For Harry and Dumbledore, there is no calm before the storm in this naturalistic drama with few set pieces.

"And once we have that particle simulation, it goes into Chris Horvath's tool, which is a hardware-accelerated approach, so it's using the GPU. We had a small GPU farm of 30 NVIDIA cards and the next phase takes the fuel and burns it and you get super high detail out of it. So it actually takes the simulation that says, "I'm fire, I'm burning, I have this much buoyancy, I have this much heat, I have this much smoke." So it has these parameters that are somewhat intuitive that you can set. We get the extra detail by running in two dimensions rather than three so it runs a lot faster, and you can do very high-resolution simulations because it's done in two dimensions. We basically slice it up into 2D slices and compile them together, so we're putting our time and money into screen space and not depth. We could turn this around in an afternoon. It's turned out to be pretty versatile and other shows are starting to pick it up."

Horvath and Geiger will present a technical paper on the "Directable, High-Resolution Simulation of Fire on the GPU" at SIGGRAPH 2009 in New Orleans, Aug. 4 in Hall E 3.

Not surprisingly, the Inferi were very tricky. According to Tim Burke, the overall visual effects supervisor, "David was pretty keen on giving them an unnerving presence but didn't want in any way to create a zombie type of character. He wanted a human character you could empathize with, that even evoked sadness. At the end of the day, these are dead souls. We came up with a design that went beyond what was possible with an emaciated human: very skinny with all the bones and ribcage showing and spending all the time underwater with the effects of gray skin. We developed this concept with ILM. They developed different variations of heads in ZBrush so we could populate this world with hundreds of these characters. And we started looking at how they moved and having done some studies of movement and getting to the point where we didn't have any reference material, we turned it over to ILM to work on their animation skills and they came up with pretty normal human type movement."

Chu acknowledges that they were given great direction as far as skin texture, and that Aaron McBride, the visual effects art director, was instrumental in coming up with different ideas to play with so that they would look as human as possible and you could have an emotional connection to them as victims.

"When they first appear, David wanted it to appear that the Inferi were not attacking Harry but welcoming him," Chu explains. "We explored a spider-like movement or walking slowly. We decided on a slow encroachment onto the island, since that was in keeping with [the slumbering state] they were in. We used Maya for animation and developed some new custom sim for the water that reacts to the Inferi that are above and below the water.

"We built about a dozen or so Inferi with different facial shapes and heads and we were able to create variations. We used our in-house tool, Zeno, to do the facial work, with lots of creature development work for the skin, muscle and hair."

When the Inferi emerge from underwater, Chu admits that it's going to be one of those, "OMG, what the hell: I thought it was bad above water," moments.

Bill Desowitz is senior editor of AWN and VFXWorld.