In the sixth Harry Potter film – the Half-Blood Prince – things have taken a dark turn, with Draco Malfoy now a Death Eater and the horcruxes starting to become important. It was also the most expensive Harry Potter movie, with a budget of $250 million. A fairly large portion of both time and money was spent getting just one special effect right – Dumbledore’s tornado of fire which he uses to rescue Harry from the Inferi.
In the sequence, Harry and Dumbledore have traveled to a magical cave in order to retrieve a horcrux (which contains part of Voldemort’s soul). Dumbledore is forced to drink a potion which greatly weakens him, and Harry tries to fetch him some water.
However, under the water, lie the Inferi – zombie-like creatures, who start to swarm him and drag him under. Dumbledore manages to find the strength to conjure fire – the only thing that destroys them. Harry is seen rising to the surface, which is covered in fire and it was this meeting of fire and water that provided such a challenge to ILM, who were in charge of creating the effect.
“We collected a bunch of references, including flares that burn underwater, and showed them to the Potter folks,” said visual effects supervisor Tim Alexander, speaking in 2009 to Wired. Once Half-Blood Prince director David Yates signed off on a visual template, ILM deployed supercharged Linux machines, each loaded with 16 processors and 4 gigs of RAM. “We emulated all these fire parameters: heat ripples, smoke, buoyancy, viscosity, opacity, and brightness,” Alexander said.
Processing the massive particle simulations for the 100- by 300-foot firewall was burning up days of data crunching for each frame. So computer graphics artist Chris Horvath spent eight months obsessing over a faster way to conjure impressive flames. “Chris figured out that a lower-resolution particle set still had a fluidy flow,” Alexander said “The effect looks as if you sprayed propane and then lit it.”