I’ve been saying for years that the best CGI scenes are the ones you don’t notice.

Almost exactly 18 years ago I wrote a Reel.com piece about a sleight-of-hand car crash sequence in Steven Soderbergh‘s Erin Brockovich. No one spotted it at first, and yet, if you thought about it, the scene had to involve exceptional trickery because a single uncut shot showed Julia Roberts getting into her beater, driving toward an intersection and getting totally rammed and spun around by another car. Roberts would’ve obviously been badly hurt if she’d been driving and yet she wasn’t. So how did they pull it off?

The shot was achieved by seamlessly blending two pieces of footage — one of Roberts getting into the crummy-looking car and slowly driving off, and a second starring a Roberts dummy (wig and all) behind the wheel only this time in a radio-controlled duplicate of the same car. (Or it was all one car — what do I know?) The real Roberts transitions into the Roberts dummy right around the 41-second mark, or just as her car’s structural divide section (i.e., the metal section between the front passenger window and the back seat) passes in front of the camera.

An excerpt from my original article: The alchemist was Tom Smith of Cinesite, a leading visual-effects company known for creating certain highly convincing illusions in Wild Wild West, Message in a Bottle, Armageddon and For Love of the Game.

“It’s what we call the A and B side of a shot,” Smith explains. “The first shot is of Julia getting into the car, starting it up and driving into traffic. Right when the camera is looking at her as she passes by, we stretched and blended this with footage of the Julia dummy behind the wheel of the radio-controlled car. The transition happens over a small number of frames, then we stayed with the dummy car all the way until the accident.”

Viewers might be able to notice the transition between the two Julias and the two cars when they watch this scene next fall on the DVD, says Smith. “There are very subtle differences,” he says. “The Julia dummy has a couple of hairs in the wrong position, but you can barely tell.”

They did the photography in L.A.’s San Fernando Valley last summer, on a Saturday. Roberts’ car pulls away from a curb on Victory Boulevard near Lankershim. The collision happens at the Lankershim traffic light. Roberts did her photography in the morning, and the dummy and the radio-controlled car in the afternoon. A stunt driver — a guy — drove the “attack” car on Lankershim at around 35 mph. He hit Roberts’ car harder than the crew expected and nearly pushed her car onto a nearby sidewalk.

“When that happened it was like, whoa!” says Smith. “I was saying, ‘Yup, that’s real enough! I can believe that!'”

Smith is justifiably proud that the motion-control “blending” is almost impossible to spot. “I have a sort of well-known editor friend who saw my name on the credits,” he says. “And called me and said, ‘What did you do on it?'”

Smith and his Cinesite team finessed that shot of Will Smith hanging underneath the train in Wild Wild West. They engineered the “destruction of Cairo”scene in The Mummy. They added extra wind and rain to the shot where Kevin Costner dives into the ocean in the middle of a storm to save a drowning woman in Message in a Bottle. They added several thousand spectators watching Kevin Costner pitch at Yankee Stadium in For Love of the Game.

The Brockovich job came about because Soderbergh had used Cinesite on a job for The Limey, which he directed last year. Cinesite was hired to digitally clean up footage from Ken Loach’s 1967 film Poor Cow, which was used as a flashback device in conjunction with Terence Stamp‘s lead character.