Chroma Key
A compositing technique that treats a specific color (typically green or blue) as transparent, allowing the background to be replaced with different footage or imagery.
Chroma key is a visual effects technique that isolates a subject by detecting and removing a specific color range, then compositing it over alternative footage. The term derives from "chroma" (color), as color information serves as the key signal for transparency.
Green screens are used because green is far from human skin tones in hue, reducing color contamination. Blue screens are an option for night scenes or when subjects contain green elements.
- Color difference method: Key signals are generated by computing channel differences. For green screens,
key = G - max(R, B)produces a grayscale matte defining transparency - Spill suppression: Reflected green light contaminating the subject is corrected through hue shifting or despill algorithms
- Edge refinement: Hair or translucent materials require soft-key techniques rather than simple threshold extraction
- Lighting requirements: Uniform illumination affects key quality. Luminance variation should be kept small
Digital workflows convert to HSV or YCbCr color spaces before keying for robustness against lighting variations. Deep learning matting can achieve foreground separation without colored backdrops, though chroma key continues to be used in controlled production environments.