The green screen

A green screen doesn't replace the set. It replaces a colour. All the work is making sure that colour only exists behind you.

The principle

The camera films a person in front of a single, even colour. The software finds every pixel of that colour and makes it transparent. Another image slides in behind. That's keying, or chroma key.

It's the same idea as the TV weather map, film special effects, or the background of any video creator.

Why green, and why sometimes blue

Green is the colour furthest from human skin, whatever its tone. And camera sensors have twice as many green pixels as red or blue: green is the most detailed colour, so the easiest to cut out cleanly.

Blue is used when the subject wears green, for night scenes, and gives less coloured reflection on skin. But it needs more light.

Absolute rule: no clothing, accessory or object in the colour of the screen. It would become transparent. Watch out for shiny things too, glasses, watches, jewellery: they reflect the screen.

The three laws of lighting

Lighting makes 80% of the result. Software can't save a badly lit screen.

1. The screen must be even. No darker corners, no creases, no glare. Two soft lights at 45 degrees on either side of the screen, rather than one in front.
2. Light the subject separately, with their own lights, as for any video.
3. Keep the subject away from the screen: at least 1.5 to 2 metres (5 to 6.5 ft). Too close, the green reflects onto shoulders and hair, that's spill, and the subject's shadow falls on the screen.

Camera settings

• Fast shutter speed (1/100 s or faster): motion blur mixes subject and green, and the key gets messy.
• Focus on the subject. A slightly blurred screen keys even better.
• Don't overexpose: a blown-out green becomes almost white.
• The highest resolution possible, and a lightly compressed format if the camera offers one.

The software

Free software is enough: OBS for live, DaVinci Resolve, CapCut or Shotcut for editing. The setting always goes in the same order:

1. pick the colour to remove with the eyedropper, on a mid-tone area of the screen;
2. raise the tolerance until the whole screen disappears, no more;
3. soften the edges a little;
4. turn on spill suppression, which removes the green reflection on the outlines.

Tips from people who do it every day

• Iron the fabric, or stretch it on a frame. Every crease makes a shadow, and every shadow is another shade of green. A wall painted matte green beats a creased cloth.
• Frame wide, crop later. If the subject goes off the screen, even by a finger, it's lost. Frame the shot, then mask everything outside the screen with a rough mask (garbage matte).
• Fine hair is the real test. A light placed behind the subject, towards the top of the head, separates hair from the screen and makes the key much cleaner.
• The final background decides the lighting. If the new background is a sunset, light the subject warm and from the side. A subject lit like an office, dropped onto a beach, fools no one.
• No green screen? A blue sheet, a plain wall, a clear sky behind you. The software accepts any colour, as long as it's even and absent from the subject.
• New AI tools cut out subjects without a green screen. They get hands and hair wrong when moving: the green screen stays more reliable as soon as you move.