COVER STORY: 'HARRY POTTER AND THE HALF-BLOOD PRINCE'

SAN FRANCISCO — The climactic sequence in Harry Potter and the Half-Blood Prince is one of the most (if not the most) complex compositions of live actors, greenscreen, CG characters, environment, simulated natural phenomena and simulated fantasy phenomena I have encountered in 21 years of writing for Post.
The new Warner Bros. film, directed by David Yates, is the sixth and penultimate installment in the series based on J.K. Rowling's blockbuster fantasy novels.
There's a seventh novel yet to be filmed, so we presume that the budding wizard will need to survive the implacable mortal threat at the climax of HP6.
It's fairly easy to describe HP6's climax on paper — Rowling did it — but how, exactly, would you do this on film? Harry and wizard/mentor Dumbledore enter a foreboding watery cave on the trail of ultra-bad guy Voldemort. They land on an island in a crystalline cavern where thousands of undead humanoids — Voldemort's victims-now-minions — suddenly emerge from the waters, ensnare Harry and drag him down into the murky depths.
Dumbledore reacts with a new trick. In the novel he engulfs the aggressors in vast sweeping arcs of flame that he controls, creating a vortex of fire. Dumbledore even hurls submarine fireballs at the enemy as they retreat underwater. For this sequence's effects Yates called ILM.

FIRE ENGINE

The culminating sequence is about seven minutes long and the fiery climax is only about eight shots. Despite their brevity, says Tim Alexander, ILM's VFX supervisor on the film, the fire shots "chewed up a great deal of our time and energy. Fire's pretty difficult and we didn't have a solid solution for it." Still, the decision was to forsake ILM's successful history in filming practical pyro effects and to go with all-CG flames.
ILM's first approach was the standard "brute force" method: Simulate particles — "a lot of a lot of them" — into a 3D volume to try to emulate the look of fire. However, Alexander says, "You have to have such a high density of particles to create the detail that you need in fire that it very quickly becomes unwieldy and slow."
He ended up using a new approach developed by ILM's Chris Horvath. Alexander credits it and the resulting fire effects as ILM's biggest achievement in the film. "We still did a particle sim, but it's a very low particle-count, like fuel flying around in the air, just to tell us where it's going to burn." The team put this low number of particles into a tornado configuration, made it look good, and then ran it through Horvath's secondary engine that actually simulates the fire. ILM calls this the "Verte" engine.
Horvath's engine calculated how the particles would burn "but he does it in two-dimensional slices so he can do very high-resolution simulations very quickly," Alexander says. The fire effect's 3D volume is "sliced" like a loaf of bread, focusing on the X and Y dimensions, but not the Z axis. "You can get really nice detail around the edges of the fire and you can get speed out of it because you're not worrying about depth."
Since the fire itself is moving, the particles move from slice to slice in 3D. "If you look at a fire, it's actually pretty difficult to tell how deep it is. By piling up a bunch of slices together, you still get dimensionality to it but you're giving your time and energy to the screen space, where you need it, like the edges of the flame where you want to see those little licks.
"Chris wrote the whole engine on the Nvidia video card, so it's hardware-accelerated," Alexander says. "The speed of these things is immense. Once you have your low-particle-count simulation, then you feed that into the Verte, which is in the GPU. In one day we could look at two or three takes of the fire, so we got more turnaround with it. The engine is very flexible because all it wants to know is where you want it to burn, so we could do all types of looks with it. We created a flame tornado, but we could also do fireballs with it in the underwater sequence — it's done with exactly the same engine."

DUAL ROLE

Robert Weaver is a veteran ILM TD and sequence supervisor who recently was promoted to associate VFX supervisor. This position is perfect for Weaver since he can now meld his aptitude for technical direction and workflow with his experience in helping to achieve the right "look" for a sequence. He worked on Backdraft (1991), where threatening house fires were portrayed as nearly sentient characters. Weaver also worked with Alexander on The Perfect Storm (2000), where ILM perfected the art of breathing life, and thrills, into huge waves that existed only in the computer — work which won a VFX BAFTA award.
On the new Potter, Weaver served as associate VFX supe and digital production supe — the former including involvement in esthetic, creative decisions and giving direct feedback to artists, and the latter involving management of the technical pipeline.
"It's a nice balance to do both," Weaver says, to be able to "convey the director's vision and be able to put that to film." This is the third Potter film in which Weaver has worked with Tim Alexander and Weaver was comfortable bouncing artistic ideas off him and vice versa.
Appropriately enough, Alexander and Weaver worked on some simulated fire shots back on 2005's Harry Potter and the Goblet of Fire (the fourth film in the series was BAFTA-nominated). But, Weaver says, "Looking back, it was somewhat crude compared to what we can do now." The team at the time was pushing the fire sim through ILM's purpose-built "smoke engine" and "it was a constant battle to get the physics correct for fire." ILM augmented those CG fire effects with practical pyro shot on set.
Achieving the look of realistic fire in CG means being able to "choreograph a force of nature," Weaver says. "Back on The Perfect Storm, water had never been done to that scale successfully. Fire is, to us, the new water. It's so difficult to achieve and I feel that this time around, we nailed it."
Alexander and Weaver give some of the credit for the success of the new Verte engine to new super-fast video cards from Nvidia — Horvath was able to build ILM's fire engine on a GPU rather than a CPU. Weaver says doing the effect on the GPU "gave us unbelievable speed-ups over the typical CPU. We built a GPU farm that could handle these massive simulations. What would take a day to run on a CPU, we were able to simulate in 40 minutes. The graphics processor is ideal for handling millions of instructions in split-seconds."
With help from Yates's visual effects supervisor Tim Burke on the set, actor Michael Gambon's work as Dumbledore — kind of like a man dramatically waving a giant wand with a sparkler on the end — meshed well with the CG effects and did not require a digital double.

COLLABORATION

HP6 was produced in England and the majority of the film's VFX were produced in London at effects houses such as MPC and Cinesite. ILM was not the VFX house of record this time (they did about 142 shots), but their centerpiece is the film's fiery climax. ILM literally spanned the globe, working with the production team in England on HP6 and also with their ILM satellite shop in Singapore, which handled about 15 to 20 shots in the cavern sequence.
One challenge was to schedule daily video conferences with the UK-based Burke and ILM Singapore.

THE INFERI

When Harry and Dumbledore land on the subterranean island they go after a talisman (a "Horcrux") that represents a part of Voldemort's soul. But once this MacGuffin is threatened, an army of Voldemort's victims springs to life to defend it. Called Inferi, these folks are a little sympathetic and doubly victimized — first killed by the evil wizard, then kept in an undead state to do his bidding. Yates emphasized that the Inferi not look like grotesque zombies — rather, they appear very skinny and sickly and make Gollum seem like a he was on steroids. Yates stressed that this sequence should not feel like a horror movie — he wanted audiences to feel sorry for these emaciated, water-logged people. "We really investigated how to not make them look like zombies," Alexander says.
For these characters, ILM needed to avoid Gollum and his ilk, including recent zombies such as those found in I Am Legend. "You pull reference from all those movies so everybody's conscious of what's out there and what you don't want to do," Alexander says, "and try not to make a dupe of those."
Yates cast a few live actors to act out some of the Inferi movements — like overwhelming Harry and dragging him underwater — and ILM animators then made the undead individuals move in a very methodical, creepy way. The actors fight with Harry and pull on him but ILM subsequently painted them out and replaced them with CG Inferi.
Marc Chu, ILM's animation supervisor/director, wanted to take on as much animation as possible — the shots with up to 100 Inferi in them are actually all keyframed and handled by the animators. "Once we get underwater and start to get into thousands of them, that's when we went for a particle-spray approach," Alexander says.
In fact, everything about the cavern sequence is CG — even the eerie liquid that Dumbledore must drink from the Horcrux.

WORKFLOW

For starters, the HP6 team gave ILM greenscreen plates shot on a very limited set. The cave setting is made of crystals — big ones — that needed to be CG. With a crystalline set, "you've got to deal a lot with refraction and reflection," Weaver says. "It's not just a matte painting."
ILM had to build an entire CG environment where the camera could go anywhere and still look like it was in a crystal cave. Just the crystalline background scenery took seven to eight weeks. The crystal surfaces reflect Dumbledore's fire as well as the characters in the action and the transparency allows you to glimpse action through the crystals.
"We had both fire and water going at the same time," Weaver says, "and the two interacting. We had escape bubbles, with the fire going under the water."
Representing the thousands of Inferi under the water involved normal-mapping: applying a cycle of animation to the character rigs and rendering it, flatly lit, on a card as a sprite. "When we go under the water, with thousands of these creatures," Weaver says, "the normal-mapping allowed us to re-light the Inferi. This was an innovative way of handling a crowd pipeline that we hadn't done before and it worked out incredibly well."

WATER

ILM created the water for the sequence as well and, as Dumbledore whips up his fire and it rotates around the island in a vortex, it also whips up the surface of the water. "The lake starts to boil," Weaver says. "We had to create the look of boiling water and the Inferi would be refracted through the bubbles. The fire also penetrates through the water's surface, generating a lot of bubbles.
"Raytracing is very expensive," Weaver says, and you have to be really smart about how and when you do it so you don't spend days working on a two-second shot.
HP6 was "a very difficult water show" for ILM. Even though ILM's shot total is on the low side at around 142, with a staff of about 80, "we actually did more fluids on Harry Potter than we had done in the past on any other show," Alexander says. And Alexander, like Weaver, worked on The Perfect Storm which, he says, "was one of the hardest shows that I have ever worked on."
The look of today's ILM water and fire effects is more realistic than it ever was but, Alexander says, "I don't think it's any easier. We keep making things more difficult for ourselves because we want to achieve more levels of realism and more detail."
"We were all very excited, not just with the results but with the process," Weaver says. "It was a huge challenge."
The team had planned on shooting at least some practical pyro for Dumbledore's conflagration but found they didn't need to. "We tamed fire. It's become a creative tool," says Weaver.
ILM expects to see new clients whose movies require fire that can be choreographed. Maybe a Backdraft 2?