DB: In the story, it's this weird concoction of a Chernobyl-like Russian energy drink and a little bit of Vodka and then a mystical force that transforms this hot tub into a time machine. What happens is the water starts to swirl around and swirl around and swirl around and our characters get taken under the surface of the water and then a huge water vortex shoots up into the sky and then it all crashes back to earth and then we travel through time through this crazy vortex of energy and water. And 90% of our effort went into those 15 shots because we had to do a pretty complicated water simulation with high level shader work and physics to make it look real and be also fun and violent, which is what Steve was looking for.

BD: What was your solution?

DB: We had one of our internal comp artists here mock up a style frame for Steve, but we did it as a still just to get sign off on the structure of the thing. It ended up being a very tall water column, which was about 10-feet wide at the thinnest and maybe 30-feet wide at the widest in diameter, if you will, and literally went into the clouds, but had varying levels of turbulence and also had an internal illumination quality to help sell the fact that it had not only had a naturalistic force but also a mystical force.

BD: How did you accomplish this?

DB: We started with a sketch and Steve said bigger, taller and we showed him a color rendering and started to lock the look in on the level of a still, and then we started doing motion tests and took it into Houdini. To create the CG water vortex, our FX team used Realflow simulations of water that were rendered in Houdini with Mantra. Custom tools were built by the team to render 50 million- plus particles in a scene of spray and splashes [composited in Nuke]. But also the water vortex had to interact with the clouds as it shoots up into the sky.

Plus the water vortex shot up into the sky and interacted with cloud systems.

DB: That was a big challenge because Steve also wanted to see the four actors in the vortex, so we came up with a hybrid of 2D and 3D where we either composited the guys in or projected them onto the surfaces and did a bit of a 2.5D approach. And there were all kinds of secondary elements: water droplets and snow and various bits of debris that get sucked up -- paper and small props from around the hot tub that added a sense of motion and scale. Because, as you know, water is tough to deal with in scale if you don't have anything next to it to help you figure out what size it is. That was one of the challenges because in our early works everything felt [too] miniature. We were in real world space in the software and we knew it was like 50 stories tall but when you rendered it, it looked like it was three-feet tall. So we scaled down the internal features of it, played with different camera angles and lensing a lot to get a sense of scale, and then, as we started to add smaller supporting items, we started to have those visual cues where it started to work: "Oh, yeah, that does feel big."

And that's when Steve started really liking it. It probably took us about six months to really develop the look of the hot tub water. It was all CG, but we first shot some practical elements on set just to see what water would do in the hot tub. And the effects guys came up with this little swirling mechanism, paddle-rigged, that swirled the water around in there for us, so we got a sense of what water would do in that confined space at the speeds we were going, but we realized that it helped to some degree, but Steve wanted us to take liberties with it and really amp it up and go about 1,000 times faster than the swirling water we shot for reference. So we were playing with such a fantastical concept that the live-action plates that we shot for reference didn't really help us much.

BD: Anything else?

DB: The only other thing of significance was there was a scene where one of the guys was juggling chainsaws and they are CG. They're actually live chainsaws in the movie with snow coming off of them and it was fun. So that's one of our shots.

Bill Desowitz is senior editor of AWN & VFXWorld.