Keying — Luma / Chroma

Keying is the compositing technique that makes part of an image transparent so that another image shows through. A chroma key removes a chosen colour, usually green or blue, and works well when the background is evenly lit and the subject is well separated from it. A luma key removes areas according to their brightness, for example a black background behind white text. A linear key (fill and key), common for broadcast graphics, uses a separate key signal to set the transparency of each pixel. A green screen with uneven lighting produces spill, green light reflected onto the subject, and ragged edges that a keyer cannot fully correct.