Issue: Volume: 23 Issue: 7 (July 2000)

What a Blast!




Explosions are a mess. Blinding light flashes. Hot gases and smoke. Particles of dust everywhere. One person's disaster, however, is another's supreme challenge, at least when it comes to re-creating explosions digitally. In this regard, computer graphics experts have been grappling with a fundamental question for years: How does one realistically model and animate an event that transpires in seconds, but then breaks down into billions of individual components that propagate indefinitely through time and space?

Researchers at the Georgia Institute of Technology have found an answer. A team of animation experts in the school's Graphics Visualization and Usability (GVU) Center have successfully simulated and visualized various types of explosions using a technique that relies on both computational fluid dynamics and volumetric modeling.

The first step is the creation of a post-detonation numerical model of the explosion based on equations for compressible, viscous flow that take into account the types of extreme shocks and supersonic velocities that characterize explosions.
To depict a blast wave shattering a glass window, Georgia Tech researchers simulated the event using computational fluid dynamics and modeled the resulting data volumetrically.




The simulated fluid flow is modeled as a 3D volume with boundary conditions that vary depending on the desired explosive behavior. For example, "free" boundaries allow blast waves to travel beyond the volume as if it were arbitrarily large. "This allows us to model slow, long-term aspects of explosions, such as fireballs and dust clouds," says Gary Yngve, who developed the system with GVU colleagues James O'Brien and Jessica Hodgins. In contrast, "hard" boundaries are finite, and thus are used to create smooth surfaces depicting the volume boundaries. To vary the visual effects, a user can initialize the explosion at different pressures or temperatures, says Yngve. "Different initial shapes of the explosives create dramatically different blast waves."

Unlike heuristic or analytical graphical methods, says Yngve, "physically based simulations allow the computation of complex scenes with multiple interacting explosions and objects." In addition, he says, "simulations can be used in an iterative fashion, allowing the director many chances to modify or shape the effect."
A fireball makes its way around a corner with the help of techniques for simulating the two-way coupling between solids and liquid (the fire).




Because the secondary effects of an explosion, such as the refraction of light, fireballs, and dust clouds, don't significantly impact the simulation itself, the rendering of such effects are decoupled from the simulation. Thus, they can be generated and edited as a post process, providing more opportunity for creative freedom in the visual display.

The animation system also incorporates techniques for simulating two-way couplings between solid objects and surrounding fluids, which is necessary for generating certain explosive effects. The fluid-to-solid coupling, for example, lets the researchers model phenomena such as a projectile being propelled by an explosion. The reverse coupling, from solid to fluid, can be used to model the compression of a piston or the shockwave formed as a projectile moves through the air supersonically. In order to facilitate this bi-directional coupling, the system represents objects two ways: geometrically, in the form of a polygonal mesh that is used to apply forces to the object from the fluid, and as a 3D voxel model that is used to displace fluid based on the motion of the object.

Ironically, one of the researchers' development goals for this system was to make it flexible enough to generate less-than-realistic effects. "Explosions used in feature films often include far more dramatic fireballs than would occur in the actual explosions they're supposed to mimic," says Yngve. "We can reproduce this effect by using more tracer particles and adjusting the rendering parameters of the fireballs. Also, we can add noise to the velocity fields or particle positions during post-processing to make the explosion look more turbulent." Similarly, he notes, explosions in space are often portrayed as more colorful and violent than their real-world counterparts. This can be achieved in the animation system by manipulating such variables as velocity, gravity, and thermal buoyancy.
To animate explosive effects such as a projectile propelled from a chamber, Georgia Tech researchers simulate the event using CFD. At right is a cross-section of the same 3D volume in which "hotter" colors indicate higher densities.




The GVU explosion-animation system is still a work-in-progress. The researchers hope to enhance it through improved rendering, a faster integration scheme, and better two-way coupling between the explosion and objects in the environment. Additionally, it could benefit from the inclusion of a physically based model for smoke creation, since smoke is often a visible feature of an explosion that includes a fireball, as well as an automated method for modifying the textures of objects to show soot accumulation and scorching.

More information on this research is available on the Web at http://www.gvu.gatech.edu/animation/explode.

Diana Phillips Mahoney is chief technology editor of Computer Graphics World.
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