Green screen works using Chroma key. This is when a single colour, distinctly different to what the person or people are wearing, is made transparent, allowing another image to be placed over it. A green screen is just that, a green screen, but without “Chroma Keying” it is not much use. Chroma Keying is the process by which a specific colour element (chroma) is removed from a video scene and replaced (keyed) with a different element. Essentially it’s the way video producers remove one background and replace it with another. You will have seen hundreds of examples of this in films, such as the kids cycling across the sky in ET, and in TV, as it’s the process used when you see someone presenting the weather in front of a moving map. For us at VirtualStudio.TV this is how we place our presenters within our 3D virtual studios to create Internet Videos and Virtual Events.
Green or blue screens have become the industry colour standard for Chroma Keying since it was invented in the 1940’s. Unlike other bright colours such as Yellow and Red, neither are found within any skin tone and this is very important. For effective Chroma Keying the distinction between what you want to keep (the presenter) and what you want to remove and replace (the green background) has to be made. Therefore using a green or blue screen means there is no chance of the background mixing with the skin tone of the subject. If they did mix the subject's skin would be partly covered by a background as the computer could not distinguish clearly between what to keep and what to remove. There are some more advanced technical reasons why people choose either green or blue screens in their productions.
In Harry Potter, many of the sets and props were real. However, green screen had been used when filming scenes such as quidditch, the train ride and the people in the paintings.
Green screen used correctly:
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