The Basics: What is a green screen studio and how does it work?

If you’ve ever watched a superhero drama, a sports programme, a news item or a weather report, then you’ve almost certainly seen the magic of green screen production in action. It’s one of the most widespread visual effects tools in the TV and film industries.

The technique enables productions to record actors or presenters in a studio in front of a plain green backdrop, then digitally replace the green background through a process called chroma key compositing (or chroma keying).

Green screens are widely employed not just for their ability to help productions save money, but because they offer so many creative possibilities. All of a sudden, a green background can be transformed into an interactive weather map, a bustling city or a stunning landscape. The set-up allows you to put your actors into any environment imaginable, to make a studio space appear much larger or to switch quickly and easily between sets.

Chroma key technology might seem cutting edge, but it was actually developed in the 1930s (initially with blue screens). Its first proper use was in 1940 feature The Thief of Bagdad for scene where a genie escapes from a bottle, which won the Academy Award for Best Special Effects that year. Its popularity then took off in the 1980s with the advent of affordable computer graphics.

A green screen is literally a large green backdrop, often a green cloth, that is hung in the studio. It’s important to make sure it’s as flat as possible with no wrinkles. The floor is also painted green. Productions either cover the whole set with a green backdrop or focus on specific areas, combining physical and virtual elements.

The term ‘green screen’ is a bit of a misnomer, as other colours could be used. As mentioned, blue was originally the most popular colour.

The most important thing is to use a colour that is different from the parts of the picture that you want to keep. This is why bright green is now so prevalent: human skin or hair doesn’t have any green elements so it’s easy for the chroma key software to distinguish between the actors and their background.

Green is also preferred to blue because actors and presenters often wear blue suits or blue jeans which would cause them to disappear along with the background.

To work in practice, chroma key software or hardware is configured to be able to recognise a specific shade of green in a filmed image, removes it and then automatically layers (or composits) a new image in its place.

The effect can be achieved live in a studio or in post-production after filming has taken place.

A chroma keyed image comprises three components:

• The background signal – as the name implies, this is the image that appears behind all the other images in the final output.
• The key signal – this is the signal that is generated by the keyer hardware or software from all the parts of the filmed scene that are green and is used to “cut out” a part of the background image to be filled by…
• The fill signal – this is the signal that appears super imposed over the background image although it’s actually inserted into the background signal at the appropriate points.

Getting a good quality key signal is important for achieving the best final output. We have all seen the results of a poor key signal, such as a presenter ending up with green fringing around their hair.  

At dock10 we have four factors that help us consistently achieve a good key signal: our engineers who know how to set up and calibrate our green screen systems hardware; our lighting teams who make sure the scene is well lit; our virtual studio operators who are skilled in manipulating the key signal to make sure it accurately cuts the hole for the fill signal; and the pixel accurate keying software that we use in our green screen studios.

All this means we can bring to life the most creatively ambitious scenes in our green screen studios – from the latest sport results right through to brining prehistoric dinosaurs to life on screen.

But… what happens when the camera moves?  Will background move too? No, it won’t. We have a range of capture systems to look after motion but that’s a story for another day.

Michael Lodmore is duty technology manager at dock10