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Minority Report

Jul 1, 2002 12:00 PM, by Ellen Wolff

Greenscreen shoot.

There's plenty of action in Minority Report, Steven Spielberg's future-noir film for Twentieth Century Fox/DreamWorks. But in addition to having Tom Cruise do death-defying stunts, the director wanted to reveal how Cruise's character is driven by an obsession over a long-lost son. To do so, Spielberg shows Cruise alone at night, reliving memories of his child by watching a “home-movie” that — in the year 2054 — is holographically 3D.

In one such scene, Cruise watches a “smart screen” on his wall, which shows his son on a beach. Suddenly, the child steps out of the screen and walks forward. From the perspective of Cruise's character, the boy appears to be 3D. Viewed from the side, however, it's a crude illusion — the boy is semi-transparent, trailing a streaky glow as he moves forward. On the screen, there's a child-shaped black hole where the image of the boy used to be.

CG model.

Industrial Light + Magic was given the directive to make the boy look “sort of like a half-luminous shell,” explains CG supervisor Barry Armour.

“It was a fine balance to make sure you know the boy isn't really there — if it looked too real, you might think it was the actual kid,” notes Armour. “He also had to be life-size in the projection because otherwise he'd look like a miniature.”

The ILM team decided to call upon recent CG developments that utilize image-based rendering. Visual effects artists have typically used image-based rendering to construct faux backgrounds from numerous still photos blended together, saving themselves the trouble of actually modeling and rendering 3D geometry. But as far as Armour knew, the approach hadn't been applied to constructing a moving character. He spoke with Steve Sullivan, a recent Academy Sci-Tech Award-winner who is versed in computer vision techniques and who had been developing photo-modeling tools for ILM.

“My academic background was about using silhouettes from camera images to build discrete object models,” says Sullivan, who solved the problem of constructing the moving character for the film.

Background screen.

Numerous cameras needed to surround the boy to capture a useful silhouette, and Sullivan had to write software that would “carve out” the boy's shape from a multitude of filmed images, using the intersecting outlines to derive a 3D polygonal form. Once the 3D shape of the boy was determined, filmed images could then be mapped onto that shape like texture maps.

Since the holographic effect was supposed to be of “home-movie” quality, Sullivan says that “it could look a little messy and weird.”

“Usually you want something that's very literal and precise,” Sullivan explains. “These techniques inherently have all kinds of artifacts. If you wanted them to really model something, you probably couldn't use them directly. But in this case, the artifacts were interesting, so we decided to try it.”

Finished composite.

ILM's approach involved building on stage what Sullivan calls “a cauldron of green.” This 360-degree green-screen stage — essentially the photo-modeling equivalent of a motion-capture stage — had a 10 x 10 foot area around which the boy could walk. Eight DVcams were set up — one high and one low at each corner — providing 360-degree coverage.

“Those cameras could be fairly low-res because the image quality was supposed to look fairly amateurish,” Armour explains. “We then had three main cameras that were high resolution, turned on their sides so that we could get a vertical image of a higher quality.”

Unlike motion capture, which produces data points that are used to animate a CG character, this approach produced footage from the 11 cameras running simultaneously.

“These sorts of techniques capture arbitrary shapes so you don't have to put markers on anything,” Sullivan observes. “The characters can just go out there and act and do what they need to do.”

“As long as the kid didn't move out of the view of any camera, he could walk 10 feet in any direction,” Armour adds. “Of course, it was hardly trivial to light and shoot on this stage. We had the biggest amount of green I've ever seen. We had cameras pointing in every direction, so we had to make sure that the light was not interfering with the capture of the images.”

To ensure that the cameras were rigged and synced properly for the photo-modeling shoot, ILM motion-capture supervisor Seth Rosenthal oversaw their placement.

While Sullivan's photo-modeling software was new, the task of blending the texture-mapped images onto the boy's 3D shape simply required writing RenderMan shaders.

“It was fairly straightforward,” recalls Sullivan. “Getting the proper level of translucency for the holographic effect was a matter of dialing things up or down.”

Sullivan notes that ILM's compositors, who use in-house compositing software, then had a lot of freedom to experiment with the textures that they mapped and wrapped.

The finished effect, observes Armour, had many “strangely nice” artifacts.

“From the side, you can see the edges of the character going back into the screen, like streaks or slit-scan effects,” he observes. “Having some of the artifacts show through the character actually gave it more of a three-dimensional look.”

For his part, Sullivan hopes that this photo-modeling effect is subtle enough to not distract from the poignant moment in the film.

“It is supposed to be a very personal moment between a guy and his lost kid,” Sullivan concludes. “I hope that still happens.”

Credit Roll

Director - Steven Spielberg
DP - Janusz Kaminski
For ILM:
Visual Effects Supervisor - Scott Farrar
CG Supervisor - Barry Armour
Software R&D - Steve Sullivan
Sequence Supervisor - Steve Braggs
Image Capture Supervisor - Seth Rosenthal
Match Mover - Brian Cantwell
— Compositing Supervisor - Scott Frankel

© 2008 Penton Media, Inc.