Building an Island of Lost Souls

How the sharp crew at ILM helped shape the breath-taking, fiery climax for Harry Potter and the Half-Blood Prince.

Toward the end of Warner Bros.’ film Harry Potter and the Half-Blood Prince, Dumbledore and Harry travel to a secret cave to retrieve Slytherin’s locket, which contains a Horcrux, one-sixth of Voldermort’s soul. They arrive at a beach, 30-feet wide, and walk into a crystal cave. Inside the cave is an underground lake with a crystal island in the middle, which holds the locket. The dark, vast cave is protected by the Inferi, the trapped souls Voldermort has charged with defending his slice of soul. When they drag Harry into the water, Dumbledore conjures a firestorm 100-feet tall that scares the Inferi. As Harry swims to the surface, a firestorm swirls around Dumbledore and the Inferi quickly disappear.

A crew of around 60 people at Industrial Light & Magic worked for about a year to help bring this difficult, climactic scene to cinematic life, ‘We started with a small crew to develop the fire,’ says Tim Alexander, visual effects supervisor. ‘We didn’t know how to do it.’

While the fire developers worked on their magic trick, layout artists took the storyboards from the sequence and created visualizations of the parts of the cave that the camera would see. Director David Yates filmed two sections on set: the entry and the island. ILM created the rest. ‘We oriented the scene with our layout department, rendered those layouts, and did rudimentary lighting with a hardware render to give [visual effects supervisor] Tim Burke and the director a good sense of what the sequence would look like,’ Alexander says.

To build the cave, the crew developed two techniques. The first was to instance crystals using the crowd pipeline. ‘It’s a heavy approach,’ Alexander says, ‘but it gave us randomness and a crystalline structure.’ For the ceiling, they stacked pieces of simple do-nut shaped geometry and turned them into rectangular crystals with a shader. Matte paintings filled the far background.

The Inferi are CG characters that look like skinny people. Some shots have 10 Inferi; others had thousands. Animators keyframed the creatures in shots with less than 100. For shots with more than 100, the crew used particle simulation with sprites instanced onto cards. The cards have images of the animated creatures doing any of eight cycles for around 800 frames. ‘We rendered cycles and populated the whole underwater areas of the cave with sprites,’ Alexander says. Although they’re sprites, ILM didn’t bake in their lighting. Instead, the technical directors rendered multiple passes with normals so they could light the cards with 3D lighting.

To create the shot with the Inferi pulling Harry into the water, stunt actors in scuba equipment grabbed at actor Daniel Radcliffe and pulled him down into a tank of water on a green-screen stage. Radcliffe held his breath until the end of the take and then one of the scuba divers sitting just out of frame gave him a mouthpiece so he could breathe.

In the film, though, Harry/Radcliffe is in CG water. ‘When we’re underwater, it’s mostly about atmospherics,’ Alexander says. The crew rendered a water surface so when the camera points up and we see the surface turn orange from the fire, we also see water ripples. Otherwise, they achieved the illusion that Harry is underwater because his hair and clothes truly are floating. When seen from above, the water is a CG plane with a 2D simulation on top for displacement that’s rendered with refraction.

Framing the Firestorm Finale

For the firestorm, which Alexander describes as looking like a giant wildfire in the forest, the crew considered using a heavy-duty particle sim that they would render volumetrically. ‘That’s the standard approach,’ Alexander said. ‘But [CG supervisor] Chris Horvath had another idea.’

One of the reasons fire simulations had not been as realistic as they wanted in the past was that it took too much compute power and time to simulate them in high resolution. Horvath’s light-bulb idea was that because edge detail is more important than depth when simulating fire, it might make sense to do the simulation in two dimensions rather than in a three-dimensional volume. That would allow them to concentrate all the compute power on a full 2K image, rather than reduce the resolution to simulate the fire in three dimensions.

The trick would be to create the realistic motion. The answer was to start with a low-resolution 3D simulation to produce a fluid particle motion. Then, to slice the volume like a loaf of bread and do high-resolution simulations on the slices in two dimensions. The number of particles in a slice affected the temperature, and therefor, the color. ‘When it’s really hot, we get less smoke and more fire,’ Alexander says. ‘When it’s cool, we have more smoke and less fire.’ As the simulation runs’that is, as the fire burns’parameters control the type of fire, which can range from a bonfire to a lava flow.

To render the scene’the crystal cave, the water, and the flames’wasn’t as difficult as it might seem. ‘The question was how black is black,’ Alexander says. ‘It’s a dark cave with two light sources without the fire. When we see the fire, we have internal scattering and refraction on the crystals, but the cave is so huge, it gets dimmer behind the island.’

This is the third Harry Potter film that Alexander has worked on at ILM, and although the studio’s work represented only 160 shots, the shots required new technology and painstaking work. ‘One thing that was really nice is that we got involved at a design level,’ Alexander says. ‘Second, having all our shots in one sequence meant we could really dig in and do the whole thing. It was a moderate-sized Potter for us, but focusing in on one sequence with water, fire and a giant environment was really nice.’

Warner Bros.’ Harry Potter and the Half-Blood Prince opens in U.S. theaters on July 15.