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Source Filmmaker filmmaking engine is the game engine itself, with a different interface to take advantage of all its features. When you make a "film," you are setting up characters, lights and cameras in a game space, and letting action happen in 3d, while recording. The cameras record and create the film. All 3d environments basically do this, but Steam has put a movie making engine onto their game engine. Here are some of the movies they have made with the engine:
Meet the Spy a game trailer for one of their games
Counter-Strike: Global Offensive Trailer Another trailer