Step by Step: Hero

Few films can boast as many stunning visuals as the Chinese production Hero, an epic tale from writer-director Yimou Zhang (Raise the Red Lantern). One of the film's most memorable visual effects sequences portrays a fantastical sword fight between actresses Maggie Cheung and Zhang Ziyi, in which the red-robed women swirl and parry within an autumnal forest of falling yellow leaves. It's also a prime example of Zhang's otherworldly storyline because the sequence reveals the enchanted powers of Cheung's character — with the point of her sword she can whip the falling leaves into a whirlwind and use them as a weapon to defeat her opponent. When a drop of the loser's blood touches the ground, the entire forest turns from yellow to red.

The challenge of creating this supernatural whirlwind fell to Animal Logic in Sydney, Australia. “The forest fight was shot on location in Mongolia,” co-visual effects supervisor Andy Brown says. “Since the yellow leaves last only a week in this forest, the director got an update on the phone every day and as soon as they turned yellow, he shipped the whole crew there.”

For some sections of the fight, Brown notes, “They had about 20 guys sitting above the camera with a net from which they would drop leaves. Industrial-strength fans blew the leaves around. But as soon as Maggie's character starts to command the leaves with her sword, it had to be all CG.”

Back at Animal Logic the first step was to clean up the plate photography because the actresses were flown on wires throughout their duel. Although the studio is primarily a Windows environment, they did wire removal in Apple's Shake running on Linux. “We also had to do a digital grade on the background to boost the golden color,” Brown says. “And in places where the forest looked sparse, we thickened it up by adding matte-painted trees done in Adobe Photoshop.”

Despite the supernatural nature of the leafy whirlwind, the task facing the animation team was to make the CG leaves behave as believably as possible. “It was created as a particle simulation in Alias' Maya,” explains Brown. “We had to create a simulation that could be directed to a point, so that our lead animator Jeff Renton could actually control it. The leaves had to follow the path of the sword as she whipped them up from the forest floor. It had to suck in falling leaves as well.”

The sword wielded by Cheung was hand-tracked. “Because this was locked-off we tracked it by eye. Then we attached a Newtonian field that attracts particles that are near it, and they all follow the tip of the sword. We had a vortex field as well that whipped the leaves around, and Renton could switch these fields off and on. After the sword got to a certain point we could drop the particles off; they would detach themselves and continue falling,” he says.

Because these leaves were swirling around the two actresses, the Animal Logic team had to rotoscope dummy human shapes that were roughly the same size as the two actresses in each frame. These were used as collision objects for the particle system-driven leaves to react against.

The studio's research and development team created proprietary plug-ins to make this simulation feasible. Chris Cooper, 3D technical director, created a CC curve field tool that allowed Renton to manipulate particles along a spline in Maya by attaching a field to it that could push particles along and around the spline. A script written by production programmer Justen Marshall also allowed the animator to work with a manageable amount of leaves. Once the animation was locked, Renton could run the simulation with up to 250,000 particles over multiple machines. Then a RenderMan procedural written by Marshall enabled Animal Logic to attach and instance one of 10 textured leaf polygons to each particle. That provided the option of rendering a specified amount of leaves taken from the original simulation.

The leaves' appearance was modeled after leaves shot on location, as well as on some actual samples brought back to Australia from Mongolia. “Unfortunately,” says Brown, “they changed color after they were radiated by customs!”

Given the large number of CG leaves, they were rendered in different passes when possible. “We'd set up the simulation into background, mid-ground, and foreground planes. Sometimes, because we had dummy objects to get collision, we rendered out the whole thing as one,” Brown explains. Animal Logic used RenderMan-compatible Air software from SiTex Graphics.

Compositing the various layers of rendered elements was done in Shake. “It was pretty tricky,” Brown recalls. “The ground leaves and the falling leaves were separate passes, and sometimes we had a stunt double in there, so we had to make sure her face was covered with leaves too.”

The director's final signature touch was the intense color change of the forest from yellow to red that capped the scene. Zhang wanted it to be both beautiful and ominous. “When the forest turned red the director said more, more,” says Brown. “So we just cranked it up.”

Striking a balance between natural beauty and supernatural color was a task made even more challenging by language barriers. “We relied heavily on putting sketches in front of the director,” says Brown. “[Zhang] knew exactly what he wanted.”

Director - Yimou Zhang DP - Christopher Doyle Action Choreographer - Tony Ching Siu-Tung For Animal Logic: Senior VFX Supervisor - Chris Godfrey Visual Effects Supervisors - David Dulac, Andy Brown Executive Producer - Murray Pope Producer - Fiona Crawford Coordinator - Danielle Rubin 3D Leads - Ian Brown, Lindsay Fleay 3D Animators - Jeff Renton, Patricio Ducaud, Wayde Duncan-Smith, Howard Fuller, Tim Gibson, Ray Van Steenwyk Senior Compositors - Aaron Barclay, Sonia Calvert, Saki Mitchell, Charlie Armstrong, Krista Jordan Assistant Compositors - Dan Breckwoldt, Andrew McDougall, George Zwier, Pheng Sisopha Concept/Matte Artist - Rowan Cassidy Head R&D Programmer - Chris Bone Production Programmer - Justen Marshall 3D Technical Director - Chris Cooper