This summer, the US president’s home took quite a beating in the action-packed film White House Down. With VFX work by the Prime Focus World teams in London, Vancouver and Mumbai, who combined to deliver a total of 326 shots, this movie featured two explosive sequences involving a high speed limo pursuit and an attack on the White House from a trio of attacking F22 Raptor jets.
Using Houdini, they generated a wide range of VFX work for the film including smoke, explosions, water, and even subtle details such as animating leaves on trees. Houdini Mantra was also used to render a lot of the Houdini generated VFX although some of the Houdini generated sims were exported out to other rendering engines when needed.
Smoke, Explosions and Debris
Rocket trails, explosions and of course the burning of the White House are some of the key VFX features in the film. To generate smoke trails and explosions, Prime Focus World developed a system to create volumetric trails that were derived from an explosion. The setup used curves as an input and then divided each of them into a separate volumetric container for each simulation.
Most debris-related VFX was simulated using Houdini’s Bullet solver. After simulation, the data generated was then post-processed in order to smooth out and correct some of the pieces that looked jittery, allowing them to refine the shot without having to perform additional simulation work.
For the fountains in the White House gardens, Houdini’s FLIP solver was used in conjunction with a custom ripple/water surface solver, allowing them to quickly create ripples and simulate water surface interaction with an input object and/or particles.
Rendering With Mantra
To render all of the volumetric VFX along with some point rendering for particles, the team used Houdini's Mantra renderer. Normals were extrapolated either during or after running a simulation. Most of the geometry/data that they rendered was executed using delayed load assets to ensure that the IFD generation remained compact and efficient.
Deep image maps were used in one particular shot to merge a variety of elements and geometry together with the volumetrics. The compositors used fire and smoke mask channels in order to add more detail into their grades.
Prime Focus World employs a variety of tools to accomplish their day-to-day work. For this reason, the Alembic file format provides them with an ideal solution for the exchange of data from one tool to the next.
In the case of White House Down, they used Alembic as a geometry cache format to pass between Maya and Houdini. When they ran into software incompatibilities, they resolved features that were unsupported, such as Maya’s varying frame topology, by using an Alembic procedural to generate geometry directly in the Arnold renderer.
Houdini provided Prime Focus a set of tools that enabled them to generate and render a wide variety of VFX shots from dust, explosions, debris and water for the film White House Down. It also gave them the flexibility that they needed to integrate those VFX into toolsets from other vendors in order to complete an expansive set of shots.