Issue: Volume: 32 Issue: 12 (Dec. 2009)

Viewpoint


By: Mark Stasiuk and Lauren Millar
Big Strides in Small Fluid Flow


Fusion CI Studios, which specializes in complex fluid simulations, has unveiled a new tool in its arsenal, Smorganic, for small CG fluid flow. Here, the video shows the capabilities through this simulation of honey being poured.


If you’ve done any visual effects work involving fluid, you probably look at high-speed movie clips of splashes with a tear in your eye. CG fluid effects have struggled to achieve the gorgeous, rich level of detail we see constantly around us. CG has been revolutionary in achieving realistic fluid motions, optical properties, and directability. But for years, we’ve hit a wall when it comes to super-realistic levels of detail.

Studios tend to face this issue at the large-scale end of the fluid VFX spectrum, for effects like submarine breaches and floods in New York City. Driven by big-budget films, large-scale fluid VFX moved forward significantly in the past decade, through studios doing smart software development and, of course, through more industrial-strength hardware, basically all to jam in more particles or more voxels per pixel of screen space. Where fluid VFX has gone almost nowhere is in the opposite direction of scale. But that’s just where we’ve now made big strides. We’re talking about tiny fluid VFX, like “crown” splashes from raindrops falling into puddles or splats of paint hitting a wall.

Ironically, these are the most commonly needed types of fluid visual effects, and they’re what CG has been worst at achieving. Look at real versions closely, and you see features that are constantly requested for commercial projects—and invariably end up coming from tabletop shoots because CG fluids don’t make the visual bar. Worst of all, these elements are always meant to be heroic, gorgeous, and graceful. Usually this means slow motion. No cover-ups with motion blur. Nowhere to hide. Let the migraines begin.

The challenge of tiny fluid effects has two causes. First, there’s the same problem as for large-scale fluid VFX: resolution. A graceful sheet of water arcing through the air may be small in size, but it contains billions of “particles” (molecules) that grant it huge amounts of detail. When the camera is close and the frame rate is high, you see that detail, at which point the splash might as well be from a breaching submarine. Second, the physics differ from large-scale fluid VFX. When we change the scale of observation by orders of magnitude, at the big scale, forces that were insignificant now can drive the fluid motion. For fluids, small-scale forces that start to become important are intermolecular—for example, electrostatic and chemical—some of which get called simply surface tension, and these are at best only poorly simulated in CG fluid engines.


Lauren Millar and Mark Stasiuk are co-founders of Fusion CI Studios (www.fusioncis.com), which specializes exclusively in photoreal CG fluid and particle effects.

The issue becomes clear when you try to get CG fluid to do something simple: fly through air as a delicate, thin, continuous sheet, for instance—you know, like in all those reference movies. Here’s where we generally hit the resolution issue. If using a conventional SPH (smoothed particle hydrodynamics) fluid solver, such as Next Limit Technology’s RealFlow, those thin sheets just aren’t stable. In a thin sheet of SPH fluid, say one or two particles thick, you only need the particles to jostle slightly away from each other and you quickly develop holes. The solver has no capability to allow the sheet to thin out because you’re at the resolution limit—unlike in real water, where those sheets are many molecules thick, and on top of that, the micro-forces that come into play (such as electrostatics) hold the sheet together for at least a little while. The fluid simulator has trouble approaching the needed fluid resolution, and it doesn’t include those needed special forces. The result is you get fluid with a lot of Swiss cheese-like holes that is decidedly unattractive to every creative director I’ve met.


In recent years, facilities have worked on large-scale fluid simulation, and have only just
started focusing on fluid flow at the small scale.


Luckily, workstations and fluid simulators like RealFlow have gotten much faster, and many tools have also become customizable as the developers open up more of their calculation engines to users. You can write your own plug-ins to add behaviors to the base fluid simulator, which we do constantly at Fusion CI Studios to get custom behaviors. Because we do a lot of commercial work, we often face the challenges of small-scale fluid VFX, so we’ve recently developed an algorithm that prevents SPH fluids from developing into Swiss cheese.

We refer to our in-house tool as Smorganic (smooth organic) because it allows SPH particle sims to be smooth (no holes), but does so dynamically during the simulation, permitting the flow to continue in a natural way. Other techniques we developed earlier involved unnatural tactics, such as creating dominating forces to hold the fluid together, or post-simulation processing to reduce the size of holes. Those methods tended to look less organic because the final rendered fluid was no longer really a fluid, but a highly massaged particle cloud. Smorganic fluids, on the other hand, are all fluid, all the time.


Fusion CI Studios’ Smorganic tool smooths SPH particle sims dynamically during the
process, enabling small-scale fluid flow to continue naturally.


In concept, the method is simple. As the fluid moves, the algorithm searches through the particle cloud and calculates a parameter called the divergence. The divergence is the rate at which fluid is flowing away from a point. The algorithm focuses on divergent areas and determines when a gap opens up between two particles. At that point, a new particle is popped into the hole. The key to getting this to work is understanding exactly when it’s safe to add a particle without increasing the pressure in the surrounding fluid. That would lead to a fluid explosion at worst, and ugly jitter at best. By doing this insertion process gracefully, smorganic creates beautiful and delicate flows that won’t break apart and will render smooth as silk. The process is so stable that there need be no reliance on having a significant thickness of fluid in the first place to buffer the algorithm in case it can’t keep up. This means that even the thinnest possible sheets of fluid—a single particle thick—can be generated and manipulated.

New tools always inspire yet more tools. The process of tracking inter-particle displacements allows you to choose to fill in the holes to a greater or lesser extent. This means you can control precisely where and when the fluid breaks apart, and how quickly it does so. Another useful by-product of the process is the ability to track how particles are distributed around one another. With a little extra work, you can identify the edge of the fluid. This domain of fluid is otherwise hard to reliably find, but it’s extremely important for additional effects.

For example, really thin sheets of fluid in air have an edge with a distinctly thickened lip, and often there are small droplets emanating from those edges. The reality is that the lip and droplets are part of a surface-tension-driven process of the fluid sheet starting to disrupt.

Although the smorganic process doesn’t simulate that disruption (yet), we can use a knowledge of the fluid margin to create the needed visuals. By increasing the weighting at the edge for the metaball-based mesh, we can generate as sexy a thickened lip as the most botoxed starlet on the Sunset Strip. And we can emit more fluid from the lip to form droplets—as many as are wanted—to get the final level of detail. That kind of reliable directability is what we strive for in our work, as it’s the foundation to achieving the right look within tight production schedules.

Smorganic moves us closer to real fluid behaviors and demonstrates the future direction in the field: extreme directability. As a result, our clients now have a solid alternative to the tabletop shoot. Green is in these days, so we like to think of smorganic as our contribution toward a better, greener world—let’s save that water for drinking, rather than splashing it over and over again under hot spotlights.
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