Via Gizmodo.com:

Before the computing era, ILM was the master of oil matte painting, making audiences believe that some of the sets in the original Star Wars and Indiana Jones trilogy were real when they weren’t. They were the work of geniuses like Chris Evans, Mike Pangrazio, Frank Ordaz, Harrison Ellenshaw, and Ralph McQuarrie.

Matte paintings are fake sets that—most of the times—used to be made with plexiglass and oil paint. The artists used oversized panels to create the necessary detail that the camera needed to fool the audiences when the film was projected over the large surface of the theater screen. The paintings were combined with live action filmed to match the perspective of the painting. If done well, the public would totally buy into the shot.

Replaced by digital matte paintings and 3D imagery, nowadays films don’t use oil paint matte paintings anymore.

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