The combination of layers can be a physical or software-based operation.
Why is compositing used?
Compositing is the process or technique of combining visual elements from separate sources into single images, often to create the illusion that all those elements are parts of the same scene.
What is compositing application?
Compositing software is used in film and video production to combine multiple images and effects into a single image stream: for example, an actor filmed against a green screen may be combined with a computer-generated city and model robots also filmed against a green screen.