color key

color key

(1) Also called "chroma key," it is a technique for superimposing one video image onto another. Widely used to place an interesting scene behind people such as a news reporter on TV, it is also used for creating special effects such as floating a car on the ocean.

The Blue or Green Screen
The foreground image is shot in front of a backdrop, which is a single, solid color, typically blue or green. When both images are combined, the background pixel takes precedence wherever a pixel of the solid color is found in the foreground image. If the foreground image is a person and the backdrop is blue, no articles of blue clothing can be worn or the background image will bleed through in those locations.

(2) (Color-Key) In prepress, a sample of printed output that is used as a proof for the customer and a guide for the printer. Marketed by Kodak Polychrome Graphics (originally developed by 3M and later Imation), it is made by exposing the CMYK negatives onto four acetate films which are developed. When overlaid on top of each other, they simulate the printed results. See Matchprint.