Dark Magic
Issue: Volume: 30 Issue: 8 (Aug 2007)

Dark Magic

ILM creates two digital creatures and other shots for the edgier Harry Potter film
By Barbara Robertson

Many of the wizards creating Harry Potter’s magic have been doing so since Warner Bros. released Harry Potter and the Sorcerer’s Stone in 2001. But, among all the studios that have created visual effects for Harry and his friends at Hogwarts, Industrial Light & Magic is the only studio in the US to participate in all five films.

For the latest, Harry Potter and the Order of the Phoenix, directed by David Yates, both visual effects supervisor Tim Alexander and animation supervisor Steve Rawlins led a crew that created Thestrals and Dementors, flew the Hogwarts students to London, and created the only shots of Askaban prison. Both had also worked on the fourth film, Harry Potter and the Goblet of Fire. And this is the fourth film for overall visual effects supervisor Tim Burke. “It was great to have the same team back,” he says. After four films, the franchise operates like the proverbial well-oiled machine.

Because ILM had a year to create its shots, Alexander had the luxury of working with a small crew. “We became very efficient,” he says. “All the supervisors sat within earshot, so we could talk about shots right away without having a lot of meetings. We didn’t have to do dailies.”  But that doesn’t mean the shots were easy. To create the creatures, ILM had to develop new techniques and new technology.

Dark Horse

In the earlier Harry Potter films, we saw Hogwarts students riding in carriages driven by nothing. It turns out that they and we couldn’t see the Thestrals, which are horse-like creatures, pulling the carriages. Thestrals appear only to those who have witnessed a death, which Harry did in the last film. “We introduce the creatures in this film,” says Alexander.

The Thestrals resemble emaciated horses with bat wings; hairless creatures, just skin and bones. They have dark gray, elephant-like skin that looks fleshy thanks to subsurface scattering.

 “When they’re pulling the carriages, they walk,” Alexander says, “but at the end of the film, the children get on their backs and fly to London.”

To create the Thestrals, ILM started with a full-scale maquette and concept art. “The model was the size of a horse,” Alexander says. “They shipped it in a gigantic box, with the full-scale wing and tail as separate pieces. We scanned it, so we had a point cloud from which we built a model, and took lots of pictures, and then we kept it in our conference room until the end of production.”

Ken Bryan led the modeling crew that built the Thestrals and textured them. “Lana Lan, one of our modelers, built all the shapes and took the Thestral all the way through modeling, painting, and lighting,” says Alexander. “Ken did the same thing for a baby Thestral—modeled and textured it, but someone else lit it. We had the luxury of having enough time for them to do that.”

The modelers used a variety of software packages. “As long as we can get subdivision surfaces into Zeno,” Alexander says, “it doesn’t matter where we get them from.”

Because the Thestrals are thin and emaciated, the modelers created detailed skeletons to expose all the bones beneath the skin. “We typically just build a shell and then use shapes to simulate muscles and fat underneath,” Alexander says. “But in this case, we built the outer shell and also the inner pieces, the bones, and ribs for the skin to slide against.”

Skin Deep

Sliding the skin became the challenge for the simulation team. The skin needed to sink into the crevices between the creature’s ribs, yet still slide. “It was difficult,” says Alexander. “As soon as we got the skin to suck in between the bones, it didn’t want to slide any more.”
For simulation, ILM uses the Phys­Bam engine developed with Stanford University. PhysBam can simulate the movement of elements such as cloth and fluids interacting and colliding with hard surfaces. Thus, during four months of preproduction, creature-rigging supervisor Eric Wong devised ways to make the skin work within that engine by balancing how much suction to give the skin without making the forces so strong that the skin wouldn’t move over the bones.

“With PhysBam, we have an open system that allows us to attempt things like this without rewriting the simulation engine,” Alexander says. “In our case, we could use PhysBam to collide soft bodies [the skin] against hard bodies [the bones]. Eric had to do a lot of work to hook things up, but the base engine supported the types of collision and sliding we wanted to do.”

The wings were less of a problem. The Potter team leaned on the crew that had recently animated a flying dragon for Eragon for advice. More difficult was making the scary-looking apparitions feel believable and benign.

“In terms of animation, the main goal was to make the Thestrals seem like peaceful creatures,” Alexander explains. “When Harry sees them hanging out by the water, the director wanted it to feel like a National Geographic episode.” Using references of horses, animators gave the Thestrals natural motions, like little muscle flicks, as if they were getting rid of flies.

“When you’re working on films about a fantasy world, it would be easy to stray into something that’s not believable,” says Burke. “We want people to believe in these creatures. If they see Thestrals walking through the forest and think they’re real, we’ve all done our job.”

Flying to London

To create a sequence during which the children fly on the backs of the Thestrals to London, ILM first created animatics by matchmoving aerial plates and then inserting digital doubles and the Thestrals. Rawlins spent four weeks creating the animatics on location. Then, to create the illusion that the actors flew on the backs of Thestrals, the crew filmed them riding a motion-control rig.

ILM needed to mitigate the frightening appearance of the The strals with animation that reminded people of bucolic animals in a nature film.

“The animatics told the crew what angle to use to shoot the bluescreen,” says Alexander. “We exported motion-control data for the camera and the rig from the animatics. They had used the same process for the Hippogriff, but since then, it has gotten more accurate and smoother.
For the final shots, ILM combined the rendered Thestrals with the bluescreen elements of the actors and the aerial plates using Apple’s Shake for compositing and ILM’s own Comptime. “It’s a drawn-out process that works well when you stick to the animatic,” Alexander says. “And they did.”

Digital Doubles

All told, Industrial Light & Magic built 11 digital doubles and used all of them, but only in a few shots-for sequences of wizards flying on broomsticks and for the Thestral shots. “We scanned all the actors and photographed them,” says Tim Alexander, visual effects supervisor at ILM. “The doubles are very detailed, with full hair and cloth simulations, subsurface scattering, the whole shebang. But, the director ended up with more close-ups, and for those he used the bluescreen elements.”

Of all the effects work, creating the digital doubles was the least painful. “Our digital double pipeline is very mature,” Alexander says. “I don’t want to make it sound easy. A lot of people pour time and effort into it. But the technology works; there are no technical glitches.” —BR

Evil Dementors

In addition to the Thestrals, ILM created soul-sucking Dementors. The studio had first developed these creatures for the third film, The Prisoner of Askaban, but for this film, Yates wanted the fiends to be more menacing and physical.

“In the third movie, they were like ghosts made from sheets, with wind blowing behind them,” says Alexander. “This time, they wanted us to pull the sheets back to see their skulls and their chests.” Also, at one point in the film, a Dementor picks up Harry, slams him against the wall, and holds him by his neck.

The effects team started with an early version of the Dementors and used that skeleton, but redid the “sheet” so that it could perform as if the arms of an octopus reached out for Harry. “That was the original concept,” Alexander says. “But ultimately [Yates and Burke] felt so many arms were too distracting, and they dropped back to something similar to the original.”

In the final design, a Dementor has two arms and swirling cloth. To control the cloth, lead creature TD Michael Balog, who had rigged the original Dementors, developed an attractor system that, again, worked within the PhysBam engine. “We cut the cloth into 12 strips,” Alexander explains. “Animators had 12 balls to animate with; each piece of cloth was attracted toward one ball.” Using this system, a Dementor could reach for something with his cloth.

To create the shot with the Dementor attacking Harry, ILM started with film of actor Daniel Radcliffe hanging from a rig, and then matchimated his neck area. To make it look like a Dementor was pushing on his neck, the effects artists considered warping rendered pieces of the neck and using these CG elements to replace Radcliffe’s neck, but they knew it would be difficult to lock in exact matches. Instead, they faked the neck deformations using occlusion passes.

The Dementors, which suck all the happiness out of their envelope. victims, set the tone early in the film. ILM created the creatures using an innovative “attractor” system that directed the studio’s PhysBam simulation engine.

“In the digital world, we had Harry’s [Radcliffe’s] neck, additional shapes to bulge his skin, and the Dementor’s hand right up against that neck,” says Alexander. The shapes made it look like the hand was making the neck bulge. So, they rendered the occlusions on the neck—that is, the shadows created by the hand and bulging skin—in grayscale, and used those rendering passes as mattes to brighten and darken the live-action neck in compositing. The shadows created the illusion that Radcliffe’s neck changed shape.

“It’s a shadow gag,” says Alexander. “Without doing any morphing or rendering of textures in the neck, we made it look like the neck was deforming.” 

Liquid Light

Ironically, ILM also used cloth to create a Patronus effect. The only way to repel a Dementor is with a Patronus, which embodies such emotions as joy and hope—the exact opposite of those of a Dementor. A full-fledged Patronus takes the form of an ethereal animal—a different animal for each wizard.

Harry’s Patronus, for example, looks like a stag formed from liquid light. “We rendered strips of cloth that we took into Saber,” says Alexander, referring to ILM’s Inferno-based system. “As the stag ran, the cloth strips trailed behind like a meteor.”

Having a small crew and long schedule allowed people to work on the project almost as if they were in a small studio, and Alexander sees benefits in that. “Often, we overlap painting with modeling to gain time, but we were able to give people a linear schedule,” he says. “We could have one person who knows a creature and doesn’t overdo modeling when paint would do. We can find better balances. But, it’s a question of scale. As soon as we start getting more shots, it becomes difficult, and there aren’t many people who can do all the jobs equally well.” 

More Magic
In addition to ILM, 10 other studios created visual effects for Order of the Phoenix. Among them, Double Negative created Death Eaters, the character Grawp, a veil room, the Hall of Prophecies, establishing shots of Hogwarts and Privet Drive, and the Patronus effect; MPC created the atrium battle, a fireworks sequence, and a marble staircase; and Framestore CFC worked on Centaurs, Kreacher, and a talking their envelope. —BR


The camera pulls back from the prison to a high, wide angle, and we see the island prison in the middle of the North Atlantic, with big waves crashing against the rocky shores. “It’s a great design by [production designer] Stuart Craig that ILM created with full-CG architecture and seascapes,” says Tim Burke, visual effects supervisor. “They did the only shots we see of the prison in these films.” To create the prison and its watery surrounds, Industrial Light & Magic used PhysBam, the same fluid-simulation technology the studio had relied on for Poseidon and Pirates of the Caribbean. “We ran a high-resolution water simulation around the Askaban area that allowed waves to crash up against it and interact with it,” says Tim Alexander, visual effects supervisor at ILM, “and a deformed surface for areas where we didn’t need the interaction.”
This is the second time Alexander has used the simulation engine. For Harry Potter and the Goblet of Fire, he supervised a shot of a boat rising from a lake with digital water pouring off its sails. And, that was the first time ILM had used PhysBam to create a large fluid-simulation shot in a film.
“It took forever to do that shot,” Alexander says. “We were working from command lines. Now, we’ve integrated the engine into Zeno [ILM’s proprietary software]. It’s still tough to do water simulations, but we were able to concentrate more on the look and art of the shot, and not on technical issues. In [Goblet of Fire], if we thought a splash was too bright and glowy and grainy, we would have had one chance to fix it. On this film, we could see a render every other day. People have more experience working with the engine now, and it’s faster and more stable.” —BR


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