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Green Screen | Blue Screen | Chroma Key | Colour Separation Overlay

jeremy | Thursday September 13, 2012

Categories: Hot Entries, Production Zone, Video Production, Staffroom, Equipment

The green screen process works by replacing a green (or blue) colour element - typically the background - in a picture with another picture, series of pictures or animations. In a simple scenario an interviewee will be videoed against a green background, which will be substituted with relevant pictures – a process which can be done very simply in editing with Avid, or Final Cut Pro or other editing software.

This process of electronically overlaying colour in a frame started in television in the late 1970s, and began to be extensively used when colour TV became the norm for broadcasters. In ITV it was known as chromakey and in the BBC as CSO (colour separation overlay).

In both cases a blue screen was used. BBC TV studios used long rolls of sky blue felt-like carpet to cover the floor, and walls of a set to create an all blue background.

Cinema had created the same effect using mattes, but this involved exposing the same frame of film twice –once with a cut-out matte of the area that was to be replaced, and again to shoot the matte sequences. Doing this electronically was used with dramatic effect in the Star Wars films using a green screen.

The term green screen is now universal in both TV and film. Hollywood films use green screen allegedly because so many of the actors wear blue jeans. Using a blue screen an actor wearing blue jeans could find his legs dissolve into a scene from somewhere else. Also the colours blue and green are furthest away from natural skin tones – unless of course you are a Doctor Who alien! This technique uses the colour of blue or green to act as a key for another set of pictures.

Green Screen Example

An example of how green screen can be used at a basic level is to gain control over the visual backgrounds to interviews. A wild life expert talking about elephants can be videoed against a green screen, and moving pictures of elephants added behind the contributor.

Presenter in front of a green...

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