How a green screen becomes a mask
A chroma-key system does not recognize a person and erase everything behind them. It looks for pixels whose color falls within a chosen range—usually a particular shade of green—and marks those pixels as transparent. That map of transparent and visible areas is called a matte. The system then places another image behind the visible parts of the original shot.
In a simple setup, the rule is straightforward: green pixels become background; other pixels stay. More advanced keying adjusts how strongly each pixel is removed, so soft edges such as hair can blend into the replacement scene rather than appearing sharply cut out.
Why green, and why lighting matters
Green is often useful because it is distinct from the colors in most skin tones and is captured clearly by many digital cameras. But it is not universal: a blue screen can work better when the subject wears green or when the scene calls for it.
The screen must also be lit evenly. A bright patch, shadow, wrinkle, or color variation can fall outside the key’s target range, leaving visible green or a blotchy edge. Light bouncing off the screen can tint the subject’s hair or clothing; this unwanted color cast is called spill and has to be reduced during filming or corrected in the image.
The edge is the real test
The key has to separate fine details without eating into the subject. A loose color selection may leave a green fringe, while an overly aggressive one can make translucent hair, motion-blurred hands, or similarly colored clothing disappear. That is why careful lighting and a clean contrast between subject and backdrop matter: they give the system a clearer boundary to work with, making the replacement surroundings look less like a cutout.
