Evolution of compositing techniques
In physical compositing all the separate parts of the image are physically placed in front of the camera and are recorded in a single exposure. These elements have to be placed at specific distances from the camera so that they are all in sharp focus. The most common types of physical compositing are partial models and glass paintings.
Partial Models : These are typically set extensions like ceilings or upper stories of buildings etc. In order to cut costs on sets, filmmakers often build only part of the set, making up for the rest with models. A model built to match the actual set is placed closer to the camera so that it appears to be part of the set. The models have to large enough so that they can be placed at a far enough distance from the camera so that the model and the set far beyond are both in sharp focus.
Glass paintings : Large glass pane that is big enough to cover the camera frame is placed at a distance from the lens so that the pane is in focus. The entire set is painted onto the glass with gaps so that the background is visible through these gaps. While holding up the glass pane live action is shot through the glass and composited with the painted area. The painted glass can be an economic foreground or background, saving a lot of time and money that would have gone into on location shots or studio constructions.