The Model 617CEC Video Chroma Keyer (Chromakey), Chroma Keyer, or Video Color Keying (Key) & Color Separation Overlay system, is a two-input, one-output, device that replaces a selected color in a "foreground" picture with details from corresponding areas of "background" picture. The video chroma key removes the selected color (chroma key blue screen or green screen, for example), or narrow range of colors, from the foreground image to "reveal" the background image "behind" it. This technique is also called video color keying, color key, blue screen chroma key, green screen chroma key, or color separation overlay. A switch allows selection of the foreground image only, or the keyed image consisting of elements from both foreground and background images. The Model 617GS Video Chroma Key, Video Color Key & Color Separation Overlay system can also freeze the background input so that it can continue to be used for keying indefinitly.
In use, a key color is first captured by pressing the unit's "capture" button while the foreground camera is viewing only the desired color. This color can be subsequently checked by pressing the "display" button. The unit's tolerance for error in the key color can be adjusted in terms of both phase and brightness. For instance, the color phase range control could determine that only a very tight range of green is acceptable for keying or that a wider range, possibly from blue through green to yellow is acceptable. Similarly, the luma range control could determine that the key color is acceptable whether bright or dark, or could determine that only a tight brightness range is to be keyed causing the unit to put in the background image where only slight shadows are cast in the foreground.