Out of water: ILM melds water and cloth simulations for a breathtaking sequence in Harry Potter.(Industrial Light & Magic, Tim Alexander, Harry Potter and the Goblet of Fire)(Interview)

Each of the four Harry Potter films has challenged the studios that created the magical illusions to push the state of the art in visual effects. ILM's Tim Alexander describes that studio's most challenging sequence in this year's effort.

Q Of the sequences in Harry Potter that you supervised, which one raised the technical bar most?

A The boat and water shot was something else. The boat comes up from underwater and breaches. When it pulls up out of the lake, water runs down the sides of the boat and back into the lake, and rings of water sweep out around the boat. The sails unfurl, and water drips off the sails while they're poofing out.

Q Did you use a miniature?

A We had a 1/16 miniature that we used later in the shot, but the boat coming out of the water was all CG. We bid it both ways, as a miniature and as CG. The price difference was negligible, and the client wanted the control, so we did it using CG. The problem with using a miniature was that the boat would've had to be quarter-scale, and we don't have a tank deep enough to bring it up from underwater. Plus, it would be hard to pull the boat up and not have it break apart. It would have been a complicated miniature shoot.

Q What were the technical breakthroughs for the shot?

A This was the first time we were able to couple cloth and water simulations. And, we used all-new water-simulation technology from our partnership with Stanford University. It lets us increase the resolution in certain areas of the simulation grid so we can get higher-resolution sims without using as much memory, which allows us to get finer details. We bought new hardware--a four-processor Angstrom Microprocessor machine with 32GB of RAM that just sat there and chunked away on the simulation. It took two weeks to run the simulation. This was a big, big shot: It's basically one long 900-frame shot, and we had to simulate a large area of water surface.