May 2023

Green Screen Studio

Green Screen Studio

As the UK's leading television facility, dock10 has ten studios suitable for any size and style of show, ranging from 1,000 sq ft to the UK's largest multicamera studio at over 12,500 sq ft. Each studio can be set-up with either the traditional black drapes and shiny floor for a physical set or as a green screen studio where the set possibilities are literally limitless. But what exactly is a green screen studio and how does it work? Richard Wormwell, Head of Innovation at dock10, explains everything you need to know about green screen studios.

What does a green screen studio do?

A green screen studio, also called a virtual studio, uses technology to replace the physical surroundings with a digitally created photorealistic environment. This allows the presenters or actors to appear to be almost anywhere – from a room in a house to a football stadium to floating through space. Productions either replace the whole set or use the green-screen walls to replace specific areas, combining physical and virtual elements. The set-up allows you to put your actors in any environment imaginable, make the studio space appear much larger than it actually is, and to switch quickly and easily between sets at the touch of a button.

What is a green screen?

A green screen is literally a large green backdrop that is hung in the studio to form the background of a shot. Often a single green screen will be used, but in our studios all four walls, the ceiling and the floor can be ‘green screened’. Traditionally, specialists in the post production team would then edit the background, removing any green and replacing it with the desired digitally created background. This is an electronic process where a machine ‘looks’ for the specified colour in the picture (in this case green) and replaces any of this with a different picture.

Why are green screens green?

The term ‘green screen’ is a bit of a misnomer, as you could just as easily use a blue screen or any other pure colour. However, the important thing is that the colour you use must be different from the parts of the picture that you want to keep. Usually, we want to keep the actor in the foreground and change the background and this has driven studios to settle on using green. Human skin doesn't have any green elements, and presenters often wear blue suits or blue jeans, making green the colour that was least disruptive. There are other technical reasons to do with green providing a stronger signal, but the reality is any colour can be chosen if it is different to your foreground.

How does a green screen work?

In a traditional film, the green colour is replaced with desired content in postproduction, but in dock10's studios this can be achieved live in the studio using what is known as a keyer. This is a piece of hardware or software that enables us to superimpose one TV picture on top of another in real time. The vision mixing desks in all our studios have built-in keyers.

A keyer uses three input signals: the background signal is the image that appears behind all the other fill images; the fill signal is the image that appears on top of the background image; and the key signal is generated from the fill image by the keyer. Anything in the image that is green is converted to black and everything else is converted to white. When the key image is black the background image appears on the final output and when the key signal is white it's the fill signal that appears on the final output. Putting these three images though our keyer delivers the final output signal. A chroma keyer works in the same way except that the key signal is generated from the fill image.

How does a green screen studio handle multiple camera angles?

Our virtual studios use a green screen set-up with an important difference: that we track the camera positions. When a camera is operated (whether hand-held, on a pedestal, a jib, a rail cam or even a techno-crane) we know with sub-millimetre accuracy where it is within the three-dimensional space of our studio; and at 25 frames per second, we accurately read all six degrees of movement (pan, tilt, roll, forward and back, left and right, up and down) as well as lens focal length and focus depths.

We capture all this information through a small but wide-angled motion camera and LED sensor mounted on top of our broadcast cameras. These motion cameras point to the ceiling and send a constant infrared (IR) signal up to the studio grid where a series of randomly placed reflective stickers bounce the IR light back down to the motion camera. The camera's sensor reads the light hitting it and the time it's taken to travel, then by a process of triangulation we can correctly calculate the exact position of the camera in 3D space.

What happens during filming in a green screen studio?

Once we have a studio lit for a green screen production, a set of broadcast cameras all rigged with camera tracking devices, and a three-dimensional set built to run in a real-time renderer (a games engine) - all we need to do is combine these three different elements and run the signals though our broadcast infrastructure. Easy!

We start by feeding the broadcast cameras' tracking data and the virtual environment into a control interface that manages all the video and data signals; this software also allows us to generate the composite (combined) video signal using a keying technique similar to those used in high-end post production. By using a real-time imaged based keyer that runs on GPU processing power, we can produce spectacular results with contact shadows, transparent objects and sub-pixel details like hair. And because each camera is tracking all the other cameras, this allows us to generate dynamic ‘3D masks’ so that the cameras can move around the space - even right next to the presenters - without appearing on the finished broadcast. We can also ‘garbage mask’ areas that aren't lit or covered in green such as the lighting grid and the ‘fourth wall’ which allows cameras to offer seemingly impossible shots.

Where does the green screen content come from?

Existing sets created in any 3D modelling package can be imported into the system and a wide range of pre-made assets can be easily sourced, adapted and added into the design. Augmented reality can then add an extra twist.

What are green screen studios used for?

Anything from Hollywood blockbuster films to everyday weather forecasting. Green screen studios are often used for live news and sports programmes, and increasingly for entertainment shows and TV dramas. dock10 employs its green screen studio technology to create virtual sets for BBC Bitesize Daily, Match of the Day and the FIA GT World Championships.

What are the main advantages of using a green screen studio?

Green screen studios allow productions to create more imaginative sets, can help save money on transportation and storage, are more environmentally friendly, and are quicker to set up than real sets. Despite the set being virtual, you can still have multiple camera angles and superimpose additional images so realistically it is impossible to tell what is real and what is not. They allow shows to appear much larger, giving small sized studio spaces the impression of being a bigger space such as a full-sized stadium. It also gives productions the ability to develop, innovate and change their studio output more regularly allowing them to evolve and refresh their sets on an ongoing basis.