The original green screen
Exhibition shot courtesy of LACMA
"2001" was a hotbed of cinematic creativity, with 205 separate special effects shots. Aside from slit-screen, one of the film's other biggest innovations was its use of a technique called front-projection during the opening "The Dawn of Man" sequence. Shooting the footage in Africa would've been costly and time-consuming, so instead Kubrick opted for a soundstage shoot with a new front-projection technology that gave a crisper image than traditional rear-projection methods, allowing him to use a 70mm format. A large projector projected an image of the African plains, which was then reflected via a semitransparent one-way mirror onto a screen made of millions of glass beads. Since the beads only reflected light from one direction (same 3M technology used in highway signs), the projected images didn't show up on the actors and objects but only as background. The camera, then, filmed the composite image, with the projected scenery appearing realistically behind the actors. Location shoot averted.