With Tim Burton's adaptation of Roald Dahl's classic Charlie and the Chocolate Factory, a flowing chocolate river is a prevalent theme throughout the entire blockbuster film. Given its important “role,” special care was given to ensure the river’s fluid simulation effect was realistic, particularly during a dramatic white-water type ride which takes Willy Wonka and the children through a labyrinth of white porcelain tunnels in a boat made entirely of candy and crewed by oompa-loompas. The Moving Picture Company (MPC) created over 550 VFX shots for the project, 70 of which involved the chocolate river. Key elements in this sequence utilized Myrtle software's Five by Five fluid solver integrated into Houdini.

Fluid Chocolate Simulations

Dr. Peter Baldwin, founder of Myrtle software and the developer of the Five by Five fluid toolset, worked with MPC as one of a team of six Technical Directors. "It really was the product of a fevered imagination," says Dr. Baldwin. "The whole range of fluid behavior had to be exhibited - fast-flowing chocolate, vast calm areas of eddying chocolate, curved joints where the rapids poured into chambers, countless pillars in the flow - you name it, it was there. And the candy boat itself had three big splashdowns and traveled at speeds ranging from full-throttle to a leisurely glide."

Luckily, MPC was able to use a real chocolate river as reference. Tim Burton had built an enormous set where the candy boat begins its journey. The set included an impressive tract of a chocolate river, and MPC was able to get footage of VFX supervisor Chas Jarrett wearing green waders undertaking extensive 'research' with a model candy boat. This pretend chocolate had the viscosity required for the sequence and MPC was able to get a very clear idea of how it, swirled and flowed around.

The Houdini Pipeline

Peter Baldwin recalls the first shot which used MPC's new Houdini pipe. "There were some oompa loompas doing synchronized swimming around the pipe in which Augustus Gloop was stuck. We ran a 2D Navier-Stokes fluid simulation of the area around the oompas then fed the results of this into a Houdini-based ripple simulator using VOPs to get nice swirly chocolate ripples. Scripts then picked up the simulation output in the form of velocity and turbulence maps and displacements, these were then rendered using bespoke in-house software.

Each shot started with animated geo sequences imported into Houdini to produce the simulations before being handed over to the lighting department. Initial tests proved that Houdini was able to create the right look for the moving chocolate. With the Houdini pipeline in place, MPC had a lot more options for how and where elements were generated and access to Houdini's extensive range of tools to manipulate them when needed. At this point MPC began using Houdini to generate and manipulate elements for some shots.

"From previous experience it was pretty clear to me that Houdini could be extremely useful in accelerating the turnaround of several key shots in the sequence" says Dr. Baldwin. "It's a much more logical framework in which to be manipulating large simulation output, changing attributes and procedurally positioning generic elements in a scene."

"I don't know what we would have done in those shots without Houdini." says Dr. Baldwin. "In the sequence where the Candy boat goes over a chocolate waterfall and performs a 180 about turn we did some big fluid simulations entirely in Houdini of the boat going down the fall and used particles to create the waterfall itself. In those shots there are a load of elements that really worked well together."

Houdini was invaluable in the turnaround of the multi-element shots. Scenes were scripted to produce large numbers of elements. One wide shot included the interaction of oars, a bow wake off of the front of the boat, swirly ripples, trailing wakes, ripples close to the boat, pillar interactions, eddied bubbles from the back of boat, outflows from two tunnel mouths and ripples from the door entrances and every side of the room! Each time the boat animation changed, most of these elements had to be redone. By using Houdini's Hscript we could rework the shots with time left to place and balance the elements before handing the end result off to the lighting department. Houdini made it much more manageable for a single person to handle a large number of shots and to make the chocolate river pipeline a success.

Myrtle Software [www.myrtlesoftware.com]

Dr. Peter Baldwin is the founder of Myrtle Software which provided bespoke fluid simulation software to MPC for Charlie and the Chocolate Factory.

The Moving Picture Company [www.moving-picture.com]

The Moving Picture Company is a large visual effects house in London. MPC is currently sole facility for Aardman Features/ DreamWorks Pictures 2005 release The Curse of the Were-rabbit starring - Wallace and Gromit, Harry Potter and the Goblet of Fire and The Corpse Bride, a stop-frame animated film co-directed by Tim Burton.

Other projects currently in production include; Danny Boyle’s Sunshine, Stormbreaker (Geoffrey Sax) and Isolation (Billy O’Brien).

MPC has recently completed key elements for Batman Begins and the majority of VFX for Kingdom of Heaven and Charlie and the Chocolate Factory.