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LibraryTrack 6 · Video

LESSON · COMPOSITING

Chroma key

Film a subject in front of one even colour, remove that colour in post, and put anything you like behind them. Weather maps, spaceships, a skyline at night.

STEP 01

Shoot

The subject stands in front of a green or blue backdrop lit as evenly as possible.

STEP 02

Key

Software picks that colour range and makes it transparent.

STEP 03

Combine

A second picture or clip is placed behind, frame by frame.

Why green, and why blue first

  • Blue and green are used because they sit furthest from skin tones.
  • The blue screen came first: RKO Radio Pictures developed it in the 1930s, combining actor and background frame by frame on an optical printer.
  • Television moved to green because weather presenters tended to wear blue suits, and blue clothing kept disappearing into the map.
  • The eye is most sensitive to green, so the green video channel carried the strongest signal and the least noise.

On set

  • Light the screen and the subject separately; an even screen keys cleanly.
  • Keep the subject a few steps in front of the screen so its colour does not spill onto them.
  • Nothing green on a green screen: clothes, props, jewellery.
  • Shoot at a fast enough shutter that hair and hands stay sharp.
  • Record the best codec the camera has; heavy compression makes edges hard to key.

History and the reasons for blue and green from the Wikipedia article on chroma key, checked 2026.10.16. On-set tips are working practice, not a standard.