Physically based animation is an area of interest within computer graphics concerned with the simulation of physically plausible behaviors at interactive rates. Advances in physically based animation are often motivated by the need to include complex, physically inspired behaviors in video games, interactive simulations, and movies. Although off-line simulation methods exist to solve most all of the problems studied in physically-based animation, these methods are intended for applications that necessitate physical accuracy and slow, detailed computations. In contrast to methods common in offline simulation, techniques in physically based animation are concerned with physical plausibility, numerical stability, and visual appeal over physical accuracy. Physically based animation is often limited to loose approximations of physical behaviors because of the strict time constraints imposed by interactive applications. The target frame rate for interactive applications such as games and simulations is often 25-60 hertz, with only a small fraction of the time allotted to an individual frame remaining for physical simulation. Simplified models of physical behaviors are generally preferred if they are more efficient, easier to accelerate (through pre-computation, clever data structures, or SIMD/GPGPU), or satisfy desirable mathematical properties (such as unconditional stability or volume conservation when a soft body undergoes deformation). Fine details are not important when the overriding goal of a visualization is aesthetic appeal or the maintenance of player immersion since these details are often difficult for humans to notice or are otherwise impossible to distinguish at human scales.
History Physically based animation is now common in movies and video games, and many techniques were pioneered during the development of early special effects scenes and game engines. Star Trek II: The Wrath of Khan famously used particle systems in the Genesis explosion scene to create the visual effect of a flaming shockwave engulfing a planet. Despite being released before physics engines were a common feature in games, System Shock incorporated rigid body physics in its engine and was widely considered innovative for this feature and the novel sense of interaction it afforded players. Valve later developed Half-Life and used rigid body physics to create environmental puzzles for the player, such as obstacles that could not be reached without stacking boxes. Half-Life 2 featured a more advanced physics engine that incorporated constrained systems such as pulleys or levers with more environmental puzzles to showcase these features. Physics engines are now much more common in games, and their frequent appearance has motivated research in physically based animation by companies such as Nvidia.
Physically based animation in games and simulations Physically based animation is common in games and simulations where users have the expectation of interaction with the environment. Physics engines such as Havok, PhysX, and Bullet exist as separately developed products to be licensed and included in games. In games such as Angry Birds or World of Goo, physically based animation is itself the primary game mechanic and players are expected to interact with or create physically simulated systems in order to achieve goals. Aspects of physics puzzle games exist in many games that belong to other genres but feature physically based simulation. Allowing physical interaction with the environment through physically based animation promotes non-linear solutions to puzzles by players, and can sometimes results in solutions to problems presented in games that were not deliberately included by level designers. Simulations used for purposes other than entertainment, such as military simulations, also make use of physically based animation to portray realistic situations and maintain the immersion of users. Many techniques in physically based animation are designed with GPGPU implementations in mind or can otherwise be extended to benefit from graphics hardware, which can be used to make physically based simulations fast enough for gaming. GPU time is often reserved for rendering, however, and frequent data transfers between the host and device can easily become a bottleneck to performance.
Physically based animation in movies Simulations can be performed offline (as in apart from when they are viewed) in the development of special effects for movies. Speed is therefore not strictly a necessity in the production of special effects but is still desirable for reasonably responsive feedback and because the hardware required for slower methods is more expensive. However, physically based animation is still preferred because slower, more accurate methods can be costly and limiting. The physical accuracy of small details in a special effect are not meaningful to their visual appeal, restrict the amount of control that artists and directors can exert over behavior, and increase the monetary cost and time required to achieve results. It is necessary to be able to dictate the high level behavior of physically inspired effects in movies in order to achieve a desired artistic direction, but scripting physical behaviors on the level of small details can be unfeasible when fluids, smoke, or many individual objects are involved. Physically based animation generally affords more artist control over the appearance of simulated results and is also more convenient when desired effects might bend or defy physics.
Sub Topics
Rigid Body Simulation
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