Chroma-keying/virtual studios
We’ve been using blue (or green) screen in film and television for a number of years. There are a few technical details you have to consider in your planning.
Why use blue?
The choice of blue as a colour is simply because there is relatively little blue in the reflectance of any human skin tone. And you don’t get many people with blue hair. Technically we don’t need to use blue – most modern chroma-keyers are capable of keying out any colour.
What happens?
The vision mixer looks at the signal as it comes in. Wherever it sees blue, it takes that bit out and replaces it with another video source. We are not talking brain surgery here.
Standard mistakes
Obviously the presenters’ clothes cannot contain the background colour. This includes gentlemen’s ties. The real difficulty is getting even lighting across the whole blue section, without getting too much blue light reflected onto the foreground.
Panning or zooming the foreground camera should only be done if that really is the effect you want. The background will appear to stand still.
How does chroma-keying differ from a virtual studio?
Virtual studios use chroma-keying, but additionally if you pan the foreground camera (the one with a person in front of the blue screen) then the background moves as well.
This is a substantially harder engineering trick than plain chroma-keying. For the background to move in a matched manner it has to be generated by a computer. That computer has to know the exact focal length of the lens on the foreground camera, its position on the floor, its height, when it is panning, how fast etc. It then has to calculate the theoretical background, up to thirty times a second.
Judicious use of chroma-keying can make even the smallest studio turn out an expensive-looking product. You do need sufficient space to light the background evenly without reflecting too much colour onto the foreground subject.