Why do film sets use green and blue screens?
A weather presenter in a green suit would vanish into the map, which is why chroma key needs a colour nobody is wearing.
▶ Start the storyBy removing one colour and filling the gap with another picture. In chroma keying, one range of colour in the filmed footage is made transparent, so separately filmed background footage or a still image can be inserted in its place. The actor stands in front of a uniform backing, and everything of that colour is swapped for another picture. Weather forecasts do this every day: the presenter appears to stand in front of a map that is really a computer graphic, with only a green or blue backdrop in the studio.
Green and blue are chosen because they differ most in from any human skin colour, so the actor's face and hands are not swapped out too. The rule is that no part of the subject may match the backing colour, or that part is treated as backing.
Step 1: Film the subject
The actor stands in front of a uniform green or blue backing
Step 2: Pick the colour
That colour range is made transparent
Step 3: Insert the background
A separately filmed or computer-made picture shows through
Step 4: Fix the edges
Even lighting and no keep the cut clean
The rule has a funny side. In television, the blue screen of the movies was used out of habit, but weather presenters tended to wear blue suits, so green took over. Newscasters sometimes forgot the dress code, and a presenter wearing the colour of the background would seem to disappear into the .
Before bluescreen, filmmakers used a , a moving mask that blocks out the right part of the background in every frame, first with black backdrops. The first proper bluescreen travelling matte, in 1940, put a genie escaping from a bottle into The Thief of Bagdad, and the film won an Oscar for special effects.
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Recap
Chroma key removes a colour that is far from skin tones and fills the gap with another picture, so the subject must never wear the screen's colour.
💡 A trick to remember it · Whatever matches the screen goes see-through, so dress for the screen: no green on green.
Surprising fact · In Spider-Man, the two flying characters were filmed against different coloured screens so neither would lose part of a costume.
Sources (1)
No source, no claim. Every fact in this lesson (15 claims) cites at least one of these.