Blue Screen Process

The Color Difference Traveling Matte System is the most flexible of all compositing techniques. It can be used with any pin-registered camera, and with normal unfiltered set lighting lamps. The only special requirement is that one must paint the backing an appropriate blue. The blue-screen traveling matte technique prior to 1959 had as its trademark a blue halo following all moving objects (and frequently non-moving objects). The Color Difference system eliminates the blue halo and provides nearly all the advantages offered by other compositing systems but without their disadvantages or limitations.

The Color Difference Traveling Matte System properly mattes rapid motion, smoke, glassware, water, fine detail, and so forth. It also permits an actor in the FG to move in, among and behind objects in the background scene. Further, the actor's shadow can be caused to fall realistically upon the objects in the BG scene even when that scene is in reality a miniature. No other compositing technique offers this range of flexibility.

The theory of the Color Difference system is based on colorimetry, and is stated as follows: (1) Excepting the colors blue and magenta, all colors have a blue content that is equal to, or less than, their green content. (2) All the remaining colors except yellow and green have equal blue and green content.

When the blue and green content of a scene is equal, the blue and green B & W separations will be identical. Thus, there is no need to make a blue separation to reproduce such colors as reds, flesh tones, all shades of pink, white, gray, and all saturations of cyan. Since the blue and green separations (for these specific colors) are identical, one would simply use the green separation twice; once as the green printing separation, and once as the blue printing separation.

When this select group of colors appears in the foreground of a blue-screen shot, the green separation has one unique difference as compared to the blue separation. Whereas the blue screen area is essentially clear on the blue

Fig. 4. Background scene to be combined with foreground action in Fig. 1.

Male Matte Bluescreen

Fig. 2. Female matte of action in Fig. 1; also called "matte master."

Fig. 3 Male matte of action on Fig. 1. (In practice, a print from film shown in Fig. 2.)

Fig. 4. Background scene to be combined with foreground action in Fig. 1.

Fig. 1. Action as filmed in front of plain (blue) backing.

Fig. 2. Female matte of action in Fig. 1; also called "matte master."

Fig. 3 Male matte of action on Fig. 1. (In practice, a print from film shown in Fig. 2.)

Fig. 5. The final composite print; Fig. 1 plus Fig. 4, via Figs. 2 and 3.

separation, this area is quite dense (black) on the green separation. Because of this density, the blue screen reproduces as a black screen when the green separation is substituted for the blue separation. Very little cover (female) matte is needed because of the high density on the green separation in the blue backing area.

A cover matte density of 0.6 to 0.9 is generally sufficient when using an excellent blue screen such as the rear-illuminated Stewart T-matte blue. The problem with the blue separation is that it is essentially clear in the blue back ing area and requires a very dense cover matte which rarely fits.

The green separation is an almost ideal replacement for the blue separation because of its high density (blackness) in the blue-screen area and because it has the correct density for all of the foreground colors except for yellow and green.

The green separation would be a perfect blue replacement if a way could be found to add a little extra density where green and yellow objects occur. The addition of this needed extra density for green and yellow is the function of the Color Difference matte. The Color Difference matte is otherwise a clear film except for a few spots of density where a yellow or green object existed.

The Color Difference matte is made by printing with blue light through a bi-pack consisting of the original negative and the green separation positive. The only areas that are simultaneously clear on both films are those areas that were green or yellow in the original scene.

When the Color Difference matte is laid over the green separation, and their combined densities are compared to the blue separation, they will be identical in all areas except the blue-screen area, which will be black instead of clear. Thus, the Color Difference matte together with the green separation area makes a perfect replacement for the blue separation. This "synthetic" blue separation is perfect because it has all the correct densities for foreground colors while remaining essentially black in the blue backing area.

The only limitation of the system as described is that it cannot reproduce colors in which blue content exceeds green content, e.g., blue and magenta. Desaturated blues (like blue jeans) reproduce acceptably.

When it is necessary to reproduce a saturated blue in the foreground, a green backing may be substituted for the blue one. While this is a common practice in video matting, it's harder to get a good result in film because the blue record (the grainiest of the three layers) must then be used twice. Good pure-green illuminators are not widely available.

Because all three separations (with blue being replaced with the synthetic blue) are essentially black in the blue-screen region there is no need to use high-contrast, high-density cover mattes. The mattes should be made on film stocks having essentially the same gamma as the B & W separations. The male matte should be transparent to the degree the subject was transparent and should be no denser than is necessary to just prevent print-through. Such semi-transparent mattes permit the reproduction of semi-transparent objects.

When it is practical to eliminate yellow and green from the foreground objects, it is possible to simply substitute the green separation for the blue separation and achieve the full flexibility of the Color Difference system.

When it is permissible to allow a reduction of saturation of yellow objects and a shift of green objects a little toward cyan, the blue separation can be made by a mixed blue/green exposure. The blue backing area will be quite dark. Actually, it is only one stop (about 0.3 density) below that of the green separation. The use of a slightly denser cover matte (increased about 0.3) is all that is needed to prevent veiling of the background. This mixed blue/green technique is a simplification and produces acceptable results when it is not necessary to reproduce saturated yellow or green.

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