Out of Water

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.

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.

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.

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.

First, we animated the boat and got our cloth simulation to the point where we could have it in our water simulation. The boat is a straightforward model with textures from our art department.

Tim Brakensiek [creature TD] used cloth sims for the sails and for some wires, ropes, and cables. The sails are tied as the boat comes out of the water, and they unfurl mid-rising. So, most of the water mass comes off the deck, the side bars, and the tied-up cloth, but it also drips off the sails. The cloth affects the water, and the water affects the cloth.

The simulation gave us a volume and a surface that we could render. On top of it, we dumped live-action elements and particles. The simulation poured water off the boat; we got a nice, flowing surface. But it gets globby if we don’t have spray and particles at the edge. Therefore, we had to transition between the simulation surface and the spray. TD John Hansen spent many a sleepless night getting the water running right.

We had escape particles that were generated by the simulation engine; it’s a nice innovation in the new water engine. We could set a boundary condition, and once the surface reached that boundary, it started emitting particles. Those particles were rendered separately, not at the same time as the surface.

We did low-res simulations to be sure they were working. We had to lock it in fairly early: When it takes two weeks to do the water simulation, you can only get a few done. At a certain point, we couldn’t change the simulation or the animation, although we could modify the camera slightly. We did paint fixes and composite fixes later for the cloth sim when we got mismatches, because we didn’t want to redo the water.

We used all our renderers-for reflections and refractions on the surfaces, we used [Mental Images’] Mental Ray; for the particle spray, we used our P-Render, and we also used [Pixar’s] RenderMan.

It was a 20-week shot. Jimmy Mitchell [VFX supervisor] was extremely patient. We didn’t show him anything until six weeks before the shot was due. We’d been working on it for 14 weeks, trying to get it rendered, but he didn’t get cold feet.

We upgraded 230 processors in our renderfarm to 64-bit machines, to continue rendering in Mental Ray. All our machines are 64-bit, but we don’t switch the ROM to 64-bit unless we need to-a lot of companies haven’t complied their software for 64-bit. Internally, we have some 32-bit software that we haven’t migrated yet. Our systems group has gotten to the point where they can boot into 64-bit when needed.

We worked in 2k resolution. Because we had a lot of reflection on the water, it was very crisp. In the far background is a matte painting based on a live-action plate, so to match the live action, we had to blur the water a tiny bit in the composite. [In fact] that was one of our issues with the water: It was a bit too glassy, and we wanted it to be a stormy afternoon. So, we added pinching and fine deforming to the water after it was simulated, to make it look windblown. It was sort of like a displacement map on the surface of the water.

We got that free from the water sim. As the boat drags through the water, it causes the wake.

It’s a great step for us. We have a lot of work coming up on movies that will use this water technology. This was our first time out, so we found the bugs and the issues. Now, we have a good sense of what it will take in the future.