Short answer
A green screen works because it puts a colour behind your subject that almost nothing in the subject shares, so software can decide which pixels are background by colour alone rather than by guessing what a person looks like. The result follows the true edge of the subject, including every gap between strands of hair, which is why it still beats an AI cutout when the shot was planned. The whole job is won or lost in the lighting: an evenly lit screen, the subject well away from it, and no green light falling on the subject.
A green screen is not a trick and not a filter. It is a way of making a hard problem easy: instead of asking software to work out what a person looks like, you arrange the picture so that the background is a colour the subject does not contain, and then the decision is arithmetic.
This is the hub for everything about that on this site. It covers why it works, what makes it fail, and where it sits next to the AI background removers that have appeared in the last few years.
Why green
Three reasons, and none of them is tradition.
Skin is not green. Human skin, across every complexion, sits in a band of oranges and reds. Green is the furthest common colour from that band, which means a key set to remove green is unlikely to start removing a face. Hair, eyes and most clothing are also not green.
Sensors see more green. The colour filter over a digital sensor has twice as many green cells as red or blue, because human vision is most sensitive to green. The green channel is therefore the least noisy channel in the file, and a key built on the cleanest channel has the cleanest edge.
Green is cheap and bright. Green paint and green fabric reflect a lot of light, so a screen can be lit to a useful brightness without an enormous amount of equipment.
Blue screens exist for the obvious reason: when the subject contains green, whether a costume, a product or a logo, blue becomes the colour the subject does not share. Blue was also the film standard, because the blue layer of colour negative held the finest grain. Any colour can work in principle, and the practical choice is always “the colour least present in the subject”.
What a keyer actually does
For each pixel, a keyer measures how close that pixel’s colour is to the screen colour, and turns that distance into transparency: close means gone, far means kept, and the band in between is a soft edge.
That soft edge is the entire point. A pixel on the boundary of a strand of hair is a genuine mixture of hair and screen, and the right answer for it is genuinely part transparent. A keyer that decided each pixel was either in or out would give you the jagged, cut-with-scissors look that people associate with bad compositing.
The detail that separates a good keyer from a bad one is what “close to the screen colour” means. Measuring raw colour distance fails on shadows: a shadow across a green wall is a long way from bright green, so it survives the key and turns up in your cutout as a grey lump. Measuring colour relative to brightness fixes it, because dark green and bright green are the same colour with different amounts of light on them. Chroma key explained goes through the arithmetic properly.
The lighting is the whole job
Everything else is adjustment. If the screen is lit well, almost any keyer will produce a clean result; if it is not, no keyer will rescue it.
Light the screen separately from the subject. Two lights on the screen at 45 degrees from either side, aimed to overlap in the middle, gives an even field. The subject is then lit with their own lights, which are not pointed at the screen.
Get the subject away from the screen. A metre is a minimum, two or more is better. This is the single highest-value change in any setup: it stops green light bouncing onto shoulders and hair, and it lets the screen fall out of focus, which hides creases.
Do not overlight the screen. It wants to be evenly lit, not bright. A screen brighter than the subject throws more spill and can flare around the edges of the subject.
Pull the fabric tight. Creases make shadows, and shadows are the hardest part of a screen to key. Clamps, a steamer or a roll of paper instead of cloth all solve it.
Avoid shine. A glossy screen reflects your lights as hot spots, which are a different colour from the rest of the screen and key badly. Matt fabric or matt paint.
How to set up a green screen has the full practical version, including what to do in a small room where none of this is possible.
Spill, the thing nobody warns you about
Green light bouncing off a large green surface lands on whatever is in front of it. It shows along the tops of shoulders, under the chin, around the edge of an ear and right through any loose hair.
Spill is not a keying failure. The pixels really are greenish, because green light really did land on them. What a keyer has to do is subtract the screen’s hue from those pixels while keeping the brightness the camera recorded, which leaves hair looking like hair instead of a grey smear. Pulling the saturation down, which is what many tools do, removes the colour and the life together. Green screen spill covers preventing it and fixing it.
Judge a green screen shot before you take fifty of them
- Set up the screen and the subject and take one frame.
- Open it and look at the screen alone: is it roughly the same brightness across the whole frame, with no dark corners and no hot spots?
- Look along the top of the subject’s shoulders and around any loose hair for a green tinge.
- Look for a shadow of the subject falling on the screen.
You should see: An evenly lit screen, no visible shadow of the subject on it, and no obvious green edge on the subject.
If you do not: A shadow on the screen means the subject is too close. A green edge means the same thing. Dark corners mean the screen lights are too close or too few.
Photos are easier than video
Most green screen advice is written for video, where every frame has to work and the settings have to hold while somebody moves. A still photograph is a much gentler problem: one frame, one set of settings, and you can reshoot.
It is also a widely useful one. Product photography on a coloured backdrop, headshots that need to drop onto different brand colours, cutouts for a poster, stock imagery that has to sit on several backgrounds, stream avatars and thumbnails are all still images keyed from a screen.
If what you have is footage rather than a photograph, removing a green screen in any app maps the controls across CapCut, OBS, Premiere, After Effects and the rest. The tool on this site is built for the stills case: it keys a photograph at full resolution and hands back a PNG with a real alpha channel, which is what a slide, a document, a listing or a video editor wants.
When a green screen is not the answer
If the photograph already exists and was taken against a bookshelf, no key will help, and an AI background remover is not a compromise but the right tool. Models trained to separate a subject from its surroundings are now very good, and they work on pictures nobody planned.
The honest comparison is in green screen versus AI background removal. The summary is that a key wins on hair and on any semi-transparent edge, because it can see the backing through the gaps; a model wins on every photo that was not shot for it, which is most photos.
They also combine. If a keyed cutout has a small area where the screen did not reach, or a prop the same colour as the screen, painting that area back by hand is a minute’s work in any tool with a brush.
What to wear, and what not to
The screen decides what the subject cannot contain, and it is worth saying out loud before the shoot rather than after it.
Against green, avoid green clothing, obviously, but also avoid anything that reflects green: shiny fabrics, patent shoes, glossy plastic, metallic jewellery and glasses with strong anti-reflective coatings all pick up the screen. Fine detail at the edge of the subject is where a key is most delicate, so loose knitwear and heavily textured fabrics are harder than a plain shirt.
Hair is the classic difficulty and the classic reward. Flyaway strands key beautifully when the screen behind them is clean and evenly lit, because each strand’s pixels are a genuine mixture of hair and backing. The same strands are impossible when the screen behind them is in shadow. If the shot depends on hair, put a light behind and above the subject aimed at their shoulders, which separates the hair from the backing and gives every strand an edge.
Transparent and translucent things, a glass, a bottle, a veil, a pair of sunglasses, are worth a moment of thought. A key will handle them better than any model, because it can see the backing through them and work out how much is showing, but it will also key out whatever is visible through the glass. If the glass is meant to look like glass in front of the new background, that is exactly right. If it is meant to keep something behind it, shoot it against something other than the screen.
What to produce at the end
A PNG with an alpha channel, at the size the camera recorded, is the right master. From that you can composite onto any background at any time. Flattening onto a colour immediately is the common mistake: it throws away the soft edge, and the next person who needs the subject on a different background has to start again.
If the destination cannot handle transparency, produce the flattened version as a second file and keep the PNG. What is a transparent background covers which formats hold alpha and which quietly drop it.
The free way
Key a green screen photo in your browser
Drop a photo taken against green, blue or any other backing colour. The screen colour is found from the edge of the frame, shadows on the screen are keyed out with it rather than left behind, and the green cast on hair and shoulders is subtracted at the brightness the camera recorded. You get a transparent PNG at full resolution, with nothing uploaded.
Green screens and chroma keying: every guide
Why a coloured backing still beats a model on hair and glass, how to light one so the key is easy, what the sliders in every app are really doing, and how to get the green off the edges without turning hair grey.
- How to set up a green screen that actually keys cleanlyDistance, lighting, fabric and camera settings, in order of importance
- Chroma key explained: how software decides what is backgroundWhat a keyer measures, and why naive keyers leave the shadow behind
- Green screen spill: why the edges go green and how to fix itThe green rim: preventing it in the room and removing it properly
- Green screen or AI background removal: which to useWhere each wins, what each costs, and how to use both
- Removing a green screen in CapCut, OBS, Zoom, Photoshop and CanvaCapCut, OBS, Zoom, Premiere, After Effects, Photoshop and Canva
The tools for this: Key a green or blue screen out of a photo, Remove a background with no screen at all.
Questions
Why are the screens green?
Because green is the colour furthest from human skin, digital sensors record more green than red or blue so the channel is the least noisy, and green fabric and paint reflect a lot of light for the money. Blue is used when the subject contains green, and was the film standard before digital.
Do I need an expensive screen?
No. What matters is that it is evenly lit and not shiny, not what it cost. A matt green bedsheet pulled tight, a roll of green paper or a painted wall all work. A creased or shiny screen is the usual reason a cheap setup fails, and that is a lighting and tension problem rather than a fabric problem.
How far should the subject stand from the screen?
As far as the room allows, and at least a metre or two. Distance does two things: it stops green light bouncing off the screen onto the subject, and it lets you light the screen and the subject separately so neither one spoils the other.
Can I use a green screen in a photo rather than a video?
Yes, and it is easier than video because you only have to get one frame right. Photographers have used coloured backdrops for cutouts for decades. The tool on this site keys a photo and gives you a transparent PNG at full resolution.
Is a green screen better than an AI background remover?
If you can plan the shot, yes: a key follows the real edge wherever the backing shows through, which no model can match on hair. If the photo already exists and was taken in a kitchen, a model is the only option and a good one.
Why is there a green edge around my subject?
That is spill: green light from the screen bouncing onto the subject. Move the subject further from the screen, light them separately, and use the spill suppression in your keyer, which should subtract the green without changing how bright the pixel is.