Definition: Chroma key; “Green screen”

Definition: Chroma key; 'Green screen' | Glossary entry

Chroma key

The technique of Chroma key photography (and Chroma key video/TV) is used to photograph a subject on a single green backdrop of uniform colour. In post production it is possible to select the subject in the image from the single colour and then cut out the subject. Then the isolated subject can be pasted into another background. The new background can be any other photograph.

Doing the post production work to isolate the subject and place it into the new scene is called ‘chroma key compositing’ or ‘chroma keying’.

The background colour in the photography of the subject is normally green. However, chroma blue is sometimes used if the subject is green. Any colour can be used, but green or blue colours are common as they differ strongly from human flesh tones.

The technique is most often used for portraiture, advertising and graphic art so that subjects are placed in location environments without needing to be on location.

In television, chroma key techniques are used extensively. An everyday example is weather forecasting. The subject (weather presenter) stands in front of a chroma key backdrop screen. The background is digitally ignored by the software which isolates the subject from the chroma key colour. In the transmission the two images (presenter and weather map) are blended into a single image in the broadcast version. Modern TV technique is highly sophisticated. Subject and background can be combined and broadcast in one live transmission.

Some TV programmes use the technique extensively with little use of scenes or props. The TV series Sanctuary  External link - opens new tab/page is almost completely a chroma key production.

Chroma key techniques are extensively used in professional photography to save location costs. It is also used in cinema, special effects work and action photography. The use of computer generated graphics and action is often used to create action shots where the subjects are filmed in the chroma key environment before being substituted into the computer graphics environment.

Software for doing chroma key compositing in digital still photography (and amateur videography) is widely available. However, simple techniques in quality photo-editing software (such as Photoshop) can be used to create still chroma key composites without purchasing specialised software applications.

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Damon Guy (Netkonnexion)

Damon is a writer-photog and editor of this site. He has run some major websites, a computing department and a digital image library. He started out as a trained teacher and now runs training for digital photogs.
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