Color transparency technique — a single color (green or blue) is keyed transparent and replaced with background footage. Foundation of green-screen compositing.
You set up a camera in front of a green or blue wall, your actor stands in front of it, and in editing, this color disappears — another backdrop, a storm, outer space appears in its place. This is Chroma Keying, and it works because the color itself becomes the selection criterion. The computer recognizes all pixels of this color and declares them transparent, allowing underlying layers to become visible.
On set, you need three things for clean keying: first, the right color — green stands out better against human skin and hair than blue, which is why it has been the standard for decades. Second requirement: even, hard lighting of the background. Shadows on your green screen are the enemy — they create green pixels with different values that cannot be cleanly keyed out later. Third: distance between the talent and the screen. The further away, the less green spill light reflects back onto the actor's clothing and hair — this spill is extremely annoying to eliminate later. Many DoPs work with at least 1.5 to 2 meters of distance.
In post-production, the keying itself is done using specialized software: Nuke, After Effects, or dedicated keyers like Primatte. The compositor defines a threshold — everything above this green intensity becomes transparent. This sounds simple, but it's an art. Keyed too aggressively, and you'll cut away hair or thin fabric edges. Too cautiously, and green remnants remain visible as fringes. Professionals use so-called matte cleanup techniques: Dilate, Erode, Color Correction on the keyed area — small operations that refine the edge image.
An important point: Chroma Keying is not the same as Green Screen Compositing. Keying is only the first step. This is followed by color and light corrections to marry the keyed element with the background — the talent must have the same luminance, the same hue as the environment into which it is inserted. A beginner's mistake is to composite a sharply lit actor in front of a dark blue lit background — this immediately looks artificial and detached. That's why professional sets work with targeted key light and kicker light to simulate spatial consistency long before editing begins.