{"course":{"slug":"image-editing","title":"Editing an Image","url":"https://addaly.com/learn/image-editing","level":"beginner","track":"create"},"lesson":{"slug":"blend-modes-are-arithmetic","title":"Twenty-seven blend modes, six ideas","url":"https://addaly.com/learn/image-editing/blend-modes-are-arithmetic","position":13,"of":78,"minutes":10,"answer":"Every blend mode is a short per-channel formula, so white vanishes in Multiply, black vanishes in Screen, mid-grey vanishes in Soft Light.","keyIdea":"Every blend mode is a short per-channel formula, so white vanishes in Multiply, black vanishes in Screen, mid-grey vanishes in Soft Light, and setting a tonal adjustment to Luminosity separates contrast from the saturation shift that normally comes with it."},"body":{"format":"markdown","text":"## Each mode is a formula run per pixel, per channel\n\nThe blend mode dropdown is intimidating because it is a long list of names with no obvious logic. Underneath, each one is a short piece of arithmetic taking the pixel below (the **base**) and the pixel above (the **blend**) and producing a result. Learn six formulas and the list collapses.\n\nWork in 0-to-1 rather than 0-to-255 and the maths reads more cleanly.\n\n**Multiply**: `result = base × blend`. Anything times 1 (white) is unchanged; anything times 0 (black) is black. So **white is invisible in Multiply** and everything else darkens. This is the mode for shadows, for a scanned signature on paper, for ink on a mockup.\n\n**Screen**: `result = 1 − (1 − base) × (1 − blend)`. Multiply performed on the inverted values, then inverted back. **Black is invisible in Screen** and everything else lightens. This is the mode for light: flare, glow, fireworks on a night sky, smoke on black.\n\n**Overlay and Soft Light**: conditional. Where the base is dark they multiply, where it is light they screen. The result is that contrast increases around a pivot. In both, **mid-grey (128) is the neutral value** and leaves the base unchanged. That fact is the entire basis of the dodge-and-burn layer technique you will meet in the tone module: fill a layer with 50 per cent grey, set it to Soft Light, and painting slightly lighter or darker grey lightens or darkens the picture underneath with nothing destroyed.\n\n**Difference**: `result = |base − blend|`. Identical pixels give black. Useless as an effect and invaluable as a tool: stack two versions of a photograph, set the top to Difference, and anything that is not pure black is a place where they differ. This is how you align two frames precisely, and how you check whether an \"identical\" re-export is identical.\n\n**Colour and Luminosity**: these two split the image rather than combining it numerically. Luminosity takes the brightness of the blend layer and the colour of the base; Colour does the reverse. Set a Curves layer to **Luminosity** and it changes tone without the saturation shift that a tonal move normally causes. Set a retouching layer to **Colour** and you can repair a colour blotch without touching the skin texture underneath.\n\n**Hue and Saturation** modes complete that family and are used far less often.\n\n## Why the contrast modes shift colour, and the fix\n\nApply a strong S-curve and the colours get more saturated as well as more contrasty. That is not a feature somebody added; it falls out of the arithmetic. Steepening the curve moves each channel independently, and moving three channels apart from each other by different amounts is, by definition, an increase in saturation.\n\nIf you want the contrast without the colour shift, set the Curves layer's blend mode to **Luminosity**. The tone change goes through and the hue and saturation of the base are kept. This one habit fixes the commonest complaint about heavy editing (that the skin went orange) without a single extra adjustment.\n\n## Opacity and Fill are not the same control\n\nTwo sliders sit next to each other and most people never learn the difference.\n\n**Opacity** fades everything about the layer, including any layer styles attached to it. **Fill** fades the layer's pixels but leaves layer styles at full strength. Drop Fill to 0 on a shape with a drop shadow and you get a floating shadow with no object. It is a specialist control, and knowing it exists saves an argument with a file that is behaving strangely.\n\nFor most blend-mode work, Opacity is the dose control: pick the mode for the effect, then dial the strength.\n\n## The free tools all have this\n\nGIMP, Krita, Photopea and Affinity all implement the standard set, and most of them match Photoshop's results closely because the formulas are published. Krita has the largest list of all, including several that exist nowhere else.\n\nTwo real differences to watch for. GIMP since 2.10 offers both \"legacy\" and current versions of several modes, because it moved to computing them in linear light (the same issue from the gamma lesson) and the two give visibly different results for Overlay and Soft Light in particular. And **Soft Light is not one formula**. Photoshop, the W3C compositing specification and older GIMP each use a slightly different curve. A file blended in one application and opened in another can differ, subtly, in exactly those layers. If a layered file has to survive a handover, flatten the Soft Light layers or note what made them.\n\n## A short catalogue you will actually use\n\n- **Multiply**: shadows, ink, darkening a texture onto a surface.\n- **Screen**: light, flare, glow, dust and rain plates shot on black.\n- **Soft Light**: dodge and burn on a grey layer; gentle contrast; High Pass sharpening.\n- **Overlay**: the harder version of the same, and the standard for High Pass sharpening.\n- **Colour**: recolouring an object, fixing a colour cast on one region.\n- **Luminosity**: tonal adjustments without saturation drift.\n- **Difference**: alignment and comparison, then switch it back off.\n\n## The honest limit\n\nAll of these run on the stored, gamma-encoded numbers, per channel, with no knowledge of what the picture contains. Multiply is a reasonable model of a filter absorbing light and Screen is a reasonable model of two lights adding, but only in linear space: run on encoded values, as they are by default, they are approximations that look plausible rather than physical simulations.\n\nThe practical consequence is that a composite built entirely out of blend modes will rarely look truly photographic on its own. Blend modes get you to a result quickly; matching light, shadow, grain and edge quality is what makes it convincing, and that is the compositing module later in this course."},"check":{"question":"A retoucher applies a strong S-curve to a portrait and the skin becomes noticeably more orange as well as more contrasty. Setting the Curves layer's blend mode to Luminosity removes the colour shift. What does that reveal about the cause?","options":[{"text":"The curve was applied to the red channel only, and Luminosity mode redistributes the change across all three.","correct":false,"tempting":"Assumes a channel-specific edit: the shift appears with a composite RGB curve, and Luminosity does not redistribute anything between channels."},{"text":"Luminosity mode reduces the strength of the adjustment, so the colour change is still present but below the visible threshold.","correct":false,"tempting":"Treats a channel-separation mode as an opacity control: the tonal effect is delivered at full strength, which is exactly why the trick is useful."},{"text":"The monitor's profile exaggerates saturated skin tones, and Luminosity mode bypasses colour management.","correct":false,"tempting":"Blames display management for an effect that survives into the exported file and appears identically on any screen."},{"text":"Steepening the curve moved the three channels apart by different amounts, which is itself an increase in saturation, and Luminosity keeps the base image's colour while taking only the new brightness.","correct":true}]},"prev":{"slug":"masks-not-erasers","title":"The eraser is the tool that costs you the second revision","url":"https://addaly.com/learn/image-editing/masks-not-erasers"},"next":{"slug":"the-layer-stack-and-order","title":"The same three edits in a different order give a different picture","url":"https://addaly.com/learn/image-editing/the-layer-stack-and-order"},"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) 'Twenty-seven blend modes, six ideas', in Editing an Image. Edited by M. Mun. Addaly. Available at: https://addaly.com/learn/image-editing/blend-modes-are-arithmetic (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":"Twenty-seven blend modes, six ideas","courseTitle":"Editing an Image","module":{"index":2,"slug":"the-non-destructive-workshop","title":"The non-destructive workshop"},"answer":null,"keyIdea":"Every blend mode is a short per-channel formula, so white vanishes in Multiply, black vanishes in Screen, mid-grey vanishes in Soft Light, and setting a tonal adjustment to Luminosity separates contrast from the saturation shift that normally comes with it.","hero":{"kind":"plate","image":null,"alt":null,"caption":null},"blocks":[{"t":"h2","id":"each-mode-is-a-formula-run-per-pixel-per-channel","runs":[{"text":"Each mode is a formula run per pixel, per channel"}]},{"t":"p","runs":[{"text":"The blend mode dropdown is intimidating because it is a long list of names with no obvious logic. Underneath, each one is a short piece of arithmetic taking the pixel below (the "},{"text":"base","b":true},{"text":") and the pixel above (the "},{"text":"blend","b":true},{"text":") and producing a result. Learn six formulas and the list collapses."}]},{"t":"p","runs":[{"text":"Work in 0-to-1 rather than 0-to-255 and the maths reads more cleanly."}]},{"t":"p","runs":[{"text":"Multiply","b":true},{"text":": "},{"text":"result = base × blend","code":true},{"text":". Anything times 1 (white) is unchanged; anything times 0 (black) is black. So "},{"text":"white is invisible in Multiply","b":true},{"text":" and everything else darkens. This is the mode for shadows, for a scanned signature on paper, for ink on a mockup."}]},{"t":"p","runs":[{"text":"Screen","b":true},{"text":": "},{"text":"result = 1 − (1 − base) × (1 − blend)","code":true},{"text":". Multiply performed on the inverted values, then inverted back. "},{"text":"Black is invisible in Screen","b":true},{"text":" and everything else lightens. This is the mode for light: flare, glow, fireworks on a night sky, smoke on black."}]},{"t":"p","runs":[{"text":"Overlay and Soft Light","b":true},{"text":": conditional. Where the base is dark they multiply, where it is light they screen. The result is that contrast increases around a pivot. In both, "},{"text":"mid-grey (128) is the neutral value","b":true},{"text":" and leaves the base unchanged. That fact is the entire basis of the dodge-and-burn layer technique you will meet in the tone module: fill a layer with 50 per cent grey, set it to Soft Light, and painting slightly lighter or darker grey lightens or darkens the picture underneath with nothing destroyed."}]},{"t":"p","runs":[{"text":"Difference","b":true},{"text":": "},{"text":"result = |base − blend|","code":true},{"text":". Identical pixels give black. Useless as an effect and invaluable as a tool: stack two versions of a photograph, set the top to Difference, and anything that is not pure black is a place where they differ. This is how you align two frames precisely, and how you check whether an “identical” re-export is identical."}]},{"t":"p","runs":[{"text":"Colour and Luminosity","b":true},{"text":": these two split the image rather than combining it numerically. Luminosity takes the brightness of the blend layer and the colour of the base; Colour does the reverse. Set a Curves layer to "},{"text":"Luminosity","b":true},{"text":" and it changes tone without the saturation shift that a tonal move normally causes. Set a retouching layer to "},{"text":"Colour","b":true},{"text":" and you can repair a colour blotch without touching the skin texture underneath."}]},{"t":"p","runs":[{"text":"Hue and Saturation","b":true},{"text":" modes complete that family and are used far less often."}]},{"t":"h2","id":"why-the-contrast-modes-shift-colour-and-the-fix","runs":[{"text":"Why the contrast modes shift colour, and the fix"}]},{"t":"p","runs":[{"text":"Apply a strong S-curve and the colours get more saturated as well as more contrasty. That is not a feature somebody added; it falls out of the arithmetic. Steepening the curve moves each channel independently, and moving three channels apart from each other by different amounts is, by definition, an increase in saturation."}]},{"t":"p","runs":[{"text":"If you want the contrast without the colour shift, set the Curves layer’s blend mode to "},{"text":"Luminosity","b":true},{"text":". The tone change goes through and the hue and saturation of the base are kept. This one habit fixes the commonest complaint about heavy editing (that the skin went orange) without a single extra adjustment."}]},{"t":"h2","id":"opacity-and-fill-are-not-the-same-control","runs":[{"text":"Opacity and Fill are not the same control"}]},{"t":"p","runs":[{"text":"Two sliders sit next to each other and most people never learn the difference."}]},{"t":"p","runs":[{"text":"Opacity","b":true},{"text":" fades everything about the layer, including any layer styles attached to it. "},{"text":"Fill","b":true},{"text":" fades the layer’s pixels but leaves layer styles at full strength. Drop Fill to 0 on a shape with a drop shadow and you get a floating shadow with no object. It is a specialist control, and knowing it exists saves an argument with a file that is behaving strangely."}]},{"t":"p","runs":[{"text":"For most blend-mode work, Opacity is the dose control: pick the mode for the effect, then dial the strength."}]},{"t":"h2","id":"the-free-tools-all-have-this","runs":[{"text":"The free tools all have this"}]},{"t":"p","runs":[{"text":"GIMP, Krita, Photopea and Affinity all implement the standard set, and most of them match Photoshop’s results closely because the formulas are published. Krita has the largest list of all, including several that exist nowhere else."}]},{"t":"p","runs":[{"text":"Two real differences to watch for. GIMP since 2.10 offers both “legacy” and current versions of several modes, because it moved to computing them in linear light (the same issue from the gamma lesson) and the two give visibly different results for Overlay and Soft Light in particular. And "},{"text":"Soft Light is not one formula","b":true},{"text":". Photoshop, the W3C compositing specification and older GIMP each use a slightly different curve. A file blended in one application and opened in another can differ, subtly, in exactly those layers. If a layered file has to survive a handover, flatten the Soft Light layers or note what made them."}]},{"t":"h2","id":"a-short-catalogue-you-will-actually-use","runs":[{"text":"A short catalogue you will actually use"}]},{"t":"ul","items":[[{"text":"Multiply","b":true},{"text":": shadows, ink, darkening a texture onto a surface."}],[{"text":"Screen","b":true},{"text":": light, flare, glow, dust and rain plates shot on black."}],[{"text":"Soft Light","b":true},{"text":": dodge and burn on a grey layer; gentle contrast; High Pass sharpening."}],[{"text":"Overlay","b":true},{"text":": the harder version of the same, and the standard for High Pass sharpening."}],[{"text":"Colour","b":true},{"text":": recolouring an object, fixing a colour cast on one region."}],[{"text":"Luminosity","b":true},{"text":": tonal adjustments without saturation drift."}],[{"text":"Difference","b":true},{"text":": alignment and comparison, then switch it back off."}]]},{"t":"h2","id":"the-honest-limit","runs":[{"text":"The honest limit"}],"role":"limit"},{"t":"p","runs":[{"text":"All of these run on the stored, gamma-encoded numbers, per channel, with no knowledge of what the picture contains. Multiply is a reasonable model of a filter absorbing light and Screen is a reasonable model of two lights adding, but only in linear space: run on encoded values, as they are by default, they are approximations that look plausible rather than physical simulations."}]},{"t":"p","runs":[{"text":"The practical consequence is that a composite built entirely out of blend modes will rarely look truly photographic on its own. Blend modes get you to a result quickly; matching light, shadow, grain and edge quality is what makes it convincing, and that is the compositing module later in this course."}]}],"check":{"question":[{"text":"A retoucher applies a strong S-curve to a portrait and the skin becomes noticeably more orange as well as more contrasty. Setting the Curves layer’s blend mode to Luminosity removes the colour shift. What does that reveal about the cause?"}],"options":[{"text":[{"text":"The curve was applied to the red channel only, and Luminosity mode redistributes the change across all three."}],"correct":false,"tempting":[{"text":"Assumes a channel-specific edit: the shift appears with a composite RGB curve, and Luminosity does not redistribute anything between channels."}]},{"text":[{"text":"Luminosity mode reduces the strength of the adjustment, so the colour change is still present but below the visible threshold."}],"correct":false,"tempting":[{"text":"Treats a channel-separation mode as an opacity control: the tonal effect is delivered at full strength, which is exactly why the trick is useful."}]},{"text":[{"text":"The monitor’s profile exaggerates saturated skin tones, and Luminosity mode bypasses colour management."}],"correct":false,"tempting":[{"text":"Blames display management for an effect that survives into the exported file and appears identically on any screen."}]},{"text":[{"text":"Steepening the curve moved the three channels apart by different amounts, which is itself an increase in saturation, and Luminosity keeps the base image’s colour while taking only the new brightness."}],"correct":true,"tempting":null}],"order":[2,1,3,0],"why":null},"prev":{"slug":"masks-not-erasers","title":"The eraser is the tool that costs you the second revision","minutes":10},"next":{"slug":"the-layer-stack-and-order","title":"The same three edits in a different order give a different picture","minutes":9}}}