The same principles are involved in color television except that four camera tubes are incorporated inside each camera: one each for the basic colors red, blue, and green, and one black-and-white tube for use in aligning and resolving the three colors. In color television receivers, three cathode guns instead of one are used to scan the phosphor screen, electronically "mixing" the palette according to the distribution of hues in the televised scene.
De-Beaming
The electron beam scanning the photocathode screen in the television camera requires a certain strength, a certain amount of electric current, in order to reproduce the image completely with sharp definition and contrast. Controls on the camera called "gain control clippers" are provided to assure that the beam is receiving proper energy to reproduce the image. By deliberately starving the electron beam of its required current, highlight details are washed out of the picture, causing the image to be retained or smeared in the camera tube. Any motion occurring in the brighter areas of the televised scene will produce a lingering smear of the image similar to the phenomenon of retinal persistence in human vision, but slower and longer lasting. Accidental beam-starving often is noticeable in musical programs when brass instruments develop flaring jelly-like trails as they move. Deliberately causing and exaggerating this effect is known as "de-beaming" or "rolling off the beam."
In color television, beam energies can be controlled in any of the three primary color tubes inside the camera simultaneously or separately. This means that the smear will be in one or all of the three colors and their combinations. Thus a human face or figure can be made to have brightly-colored outlines or ghost images that seem to stick to the screen as the figure moves. In addition, the three color tubes can be deliberately de-aligned from the coordinating black-and-white tube, producing three separate color images moving together in time but spatially differentiated, as sometimes occurs accidentally in offset color lithography.
Keying and Chroma-Keying
The video equivalent of cinematic matting is called "keying." As in cinematography, the purpose is to cause one image to be inserted into another image so that the background image is effectively obscured by the insertion. Cinematic matting is mechanical whereas video keying is electronic. There are two basic methods of keying: "inlay keyed insertion" (static mattes and wipes), and "overlay keyed insertion" (traveling mattes). Inlay keying involves a picture tube displaying a plain white raster on its screen, which is seen through a transparent masking plate (or "cel") by a lens focused onto a photo-tube that triggers a switching circuit. We select part of Camera One's picture to be matted out and make an opaque mask (cardboard, etc.) to cover the corresponding area on the cel over the inlay tube's raster. The switching circuit automatically blanks out that area in Camera One's picture, allowing the rest to show through wherever the circuit "sees" the inlay tube's raster. Camera Two's picture is automatically inserted into the matted area. Numerous wipes are possible simply by moving a mask over the inlay tube's raster. These wipe masks may be manually or electronically operated. Or they can be photographed on motion-picture film, which is then run through a telecine projector whose video signal triggers the switching circuit.
In overlay (traveling matte) keying, the switching circuit senses the scale of grays in a televised scene. Clipper controls on Camera One are adjusted to select the particular gray-scale level at which a keyed insertion from Camera Two is desired. This level of luminosity is known as the "switching tone." If a white switching tone is selected, Camera Two's picture will be inserted into Camera One's picture wherever the circuit "sees" the switching tone or a lighter one. If a dark tone is selected, the insertion will be made wherever the circuit "sees" that tone or a darker one. The shape of the insertion is determined by the shape of the switching tone areas in the scene. There must be a marked tonal difference between the inserted subject and its surroundings for the switching circuit to operate effectively. For example: Camera One shoots a dancer in black leotards against a white backdrop; Camera Two shoots a striped pattern. If a white switching tone is used, the dancer will be seen against a striped background. If a black tone is used, the dancer's body will be filled with stripes and the background will remain white.
Ordinary use of keying as described here usually results in the same sort of unconvincing, tacky visual effects as are generally produced by traveling mattes in movies: that is, a scene in which two images are trying unsuccessfully to be one. The problem lies in general insistence on "clean" mattes. Tonal differences of at least fifty percent on the gray scale must exist between the subject and surroundings, otherwise the switching circuit reaches points where it cannot distinguish between forms. This results in image-breakthrough and ragged "fringing" of matted edges, destroying the desired illusion of "objectivity." In synaesthetic videographics, however, keying is employed purely for its graphic potential in design information. Since there's no attempt to create the illusion of a "foreground" figure being inserted into a "background" field, image-breakthrough and edge-fringing are no longer a problem. In fact, they are deliberately induced through a technique called "tearing the key."
If there is no second video source, all areas of a scene above a white switching-tone turn black and all areas below a black tone turn white. If the scene contains a wide range of gray-scale tones with little contrast a great deal of image-breakthrough and edge-fringing will occur to the point where one cannot distinguish between the two. Electronic metamorphosis has occurred. If the scene is a medium close-up of faces in low contrast and a white tone is used, all facial highlights will turn black while all lower gray-scale values will remain normal. If a black tone is used, facial shadows will flash white while lighter values reproduce normally.
If the clipper, or sensor of the gray-scale level, is adjusted up and down the scale instead of being left at one level, the result is a constantly "bleeding" or randomly flaking and tearing image. This is called "tearing the key." In the scene just described, this would result in a constant reversal of dark and light tones and a general disintegration and reappearance of the image. If a second video source is used, which happens to be the same image we're seeing, except through another camera, the result is a bizarre solarization effect of flashing outlines and surfaces, or a composite in which an image appears to be inside of itself.
Gray-scale keying is possible also in color television, flaring and intermixing colors based on their gray-scale luminosity. However, Chroma-Key, although limited in some ways, provides certain advantages in color video work. Chroma-Key does not sense gray-scale luminosity but rather color hues. Any combination of the red, blue, and green primary tubes can be selected as the keying hue. Whenever a background is a particular hue, it will be keyed out and a second video source will be inserted. Any combination of colors in the spectrum can be used, but blue is normally employed because it is most opposite to skin tones and therefore provides the widest margin for "clean" mattes. If a blue-eyed girl is in front of a blue background and the Chroma-Key is set for blue, "holes" will appear in her eyes into which any other video source— including another image of herself— can be inserted.
In July, 1968, WCBS-TV in New York featured the Alwin Nikolais Dance Company as part of its Repertoire Workshop series. The dance composition, Limbo, was designed especially for Chroma-Key effects and thus provides an excellent example of a certain approach to this technique. In one scene of Limbo a man is threatened by disembodied hands and arms. He is tossed aloft by them and, according to the program description, "all of life's little problems are thrown at him." To achieve this effect the principal dancer and the chorus were positioned in front of a blue backdrop, all on the same camera. The chorus members were dressed completely in blue except for their hands and arms. Using a blue Chroma-Key, this meant that wherever there was blue in the picture, the background camera shooting smoke would show through. Thus the hands and the principal dancer appeared to be floating through smoke clouds. At one point, the hands appeared to pull confetti and streamers out of nowhere and throw them in the air. The colored confetti was concealed with blue confetti covering the top of the pile. It was invisible until it was pulled out in the open.