Vision Mixer: Special Effects
The use of special effects facilities on switchers can improve the scope of production capabilities.
The wipe
Available on most switchers, this can best be described as one picture gradually replacing the original picture. The direction of entry can be horizontal, vertical, diagonal, circular, diamond shaped and so on. The ‘edge’ of the wipe can be sharply defined (hard) or electronically defocused (soft).
The split screen
This is a wipe which is frozen at some pre-determined point. The composite picture is made up from two picture sources.
Chroma key
This is a powerful production facility whereby part of one picture, known as the foreground source, is electronically inserted in another picture, known as the background source. The foreground subject is shot against an area of plain saturated colour backing, usually blue, and a suitable switching or keying signal, derived from the foreground camera, is used to operate a fast switch. When the foreground camera sees blue only, i.e. the blue background, the keying signal will be at a maximum; this results in the background source being switched to the output of the switcher.
Obviously the foreground subject must not contain any of the keying colour, otherwise spurious keying will result. The background picture may contain all colours since it is not involved in generating the keying signal. Good results with chroma key require careful planning and lighting.
Caption key
This facility enables white lettering, from black/white captions or an electronic caption generator, to be inserted into a picture, resulting in much greater clarity than using a superimposition.
Black edger or borderline generator
Even the use of caption key will not help the visibility of captions if they are inserted against a white or similar high luminance background. A black-edger puts a black edge around lettering to make it stand out. On some switchers the thickness can be independently varied on the horizontal and vertical strokes, producing a ‘drop-shadow’ three-dimensional effect and the edge may be selected to be any colour (hue, saturation and luminance).
Colour synthesiser
A colour synthesiser is able to produce coloured pictures from black and white graphics. Both the background and the letters can be selected to any hue, saturation and luminance.
Speed of wipe is set by rate at which wipe fader is moved on effects unit. All wipe pattern directions can be reversed. Typical patterns are shown here (1).
Split screen
By stopping a wipe pattern at an intermediate position, we achieve a screen that is bisected (split-screen) or an inserted area.
Here the picture from two sources (2&3) have been combined in a split-screen (4) using a horizontal wipe.
Chroma key
The subject stands before a blue backing (5). Its camera output (Camera 1) (6) is fed to the chroma key switch (7). Whenever blue appears in this shot, the switch operates and presents the background scene on Camera 2, instead (8). The combined composite shot shows the subject within the background scene (9). Note that the floor needs to be blue for long shots.