Introduction to Keying

Keying is a technique by which separate foreground and background images are combined together to create a single composite image, eg the TV weather.  This is achieved by recording the foreground against an evenly lit colored background.  This background is usually blue or green, however in theory any color could be used.

Blue was initially chosen as the key color as it does not appear at significant levels in flesh tones.  Green is used as an alternative for darker flesh tone and for when the foreground contains significant amounts of blue.  Green also has a higher luminance value than blue and thus requires less light to produce a satisfactory output.  Chroma keying, Bluescreen, Greenscreen and Color Separation Overlay are all terms used to describe this process.

Traditionally the blue background is a stretched self colored curtain or painted studio walls and floors.  Large quantities of lighting are required to light the curtain evenly (typically 1Kw per linear yard).  This light has to be placed several feet in front of the curtain to provide an even spread.  The actors therefore have to be placed even further forward of the curtain to avoid interaction with the curtain lighting and to avoid spill from the reflected light hitting the actors.  Considerable skill and time is required to achieve a well lit background, and the amount of background light used precludes the use of subtle foreground lighting.  The setup also generates a great deal of heat and consumes large amounts of electricity.

Using Chromatte there is no need to light the background at all.  All illumination of the backing is achieved using a ring of light emitting diodes (LEDs) attached to the lens of the camera.  The LiteRing only consumes 10 watts of power, yet still produces an even illumination of the cloth wherever the camera is framed.  Creases and folds will not affect the quality of the key due to the random arrangement of the glass beads on teh surface of the cloth.  Actors can be placed close to teh backing adn there is neglible spill generated by the screen.  Your foreground lighting can thus be as subtle as you like.  Changing the background color is simply achieved by changing the LiteRing on the camera.

The process of combining the foreground with the background can be achieved in two distinct ways.

Live Keying

Normally found within the TV studio environment, a hardware keyer is an electronic device that combines the foreground cameras output with that of the background image.  it may be a standalone unit or may be integrated within the vision mixer or switcher used by the studio.  In either case the controls will be similar however, manufacturers will describe these controls in slightly different terms.  A standalone keyer tends to have more operational controls than one that is integrated into a vision mixer, although modern trends in software based control has allowed switchers to emulate standalone keyers in complexity.  Keying is achieved in real time and the composite image is then broadcasted or recorded to tape.

Typical hardware keyers and mixers: Ultimatte, Crystal Vision, Darim, FOR-A.

Keying in Post Production

In this instance the foreground image is shot against the blue or green screen and recorded to tape.  The clip is then digitized via a computer and the keying process is achieved using software keyers.  These programs are almost always part of a larger more complex software package.  The package may be a Non Linear Editor such as Final Cut Pro or Premier Pro, or a dedicated compositor such as Adobe After Effects, Eyeon’s Digital Fusion or Shake.

The keyer component could be an integral part of the original program or it may be provided by third party manufacturers such as Reflecmedia’s mattenee.  and referred to as a plug-in.  Most controls within software keyers work in a similar way to their hardware counterparts but may be referred to in slightly different terms.  A quick look in the user guide should enable you to identify the controls.  To produce the final composite image the software usually renders the sequence from by frame, in some cases this action may be done in real time.

List of software keyer plugins and compositors: Ultimatte Advant Edge, Adobe After Effects, Eyeon Digital Fusion, The Foundry Key Light.