Along Came A Spider

In Paramount Pictures' Along Came A Spider, there's a car crash that will leave stunt drivers wondering, “How did they do that?” The shot shows a speeding Corvette careen off a cement wall and flip through the air. After crashing through a guard rail, the vehicle comes to a stop at the edge of a dam and dangles precariously above a roaring waterfall.

“My best friend is a stunt driver, so we had fun conversations about this,” remarks Rhythm & Hues' Bill Westenhofer, who co-supervised Spider's visual effects. What director Lee Tamahori wanted, Westenhofer explains, was a continuous action shot, so that no one could assume that filmmakers had used a dissolve of some kind. The only solution was a computer-generated car, complete with a digital driver.

“This Corvette was easily the most complex model we've ever had to make,” observes Westenhofer. “It's one thing to build a car and light it and quite another to smash it apart. This car was two million polygons, and several versions of it were needed to get to the final ‘damaged mode.’ We did have Viewpoint scan a Corvette, but that just got us the shell. We then had to model every little piece that flew off, and all the interior parts that were revealed by the crash. We got a General Motors book to model all of those.”

Rhythm & Hues also had to animate the car's passengers, he adds, “because there was no way anyone could do a stunt this dangerous.”

Rhythm & Hues used proprietary software and Maya for modeling and animation. The team also used Houdini for particle effects showing small car parts breaking up and sparks and smoke from a loose tire screeching across the road.

While Westenhofer is pleased with the photoreal appearance of the CG Corvette, he notes that the real challenge lay in creating the “virtual environment” in which the wreck takes place. Unlike typical composited shots, where a background plate is filmed and then CG is inserted to match a real camera move, this shot had no background plate. Instead, the environment — the road, trees, streetlights, and dam — was made by mapping photographs onto CG models.

This image-based rendering approach relies on the technique of photogrammetry. “If you take photographs of objects from known positions, you can use trigonometry to figure out the location of various points in space,” Westenhofer explains. “We went to the real location, the Cleveland Dam in Vancouver and took still photographs of everything around the dam from various angles. Since this sequence was shot anamorphic, we maintained the same image quality by putting the motion-picture camera on a dolly and shooting very short bursts that we could pull single frames from later. We walked the dolly along the dam and looked at the right side, the left side, and the center, and then we moved down several feet and did it again. We also hung the camera over the side and filmed straight down at the moving water. The only motion-picture photography in this shot is that water.”

Back at Rhythm & Hues' L.A. studio, Westenhofer's team constructed geometric models of various objects in the environment, using the location photographs as survey material. Architectural plans for the dam also aided the modeling process. Westenhofer notes that the geometry necessary for image-based rendering was fairly simple, since the details were in the photographs that the team texture-mapped onto the models. “We did have to create part of the fence in CG where the car hits it, but other elements like the street lamps and the trees are nothing but photographs,” he says. The team created the texture maps for a five-second camera move from 30 to 40 images. The maps hold up as the camera moves past them, Westenhofer notes, “because we have a really good tool for mapping images in perspective onto objects.”

Most of the shot was rendered in Ren, Rhythm & Hues' proprietary renderer, except for particles and smoke that were created in Houdini, which were rendered in RenderMan. To blend the waterfall footage with the texture-mapped dam, R&H used the company's proprietary compositing tool, Icy. “Instead of just taking the water from our footage, we also took some of the banks that we'd filmed along with the water,” says Westenhofer. “We then used a soft matte to blend that into the banks of our CG model. That preserved the real edge where the water met the side of the dam. Finally, we used Inferno for lens flare and final touch-ups.”

Westenhofer found more than one advantage of image-based rendering over the conventional approach to compositing CG with plate photography. “When you're working with a background plate,” he notes, “it's often hard to know exactly where things are, like bumps in the road. But we could exactly pinpoint where certain things were and add nice lighting effects.”

“It's so challenging to shoot plates for an invisible thing that will be added later,” he continues. “Our cameraman would have had to guess where this invisible car was going to be and then follow it through the air and over the side of the dam. With a car moving fast, it's hard to visualize that and make a camera move that looks like we're following the car at real speed. Then you have to make compromises to fit the CG into the way the plate was shot. Using image-based rendering techniques, you can literally redo any camera move with a little bit of cleanup. Whenever this approach makes sense, it's a great avenue to pursue.”

Director - Lee Tamahori
For Rhythm & Hues: Visual Effects Co-Supervisors - Bill Westenhofer, Bert Terreri

CG Supervisor - Liz Kupinski
Virtual Sets - Mike Sandrik
Car Lighting and Integration - Pascal Schappuis
Effects TD - Hideki Okno
Digital stunt people - Lisa Clarity
Modeler - Nicholas Imhos
Animator - Scott O'Brien