Physically based animation
Physically based animation is an area of interest within
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
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
Simplified rigid body physics is relatively cheap and easy to implement, which is why it appeared in interactive games and simulations earlier than most other techniques. Rigid bodies are assumed to undergo no deformation during simulation so that rigid body motion between time steps can be described as a translation and rotation, traditionally using affine
Soft Body Simulation
Soft bodies can easily be implemented using spring-mesh systems. Spring mesh systems are composed of individually simulated particles that are attracted to each other by simulated spring forces and experience resistance from simulated dampeners. Arbitrary geometries can be more easily simulated by applying spring and dampener forces to the nodes of a lattice and deforming the object with the lattice. However, explicit solutions to these systems are not very numerically stable and are extremely difficult to control the behavior of through spring parameters. Techniques that allow for physically plausible and visually appealing soft bodies, are numerically stable, and can be configured well by artists were prohibitively expensive in early gaming history, which is why soft bodies were not as common as rigid bodies. Integration using
Fluid Simulation
Computational fluid dynamics can be expensive, and interactions between multiple fluid bodies or with external objects/forces can require complex logic to evaluate. Fluid simulation is generally achieved in video games by simulating only the height of bodies of water to create the effect of waves, ripples, or other surface features. For relatively free bodies of liquid, Lagrangian or semi-Lagrangian methods are often used to speed up the simulation by treating particles as finite elements of fluid (or carriers of physical properties) and approximating the
Particle Systems
Particle systems are an extremely popular technique for creating visual effects in movies and games because of their ease of implementation, efficiency, extensibility, and artist control. The update cycle of particle systems usually consists of the three phases: generation, simulation, and extinction. These phases respectively consist of the introduction of new particles, simulating them through the next timestep, and removing particles that have exceeded their life-span. The physical and visual attributes of particles are usually randomized on generation with the range and distribution of attributes controlled by the artist. Particle systems can further be made to generate particle systems themselves to create more complex and dynamic effects, and their high-level behavior can be choreographed through a framework of operators as in the canonical Sims paper. [9] Early games that rendered systems of particles suffered from clipping artifacts when particles partially intersected geometry in the environment, and this artifact was especially noticeable for large particles (which were often used to stand in for smoke). Soft particles address these artifacts through careful shading and manipulation of the transparency of particles, such that particles become more transparent as they approach surfaces.
Flocking
In physically based animation, flocking refers to a technique that models the complex behavior of birds, schools of fish, and swarms of insects using virtual forces. These virtual forces simulate the tendency for flocks to center their velocities, avoid collisions and crowding, and move toward the group. In these simulations, individual members of the flock (sometimes called boids, short for bird-oid) act without collaboration using only information about the position and velocity of their peers to create the illusion of synchronized, group behavior efficiently. were among the first games to implement flocking, which was used to model the behavior of birds and flying creatures present in outdoor levels.
Physically Based Character Animation
Characters in games and simulations are traditionally animated through methods such as
References
- ^ Baraff; Witkin (1999). "Physically Based Modeling Course Notes". Siggraph. Course 36.
- ^ Reeves, W. "Particle Systems - A technique for modeling a class of fuzzy objects" (PDF). ACM Transactions on Graphics.
- ^ Müller, M.; B. Heidelberger; M. Hennix; J. Ratcliff (2006). "Position Based Dynamics" (PDF). Proceedings of Virtual Reality Interactions and Physical Simulations (VRIPhys).
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- ^ Stam, J. (1999). "Stable Fluids" (PDF). Siggraph.
- ^ Geist, Robert; Christopher Corsi; Jerry Tessendorf; James Westall (2010). "Lattice-Boltzmann Water Waves" (PDF). ISVC.[dead link]
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- ^ Reynolds, C. (1989). "Flocks, herds and schools: A distributed behavioral model". Siggraph.
- ^ Geijtenbeek, T.; N. Pronost; A. Egges; and M.H. Overmars (2011). "Interactive Character Animation using Simulated Physics" (PDF). Eurographics.