ILM created a new fire sim tool that works at both low and high resolution for the encounter with the Inferi. All Images: © 2009 Warner Bros. Ent. Inc.

View clips and trailers of Harry Potter and the Half-Blood Prince at "Harry Potter Haven" on AWNtv!

As part of director David Yates' mandate for a more naturalistic Harry Potter, Half-Blood Prince contains some extraordinarily good fire and water sim work by Industrial Light & Magic, which also animated the Inferi creatures inside the seaside cave, protecting Voldermort's Horcrux, the device that stores a portion of his soul and allows him to remain immortal.

In fact, for the first time in the franchise, ILM was tasked with creating the vfx for an entire sequence: the crucial one in which Harry (Daniel Radcliffe) and Dumbledore (Michael Gambon) travel to the seaside cave to retrieve the Horcrux in order to defeat the Dark Lord. Inside the cave, Harry is forced to make Dumbledore drink a foul, mind-altering liquid that hides the Horcrux. Though weakened by the liquid, Dumbledore defends them from a horde of Inferi by conjuring a fiery tornado, and returns them back to Hogwarts' Astronomy Tower.

For the first time in the franchise, ILM was tasked with an entire sequence: the search for the Horcrux in the seaside cave, which entailed lots of diverse CG work.

ILM, with Tim Alexander supervising the vfx and Marc Chu supervising the animation, created all of the CG for the sequence.

"In the end, the most difficult and interesting work for us was the water and fire simulation," Alexander recalls. "We had three shots external to the cave where Harry and Dumbledore are looking back toward the entrance and that was a fully CG ocean. They actually shot a helicopter plate of it, but the water wasn't wild enough for David Yates. So they asked us to give them a much stormier ocean. And once we go into the cave, it's dominated by a giant lake and the island that Harry and Dumbledore are going to is in the middle of that lake. All of the water is computer generated, even the water that Harry scoops out of the bowl that holds the Horcrux.

"We used our proprietary water engine for the other work, but we hadn't experienced such a small scale before when dealing with the water in the bowl. We found that a lot of the settings within the engine had to be changed. The engine acted very differently and it was an interesting discovery to witness the differences between handling a cup of water vs. the Maelstrom or the Poseidon storm."

But it's the fire that required some new technology because of its scale and slithering quality. "We needed to turn it into a giant tornado, so we had to come up with a full CG solution for greater control," Alexander adds. "We started the process with two methods: brute force and another path led by [TD] Chris Horvath. And after a couple of months of development, we realized that Chris' [finesse] approach was going to give us the detail and the look that we needed to get a realistic-looking approach. The development of the process took about eight months and we had a fire crew working on it with four people with Chris leading that group.

"The fire is based on two simulations: an initial one that is very low particle count that is meant to be the fuel, as if you're shooting gas into the air and then are going to light it. This low count means the simulations can be turned around very quickly, with multiple iterations within an hour. Willi Geiger set those simulations up and they basically look like a tornado, spinning around.