How did computers first sneak into the movies, from Tron to Jurassic Park?

Slowly, and one very expensive frame at a time. The 1982 film Tron is celebrated as a milestone because it was one of the first to make extensive use of computer animation, but only fifteen to twenty minutes of the film used it, mostly digital terrain, patterns and vehicles such as light-cycles and tanks. The technology to combine computer animation and live action did not exist yet, so the computer sequences were interspersed with the filmed characters. One of the computers had only 2 MB of memory, so to limit the detail the team faded the distance into black, a process called depth cueing. Their motto was "When in doubt, black it out!"

Making each frame was slow. In the hardest sequences a single frame could take up to six hours, and there was no way to print it digitally on film, so a film camera was placed in front of a computer screen to photograph each one. Despite the groundbreaking effects, Tron got no Oscar nomination for visual effects. Its director, Steven Lisberger, speculated that voters believed the production "cheated by using computers".

Then came the 1990s. In Jurassic Park in 1993, the dinosaurs in 52 shots were computer graphics, including the final fight between the T. rex and the raptors. Some frames of the dinosaurs took two to four hours to render, and the T. rex in the rain took six hours per frame. When the stop-motion expert Phil Tippett saw the first T. rex test, he said, "I think I'm extinct." Spielberg gave the line to the characters in the script.

Two years later, Pixar's Toy Story became the first fully CGI feature film, and John Lasseter said that without Tron, there would be no Toy Story.