What did they use as its mirror?

Chrome is nothing but reflections, and tracing every one through a modelled world costs a lot. In 1976 Jim Blinn and Martin Newell put the object "at the center of a large sphere on the inside of which a picture of the environment has been painted", and looked each reflection up by its direction; their room was painted in a paint program. At MAGI, Gene Miller wanted the real world: "I photographed a christmas tree ornament in the MAGI parking lot", and wrapped it onto Ken Perlin's "blobby dog (glitches and all)". Miller and Hoffman, 1984: "A silvered glass Christmas tree ball is nearly spherical and a good reflector." In 1991 the T-1000 reflected "a cube of still photos" (Dennis Muren), and Muren "muddied it up a little bit": "That's what anchors it into the shot."

Our own blobby dog (Blinn's summed clouds), our own painted room, and our own mirror ball reflecting our own drawn lot; no photographs, no frames from any film.

What would you hang a mirror ball in front of?

Sources: Blinn and Newell, "Texture and Reflection in Computer Generated Images", CACM 1976; Jim Blinn, "A Generalization of Algebraic Surface Drawing", ACM TOG 1982; Gene Miller, notes on reflection mapping (1999), and Paul Debevec, "The Story of Reflection Mapping"; Miller and Hoffman, "Illumination and Reflection Maps", SIGGRAPH 84 course notes; American Cinematographer, December 1991; Den of Geek, 2017.
Sounds: our own, made in code (earshot, CC0).