Matte painting

A matte painting is a special effects device.

Procedure
If action requires a larger, more elaborate, or simply different environment than is available for filming, said environment could be produced by creating a painting of the scene with a certain area left blank. The blank area is later filled in with the actual filmed footage, creating the illusion that the action is taking place within the painted environment.

The procedure evolved to include the use of photographs, and later digital artwork and CGI, in place of traditional paintings. The term "matte painting" is traditionally still applied, with the artists being referred to as digital matte painters.

Notable examples
Silence in the Library won a prestigious Video Effects Society award for its digital matte paintings by Simon Wicker, Charlie Bennett, Arianna Lago, and Tim Barter. It was the only visual effects award won by Doctor Who during the Russell T Davies era, despite several nominations.