A set extension is imagery that is used to extend the size and scope of the set on which filming took place. A common example of a set extension is a matte painting. You can see a set extension early in the movie Titanic, when passengers are boarding the ship. Docks with additional crowds were added behind the on-set actors, as well as cranes and buildings, making the scene look richer and more interesting. Set extensions like this can be found in almost all ‘big-budget’ movies.