Most common, perhaps, are set extensions: digital additions to actual performing environments. In the film "Gladiator", for example, the arena and first tier seats of the Roman Colosseum were actually built, while the upper galleries (complete with moving spectators) were computer graphics, composited onto the image above the physical set. For motion pictures originally recorded on film, high-quality video conversions called "digital intermediates" enable compositing and other operations of computerized post production. Digital compositing is a type of matting, and one of four basic compositing methods. The others are physical compositing, multiple exposure, and background projection, a method which utilizes both front projection and rear projection.