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These are four examples of a physics engine simulating an object falling onto a slope. The examples differ in accuracy of the simulation:
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Aphysics engine iscomputer software that provides an approximatesimulation of certainphysical systems, typicallyclassical dynamics, includingrigid body dynamics (includingcollision detection),soft body dynamics, andfluid dynamics. It is of use in the domains ofcomputer graphics,video games and film (CGI). Their main uses are in video games (typically asmiddleware), in which case the simulations are inreal-time. The term is sometimes used more generally to describe anysoftware system for simulating physical phenomena, such ashigh-performance scientific simulation.
There are generally two classes of physicsengines:real-time and high-precision. High-precision physics engines require more processing power to calculate veryprecise physics and are usually used by scientists and computer-animated movies. Real-time physics engines—as used in video games and other forms of interactive computing—use simplified calculations and decreased accuracy to compute in time for the game to respond at an appropriate rate for game play. A physics engine is essentially a big calculator that does mathematics needed to simulate physics.[1]
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One of the first general purpose computers,ENIAC, was used as a very simple type of physics engine. It was used to design ballistics tables to help the United States military estimate whereartillery shells of various mass would land when fired at varying angles and gunpowder charges, also accounting for drift caused by wind. The results were calculated a single time only, and were tabulated into printed tables handed out to the artillery commanders.
Physics engines have been commonly used on supercomputers since the 1980s to performcomputational fluid dynamics modeling, where particles are assignedforce vectors that are combined to show circulation. Due to the requirements of speed and high precision, special computer processors known asvector processors were developed to accelerate the calculations. The techniques can be used to model weather patterns inweather forecasting, wind tunnel data for designing air- and watercraft or motor vehicles including racecars, and thermal cooling of computer processors for improvingheat sinks. As with many calculation-laden processes in computing, the accuracy of the simulation is related to the resolution of the simulation and the precision of the calculations;small fluctuations not modeled in the simulation can drastically change the predicted results.
Tire manufacturers use physics simulations to examine how newtire tread types will perform under wet and dry conditions, using new tire materials of varying flexibility and under different levels of weight loading. The simulations optimize tire operations, material selection, costs, and enhance time efficiency.
In most computer games, speed of the processors andgameplay are more important than accuracy of simulation. This leads to designs for physics engines that produce results in real-time but that replicate real world physics only for simple cases and typically with some approximation. More often than not, the simulation is geared towards providing a "perceptually correct" approximation rather than a real simulation. However some game engines, such asSource, use physics in puzzles or in combat situations. This requires more accurate physics so that, for example, the momentum of an object can knock over an obstacle or lift a sinking object.
Physically-based character animation in the past only usedrigid body dynamics because they are faster and easier to calculate, but modern games and movies are starting to usesoft body physics. Soft body physics are also used for particle effects, liquids and cloth. Some form of limitedfluid dynamics simulation is sometimes provided to simulate water and other liquids as well as the flow of fire and explosions through the air.
Objects in games interact with the player, the environment, and each other. Typically, most 3D objects in games are represented by two separate meshes or shapes. One of these meshes is the highly complex and detailed shape visible to the player in the game, such as a vase with elegant curved and looping handles. For purpose of speed, a second, simplified invisible mesh is used to represent the object to the physics engine so that the physics engine treats the example vase as a simple cylinder. It would thus be impossible to insert a rod or fire a projectile through the handle holes on the vase, because the physics engine model is based on the cylinder and is unaware of the handles. The simplified mesh used for physics processing is often referred to as the collision geometry. This may be abounding box, sphere, orconvex hull. Engines that use bounding boxes or bounding spheres as the final shape for collision detection are considered extremely simple. Generally a bounding box is used for broad phase collision detection to narrow down the number of possible collisions before costly mesh on mesh collision detection is done in the narrow phase of collision detection.
Another aspect of precision in discrete collision detection involves theframerate, or the number of moments in time per second when physics is calculated. Each frame is treated as separate from all other frames, and the space between frames is not calculated. A low framerate and a small fast-moving object causes a situation where the object does not move smoothly through space but instead seems to teleport from one point in space to the next as each frame is calculated. Projectiles moving at sufficiently high speeds will miss targets, if the target is small enough to fit in the gap between the calculated frames of the fast moving projectile. Various techniques are used to overcome this flaw, such asSecond Life's representation of projectiles as arrows with invisible trailing tails longer than the gap in frames to collide with any object that might fit between the calculated frames. By contrast, continuous collision detection such as inBullet orHavok does not suffer this problem.
An alternative to using bounding box-based rigid body physics systems is to use afinite element-based system. In such a system, a 3-dimensional, volumetrictessellation is created of the 3D object. The tessellation results in a number of finite elements which represent aspects of the object's physical properties such as toughness, plasticity, and volume preservation. Once constructed, the finite elements are used by asolver to model the stress within the 3D object. The stress can be used to drive fracture, deformation and other physical effects with a high degree of realism and uniqueness. As the number of modeled elements is increased, the engine's ability to model physical behavior increases. The visual representation of the 3D object is altered by the finite element system through the use of adeformation shader run on the CPU or GPU. Finite Element-based systems had been impractical for use in games due to the performance overhead and the lack of tools to create finite element representations out of 3D art objects. With higher performance processors and tools to rapidly create the volumetric tessellations, real-time finite element systems began to be used in games, beginning withStar Wars: The Force Unleashed that usedDigital Molecular Matter for the deformation and destruction effects of wood, steel, flesh and plants using an algorithm developed by Dr. James O'Brien as a part of his PhD thesis.[2]
In the real world, physics is always active. There is a constantBrownian motion jitter to all particles in our universe as the forces push back and forth against each other. For agame physics engine, such constant active precision is unnecessarily wasting the limited CPU power, which can cause problems such as decreasedframerate. Thus, games may put objects to "sleep" by disabling the computation of physics on objects that have not moved a particular distance within a certain amount of time. For example, in the 3Dvirtual worldSecond Life, if an object is resting on the floor and the object does not move beyond a minimal distance in about two seconds, then the physics calculations are disabled for the object and it becomes frozen in place. The object remains frozen until physics processing reactivates for the object after collision occurs with some other active physical object.[3]
Physics engines for video games typically have two core components, acollision detection/collision response system, and thedynamics simulation component responsible for solving the forces affecting the simulated objects. Modern physics engines may also containfluid simulations, animationcontrol systems andasset integration tools. There are three major paradigms for the physical simulation of solids:[4]
Finally, hybrid methods are possible that combine aspects of the above paradigms.
A primary limit of physics enginerealism is the approximated result of the constraint resolutions and collision result due to the slow convergence of algorithms. Collision detection computed at a too low frequency can result in objects passing through each other and then being repelled with an abnormal correction force. On the other hand, approximated results of reaction force is due to the slow convergence of typical Projected Gauss Seidel solver resulting in abnormal bouncing. Any type of free-moving compound physics object can demonstrate this problem, but it is especially prone to affecting chain links under high tension, and wheeled objects with actively physical bearing surfaces. Higher precision reduces the positional/force errors, but at the cost of needing greater CPU power for the calculations.
A physics processing unit (PPU) is a dedicated microprocessor designed to handle the calculations of physics, especially in the physics engine ofvideo games. Examples of calculations involving a PPU might includerigid body dynamics,soft body dynamics,collision detection,fluid dynamics, hair and clothing simulation,finite element analysis, and fracturing of objects. The idea is that specialized processors offload time-consuming tasks from a computer's CPU, much like how aGPU performs graphics operations in the main CPU's place. The term was coined byAgeia's marketing to describe their PhysX chip to consumers. Several other technologies in the CPU-GPU spectrum have some features in common with it, although Ageia's solution was the only complete one designed, marketed, supported, and placed within a systemexclusively as a PPU.
Hardware acceleration for physics processing is now usually provided by graphics processing units that support more general computation, a concept known as general-purpose computing on graphics processing units (GPGPU).AMD andNVIDIA provide support for rigid body dynamics computations on their latest graphics cards.
NVIDIA'sGeForce 8 series supports a GPU-based Newtonian physics acceleration technology namedQuantum Effects Technology. NVIDIA provides an SDK Toolkit forCUDA (Compute Unified Device Architecture) technology that offers both a low and high-level API to the GPU.[5] For their GPUs,AMD offers a similar SDK, calledClose to Metal (CTM), which provides a thin hardware interface.
PhysX is an example of a physics engine that can use GPGPU based hardware acceleration when it is available.
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