Houdini 11 Review: Flipping the Switch
A big step toward Houdini's march to be the complete VFX solution for high-end computer graphics was the addition of its own array of fluid simulation tools in the version 9 release a couple of years back. In production, the main hindrance to the widespread use of Houdini's native fluids has been simply a matter of speed. Many visual effects facilities possessed their own faster proprietary fluid solvers or turned to RealFlow. The release of Houdini 11 from Side Effects sees the implementation of the FLIP solver method (fluid implicit particle), which sees a marked increase in simulation speed over other solver types such as SPH (smoothed particle hydro-dynamics). Both FLIP and SPH represent the fluid as particles, but, to evolve the fluid over time the SPH method employs expensive nearest neighbor searches to sample the attributes of the surrounding particles. FLIP solvers avoid this by storing these attributes on a three-dimensional grid, making the sampling of the attributes more efficient and thus resulting in a faster simulation.
This makes Houdini's fluid toolset a much more enticing prospect for production, and also for the amateur artist with professional ambition who wants to tinker around. After all, the simming itself isn't the fun part; it's seeing the results of the sim that is, and this introduction goes a long way to minimizing the wait and maximizing the fun.
The below fluid scene, simulated using the flip solver and consisting of 50,000 particles interacting with four static rigid body objects took less than 10 minutes to solve 240 frames (exactly 10 seconds of film time). This equates to less than 2.5 seconds per frame, enabling the user to experiment with input parameters and quickly see the effect with very impressive turnaround time, a huge benefit for the artist (not to mention to their sanity when the art direction of the shot alters with no warning!).
Anyone who has seen any of the summer blockbusters over the last two decades knows that destruction is a staple of a visual effects artists work. Houdini 11 strives to make this facet of its toolset even more robust than it already is with the introduction of automatic dynamic fracturing based upon the commonly used voronoi noise pattern. This tool takes away the need for one of the less rewarding parts of destroying a modelers' lovingly hand-crafted work: pre-fracturing. The traditional method of digital demolition is to, pre-simulation, "break" the intact model and its contents according to predetermined impact points and/or structural weak spots. If the pre-fracturing process is done correctly, the individual sections will be perfectly co-planar so the model appears intact when rendered. Then during simulation applied forces, such as gravity, take their affect and the structure falls and smashes. With this new toolset, the user can simply have their object fall to the ground or get hit by, for example, a meteorite, and on impact, Houdini will analyze the structure and the current parameters of the simulation as inputted by the user and fracture the geometry accordingly and the structure is smashed apart.