed area, then correct the tone separately on low. This is how a badly damaged region of a photograph is rebuilt."}]},{"t":"h2","id":"the-honest-caution","runs":[{"text":"The honest caution"}]},{"t":"p","runs":[{"text":"Frequency separation is powerful enough to make bad retouching invisible to the retoucher. The high-frequency layer preserves pores, so the result "},{"text":"reads","i":true},{"text":" as skin even after the low layer has been smoothed into something no face does."}]},{"t":"p","runs":[{"text":"Two checks. Toggle the whole retouch off and on at 100 per cent: if the person has become someone else’s idea of a person, reduce it. And view at the final output size, because at 400 per cent every pore is a decision and at 100 per cent almost none of them matter. Most over-retouching happens at zoom levels nobody will ever see the picture at."}]}],"check":{"question":[{"text":"A retoucher evens out a red patch on a cheek by lassoing it and blurring the low-frequency layer, but the lasso extends slightly past the jawline onto the dark background. A pale glow now traces the jaw. What caused it?"}],"options":[{"text":[{"text":"The lasso feather was too large, so the correction bled onto the background and lightened it."}],"correct":false,"tempting":[{"text":"Identifies a contributing setting rather than the mechanism: even a hard-edged selection crossing the boundary produces the halo, because the two layers hold complementary edge information."}]},{"text":[{"text":"Blurring across a boundary mixes skin and background in the low layer, while the high layer still carries the sharp edge signature, so the recombination shows a glow."}],"correct":true,"tempting":null},{"text":[{"text":"Linear Light blend mode amplifies any change on the low layer near high-contrast areas, which is why the effect appears only at the jaw."}],"correct":false,"tempting":[{"text":"Names the correct blend mode and invents a contrast dependency: Linear Light recombines the two layers uniformly and does not amplify by local contrast."}]},{"text":[{"text":"The Apply Image offset was set to 128 in a 16-bit document, which shifts values near the extremes and shows most at edges."}],"correct":false,"tempting":[{"text":"Points at a real settings mismatch between 8-bit and 16-bit workflows, which would produce a global tonal shift rather than a halo confined to one boundary."}]}],"order":[3,1,2,0],"why":null},"prev":{"slug":"retouching-honestly","title":"Three tools that all remove a spot, and they are not interchangeable","minutes":11},"next":{"slug":"portrait-retouching-with-restraint","title":"Keep what is permanent, remove what is passing","minutes":9}}}