Also known as, blue screen, chromakey, keying

Sometimes we want to separate an element of our picture and add it or composite it into another picture. In video we call this process keying. Think of a flying carpet in the sky. We would film the carpet on it's own, then cut it out (or key it out) from it's background and finally add it to a shot of moving sky.

Keying is created by filming something against a flat coloured background. Greenscreen software looks for all the pixels of that colour, or near to that colour, and makes them transparent, so you can see your chosen background behind

The great thing about this technique in education, is it allows things to happen in your film which are normally out of the question for logistical or safety reasons

The good keying list

  • Create a large area of keying colour.
  • Evenly stretched material (or smoothly painted wall).
  • Make it larger than you think (even for head & shoulder only shot).
  • Intense pigment colours are best.
  • Usually green or blue.
  • Make the background colour different from any colour in your subject. Sounds obvious, but you need to be vigilant - eg you decide to go with blue screen because the school uniform is green; then in the edit you see children wearing green badges.
  • If you have flexibility with your background colour, make it close to the colour of the new scene you intend to replace in the computer.
  • The further away the background is from your subject, the better (another reason for making the bg big).
  • If you can, have the background two metres or more away.
  • You want it far away so reflected light doesn't bounce back onto your subject, and you can light foreground and background separately.
  • And so shadows don't fall on your background.
  • Use a lot of even light.
  • You want: sharply defined bright colour not washed out video.
  • If you can, backlight your subject as this helps separation.
  • Manual focus on your subject.
  • Its difficult to do peoples legs (storyboard carefully and you won't need them)
Positioning of lights for green screen This is the ideal - less lights can work :-)

If you can't manage all the above, do as many as you can and see how you go. Run a test and see if it works well enough in your effects sequence.

Green screen before anything is removed

The green screen has now been keyed out (and the remaining bits of the studio you could see behind have been masked off)

The graphics to go behind the action

The two clips 'composited' together to make one

The shot now in sequence with a few other shots as it appeared in the short film 'Bad Conscience' created by young people at the DCA in Dundee