G.I. Joe: The Rise of Cobra is a movie where big screen heroes leap through moving trains, freefall from the stratosphere, and unleash an ongoing stream of destruction and debris. To help bring this summer blockbuster to the big screen, Digital Domain was called in to execute a wide range of visual effects shots –320 total. For many of these shots, Digital Domain chose Houdini to bring down the house, or in the case of G.I. Joe, the Eiffel Tower.

Crushing steel

In one of the most challenging effects sequences in the film, the villains shoot the Eiffel Tower with a missile filled with small nanomites designed to eat away at the metal structure until it crumples into the Seine. An attempt was made to animate this sequence by hand but Digital Domain wasn’t getting the right level of detail in the motion.

Digital Domain saw an opportunity to use Houdini’s cloth solver to create plastic deformations as part of this elaborate effect. Working with Side Effects, Digital Domain helped define brand new crumpling and tearing tools which are now included with Houdini 10. Digital Domain was able to test early versions of these tools in production to topple the tower.

“One of the strongest qualities of Side Effects is their willingness to support us every step of the way,” says Phillip Prahl, CG Effects Animation Lead, Digital Domain. “It is always great to see Houdini benefit from this partnership as new tools are production-tested and then packaged up to accommodate artists around the world.”

To set up the plastic deformations, a dynamic system was built by combining an elaborate sequence of Houdini Digital Assets which all shared information with each other. The overall effect started with a nanomites missile asset which passed off attributes to the tower geometry on impact. Hit points were calculated on the tower geometry and the nanomites were birthed from these points. The nanomite cloud asset then passed new attributes to the tower geometry in order to influence the plastic deformation cloth sim being run on the structure.

From there, a single Python command would trigger a cascade of simulations that added individual supports, or mini-beams, between those that were being influenced by the cloth solver. Special geometry solvers would deform them and get them to snap at the right time. The results were influenced by various parameters such as elasticity and the rate at which they sprung back to their rest or bent position. There was also extensive particle work to create the green nanomites that eat away the tower. These ended up painting a value on the beams as they moved up the tower which defined which objects would deform. If a piece became isolated from the rest of the geometry, it would be added to a separate RBD simulation and would then fall off. The painted value was also used to determine the opacity of the beam so that eaten away bits were not visible.

Apache Helicopter Shot

At Digital Domain, Houdini was used for the majority of the effects shots including a variety of particle effects, RBD sims, and volumetric effects. These tools were used to shatter glass, create debris, procedurally crack roads, and shoot down an Apache Helicopter. About 90% of these effects shots were also rendered in Mantra. Glass, wood and shattering roads were also shaded in Houdini and rendered with Mantra.

In one shot, an Apache Helicopter is shot down by a concussion canon and squashed like an aluminum can. The helicopter model was brought into Houdini and run through several RBD simulations. These sims included the bending of a rotor and the compression of the fuselage. The window panes were also shattered using procedural cracking tools and RBD simulations. After the RBD simulations were calculated, they were often used to drive separate particle simulations to create smaller debris chunks and glass fragments.

Thomas Reppen, CG Effects Animation Lead, explained, “Houdini's Digital Assets allowed us to split the Apache helicopter into several sections which meant we didn't have to re-sim the whole helicopter each time we had to make a small change."

“One of the strongest features of Houdini is how easy it is to navigate and coordinate different contexts such as dynamics and particles,” says Prahl. “We were able to use proprietary software to acquire impact data from the sims then use this information to drive a variety of other effects in the shot. The fact that we were able to write our own tools and access Houdini’s data so easily speaks to the openness of Houdini’s architecture.”

Rooftop Shattering

At one point in the film, Digital Domain had to shatter a massive glass rooftop. The sequence contained six or seven challenging shots and it soon became clear that Houdini would offer the only real solution to the problem. Impact points were selected on the glass geometry and procedural noise, such as voronoi noise, was used to generate cracks that spiraled out in a radial pattern from the impact point.

The shattering glass effect was bundled up into a Houdini Digital Asset which could be passed around amongst the many effects artists on the film. It is common for Digital Domain to create their own Houdini tools to achieve effects that will be repeated throughout a film. The concussion canon, which appeared several shots, was also turned into a digital asset.

Reppen noted, “The concussion effect was made up of many layers, but with the setup being 100% procedural and packaged up in a Houdini Digital Asset, all the artist would have to do was to specify where and when the concussion effect should occur in a shot."

“It was very valuable to us to be able to package up all of our stock-type effects and pass them around to the artists since the assets did most of the work automatically,” added Prahl “This kept the quality high on the bread-and-butter work while offering us more time to focus on the most challenging shots.”

With G.I. Joe: The Rise of Cobra, Digital Domain was given the chance to create a visual effects showpiece where the audience can revel in all of the destruction. Houdini provided the perfect platform for their VFX artists to meet production challenges and deliver maximum impact.