(Color Key) See prepress proof
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.
Green Screen Color Key
This Las Vegas scene was superimposed on every green pixel in the photo. Carefully look at the colors of the woman's blouse in both top and bottom images. The blouse was green and therefore picked up the blue sky in the background.