A Draconian Test
Issue: Volume: 29 Issue: 1 (January 2006)

A Draconian Test

In Warner Bros.’ Harry Potter and the Goblet of Fire, the young wizard’s nemesis is a fire-breathing dragon with a spiky tail that can slap the teenager around, claws on its wings so it can walk, and the ability to fly as fast as Harry on his broomstick. It’s a “horntail” dragon, the most dangerous kind, and it stars in 145 shots, all created by Industrial Light & Magic, one of seven facilities that added digital wizardry to Harry Potter’s latest adventure.

“It was our main sequence,” says Tim Alexander, ILM visual effects supervisor. In addition, the studio created a hero shot of a magical boat (see “Out of Water,” pg. 40), the world-cup Quidditch match and stadium in the beginning of the film, and a few other one-off shots.

The dragon sequence begins in an open-air arena built inside a rock quarry. There, a large golden egg glows atop a boulder guarded by a fiery dragon tethered to a long chain. To win the Triwizards Tournament, Harry must retrieve the egg and find a clue for the next task. He enters the quarry timidly, and hides behind a rock. When he scampers toward the egg, the writhing dragon attacks with its spiky tail. The young wizard calls for his broom and flies out of the dragon’s reach. But, the dragon breaks free and gives chase. Harry heads for Hogwarts with the dragon blowing fire at his feet, and the action sequence takes off.

To build the CG dragon, ILM scanned a half-scale, 30-foot-long maquette with a 14-foot wingspan. Plates covered the dragon’s long tail. Little claws hung from its wings like tiny hands. Modelers at ILM then sculpted the CG creature using one Alias Maya unit to represent one foot in real space. They relied on displacement to create wrinkles on the leathery skin and wings and to create the small plates, but modeled the tail’s bigger scales.

An angry digital dragon chases Harry Potter (played by actor Daniel Radcliffe, who was filmed on a bluescreen stage) past a matte painting of the hills outside Hogwarts. (Images courtesy Warner Bros. Pictures.)

The modelers also created shapes that the animators could dial in and out to emulate the creature’s moving musculature. “We talked about doing flesh sims, but it was better to put the controls into the animators’ hands,” Alexander says. ILM’s cloth-simulation engine, however, animated the wings. To decide how thick to make the bat-like wings, character-look developers showed Jimmy Mitchell, the film’s overall visual effects supervisor, various simulations that used combinations of flexibility and springiness until he found a setting that he liked.

While the creature-development team modeled, textured, and rigged the dragon, animation director Steve Rawlins sorted through reference material, working with Mitchell to shape the dragon’s attitude.

“A dragon is one of those mythical creatures that you always want to give a different style,” says Rawlins. “He’s an obstacle, a threat to Harry; we had to make him threatening and powerful.”

Rawlins borrowed the predatory actions and flying styles from falcons, eagles, and hawks, and created animation tests for the dragon using, especially, the birds’ tendency to use their wings to change direction. But in one sequence, the dragon chases Harry across Hogwarts’ tiled rooftops, and hangs from Dumbledore’s tower like a bat. “To display its personality, we had to determine how upright and crouched the dragon would be,” Rawlins says.

Eric Wong, the creature development supervisor, created the rig-an elegant one, states Rawlins. “He made sure the bones pivoted from the right place, and gave us enough joints to control the dragon, whether it was flying or walking, without overburdening the animators,” he says.

Animators could move one wing, and the other would match the performance, or they could animate the wings separately by turning “symmetry” on or off; the wings moved with the upper body. “Symmetry worked from the shoulder to the wings and the fingers,” says Rawlins. “When he’s flying, a wing acts like a wing, but we also think of it like an arm. When he’s walking on his hands, he’s similar to any animal walking on all fours. He folds up his fingers, puts his knuckles down, and twists out his elbow.”

The animators used constraints to flip the dragon’s claws into position and to wobble the wrist, which provided a sense of weight. They also keyframed the tail spikes to move up or down during the dragon’s performance.

To help all nine animators give the animal the same type of performance, they started with a library of pre-made poses. “They could see a pose, click on it, and the dragon would go into that pose on that frame,” explains Rawlins. “It helped everyone get up to speed quickly.” The interface Wong developed for the rig used a picture of a dragon: Animators grabbed controllers on the picture to move the joints. Then, they moved the Maya animation into various software packages within ILM’s Zeno pipeline to see the high-res model and work with the enveloping (skin), textures, and simulations.

The wing was like a skin membrane that stretched from the fingers to the creature’s hip. Weighting the skin to move with the influence of the joints stretched the membrane, but the skin didn’t flap in the wind when the dragon flew or jiggle when it planted an arm on the ground. Cloth simulation added the rippling realism.

ILM created the dragon’s fire using a novel fire-simulation engine within the studio’s Zeno pipeline that had evolved from the ongoing Physbam collaboration with Stanford University.

“The cloth simulation was difficult when the dragon closed his wings,” says Alexander. “All that cloth had to go somewhere, and it penetrated and intersected itself in the folds.” To fix these problems, the crew used corrective shapes. “In most of the sequences, he’s flying; the tough situations were at the beginning, when he’s crawling on the rocks and blowing fire,” he adds.

To help sell the shots in the arena, rigid-body simulations caused CG rocks to crack, tumble, and fall away when the dragon planted its knuckles on the ground. They also made the chain snap, fall, and hit the rocks when the dragon took flight, and swung the piece of chain that hangs from the dragon’s neck.

Animators controlled the timing that started the fire: The dragon opened its mouth, took a deep breath, and exhaled a blast of flames from vertical slits at the base of its tongue. “We had shapes that flapped a bit, as if some force was coming through,” Rawlins says. “And, of course, he blinks his eyes and snarls.” Another simulation fanned the fire.

In many of the shots, the dragon chases a digital Harry Potter; that is, a digital double of actor Daniel Radcliffe. For the double, the crew scanned Radcliffe and created digital robes. “We could use the underlying body from the previous films, but he had aged enough that we needed a new scan,” says Rawlins, “and a new broomstick.”

Animators started with matchmove animation derived from the bluescreen plates of Radcliffe riding a motion-control base, and then blended the digital Harry into the live-action Harry in shots that were too dynamic to film. “The key to making the shots work was matching the long robes,” says Rawlins. “The cloth-sim guys did the simulation, and then we sculpted the cloth into the right frames for the transition.”

For the backgrounds, the crew worked in ILM’s Zenviro camera-projection software, creating 3D matte paintings of Hogwarts, a ravine, and a bridge from photographs. On the all-CG shots, the animators worked with the camera-layout team to make sure the camera moves matched the energy in the chase scenes. “Sometimes even the background plates were heavily manipulated to add energy,” says Rawlins. “We shot still elements of crowds cheering in the stands and tiled them together to give more energy to the shots.”

In the end, Harry defeats the dragon by using a familiar action/adventure trick: He flies through an arch too narrow for the dragon, and just like a helicopter chasing a car into a small tunnel, the dragon crashes into the bridge. The triumphant Harry flies back to the arena and captures the egg.

For its part, by creating a nasty fire-breathing dragon that pushed Harry into the quarry rocks, chased him through the air, and slithered after him across rooftops, ILM mastered its challenge as well.

Barbara Robertson is an award-winning journalist and a contributing editor for Computer Graphics World. She can be reached at BarbaraRR@comcast.net.

Visit www.cgw.com this month as Barbara Robertson details ILM’s cutting-edge fire- simulation engine used for the film.