Smoke Signals.

Engineering principles help blow smoke into digital animations

No matter how you look at it, smoke is a nuisance. In reality, it burns your eyes. In virtual reality, it bothers your eyes, mostly because it rarely looks real enough to burn them. This is the problem facing film directors and game developers who want to achieve the look of actual smoke in their productions without having to deal with the physical reality of it.

Existing animation techniques, however, have typically fallen short of this goal. "Smoke is quite difficult to hand animate due to its inherent complexity, so most hand animations end up looking more like living smoke creatures than, passive, turbulent smoke--factors that point to doing CG smoke on a computer," says graphics researcher Ron Fedkiw of Stanford University. Unfortunately, creating a computational model of smoke that behaves like actual smoke typically requires supercomputer power to run the necessary computational fluid dynamics (CFD) calculations. The huge amount of simulated data must then be imported into a visualization environment for 3D graphical display and interaction.

In an effort to bring digital smoke closer to reality, Fedkiw and colleagues Jos Stam of Alias|Wavefront and Henrik Wann Jensen of Stanford, have developed a technique that borrows physics-based principles from traditional CFD but avoids the computational drain.

They are able to achieve this by taking advantage of the fact that animations for entertainment don't need the scientific precision of full CFD simulations, only the visual reality that can be extracted from such data. "Scientific CFD is all about multi-processor calculations. National laboratories like Los Alamos and Lawrence Livermore build special computers with thousands of processors precisely to handle CFD calculations," says Fedkiw. "The problem with such CFD [for animation applications] is that simulating actual physics usually requires a much faster frame rate than the human visual system can process." For example, he says, while we visually perceive natural motion when watching an animation comprising 24 to 30 frames per second, it may take thousands of frames to process the physics of what's happening during that same second. Clearly, says Fedkiw, "it's CPU-intensive, and wasteful, to compute thousands of flames per second when only 24 to 30 are needed."

To compensate for this in the scientific community, researchers have developed special-purpose integration schemes that enable a fairly fast flame rate at the expense of small-scale detail. For entertainment purposes, however, such an approach is ineffective. With respect to smoke, for example, it would allow the accurate representation of the large-scale behavior--such as the gross movement of clouds of smoke--but would lose the small details that make smoke look smokey, such as minor density variations that make certain regions more transparent. Basically, says Fedkiw, "we are left with a very fast, practical calculation that can be run on a laptop, but one with no small-scale detail, so it looks blobby or syrupy." Because of this, Fedkiw and his colleagues have modified the integration schemes for their smoke application.

To add back the ...