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extension3.12 Use chroma key or green screening*

Although you may not recognise the term 'chroma-key', you will be familiar with the effect, as it is used in a range of contexts from TV weather forecasts to sci-fi movie blockbusters. The main character, presenter or superhero is filmed against a plain, evenly lit coloured backdrop that can be removed in editing to reveal another video behind. The weather presenter who appears in front of an animated weather map is actually standing in front of a large blue or green screen; Luke Skywalker in combat with Darth Vader on the edge of a precipice was shot on solid ground in the studio; real actor Bob Hoskins interacting with cartoon character Roger Rabbit only happened on film – in the studio he performed to thin air.

blue screen

For Chroma Key video the principal subject is usually filmed against a matte blue or green background because those colours are regarded as most unlike flesh tone, although any strong colour could be used. If blue or green clothing is worn by an actor that part of the character's body will appear to be invisible in the edit.

Green is increasingly replacing blue as the favoured background colour because digital cameras record greater detail in the green channel. It is essential when filming against a screen that lighting is as uniform as possible – non-reflective materials are best - and that the surface is absolutely smooth because even minor tonal variations caused by slight shadows or reflections will spoil the illusion since the replacement footage may not register in those areas.

The video below by Stargate Studios shows how green screens are used extensively in television and movie production, even in scenes which are not obviously using "special effects".

Serif MoviePlus allows you to remove the background colour of one video source and replace it with another to create these special effects during editing simply by dragging the Chroma Key effect thumbnail onto the selected clip or group of clips. The 'How to use blue-screen effects with MoviePlus' tutorial will help you use this type of effect.

* Chroma Key is also known by other terms such as: blue-screening, green-screening, keying, matting, colour keying, and colour separation overlay.

paperclip How to use blue-screen effects with MoviePlus

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