Extract color or tone range from footage — typically green or blue screen matting. First step in compositing before layering elements, requires clean, even lighting.
You're standing in front of a green screen, the talent is in front of it, and you know: this will be keyed out later. Keying is precisely this process — isolating a specific color (usually green or blue) from your footage to make it transparent and place a new environment behind it. It's not just chroma key in the narrow sense; keying encompasses any technique where you extract a tonal value or a color channel as cleanly as possible.
On set, you quickly realize: keying doesn't start in the edit. Evenly lit green screen, consistent lighting on the talent, distance from the background — all of this influences how clean your key will be later. In compositing, you'll use common tools like Ultra Key, Keylight, or proprietary solutions from color grading software. You define your key color, increase tolerances until the green screen is gone without dissolving your talent. The subtlety lies in the edges: hair and shaders require delicate treatment, otherwise the transition to the new environment will look pasted on.
Practical Pitfalls: Green reflections on skin and hair need to be corrected separately. Transparencies in hair quickly become a nuisance — this is where rotoscoping or tracking-based corrections come in after the key. With older cameras or compressed codecs (H.264), you quickly lose color information; then keying becomes detailed work. The greater your contrast between the key color and the object, the cleaner the automatic key. This is why bluescreen works better with skin tones than greenscreen — less overlap in the color channel.
Also distinguish: luminance keying (by brightness) works with black or white backgrounds; difference keying requires two passes of the same shot (with and without the object) and is extremely precise but complex. In modern workflows, you often layer multiple keys — an aggressive one for the center, subtler ones for the edges — or combine keying with manual roto. The finished key is just the base; post-processing (despill, edge feathering, color correction) determines if it looks natural.