Editing Physics engine (section)

== Description ==
There are generally two classes of physics [[Software engine|engine]]s: [[Real-time computer graphics|real-time]] and high-precision. High-precision physics engines require more processing power to calculate very [[Accuracy and precision|precise]] 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.<ref>{{Cite book |last=Millington |first=Ian |url=https://books.google.com/books?id=d0NZDwAAQBAJ&q=is+computer+software+that&pg=PP1 |title=Game Physics Engine Development |date=2007-03-07 |publisher=CRC Press |isbn=978-1-4822-6732-7 |language=en}}</ref>

=== Scientific engines ===
{{Expand section|date=August 2010}}
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 where [[artillery]] 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 perform [[computational fluid dynamics]] modeling, where particles are assigned [[force vector]]s that are combined to show circulation. Due to the requirements of speed and high precision, special computer processors known as [[vector processor]]s were developed to accelerate the calculations. The techniques can be used to model weather patterns in [[weather forecasting]], wind tunnel data for designing air- and watercraft or motor vehicles including racecars, and thermal cooling of computer processors for improving [[heat sink]]s. 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; [[butterfly effect|small fluctuations]] not modeled in the simulation can drastically change the predicted results.

Tire manufacturers use physics simulations to examine how new [[tire tread]] types will perform under wet and dry conditions, using new tire materials of varying flexibility and under different levels of weight loading.

=== Game engines ===
{{Main|Game engine|Game physics}}

In most computer games, speed of the processors and [[gameplay]] 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 as [[Source (game engine)|Source]], 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 animation|Physically-based]] character animation in the past only used [[rigid body dynamics]] because they are faster and easier to calculate, but modern games and movies are starting to use [[soft body dynamics|soft body physics]]. Soft body physics are also used for particle effects, liquids and cloth. Some form of limited [[fluid dynamics]] simulation is sometimes provided to simulate water and other liquids as well as the flow of fire and explosions through the air.

==== Collision detection ====
{{Main|Collision detection}}
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 a [[bounding box]], sphere, or [[convex 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 the [[framerate]], 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 as ''[[Second 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 in [[Bullet (software)|Bullet]] or [[Havok (software)|Havok]] does not suffer this problem.

==== Soft-body dynamics ====
An alternative to using bounding box-based rigid body physics systems is to use a [[finite element]]-based system. In such a system, a 3-dimensional, volumetric [[tessellation]] 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 a [[solver]] 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 a [[deformation 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 with ''[[Star Wars: The Force Unleashed]]'' that used [[Digital 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.<ref>{{cite web|url=http://graphics.eecs.berkeley.edu/site_root/papers/Obrien-GMA-1999-08/ |title=Graphical Modeling and Animation of Brittle Fracture|publisher=Graphics.eecs.berkeley.edu|access-date=2012-09-01}}</ref>

==== Brownian motion ====
In the real world, physics is always active. There is a constant [[Brownian motion]] jitter to all particles in our universe as the forces push back and forth against each other. For a [[game physics]] engine, such constant active precision is unnecessarily wasting the limited CPU power, which can cause problems such as decreased [[framerate]]. 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 3D [[virtual world]] [[Second 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.<ref>{{cite web |url=http://wiki.blender.org/index.php/Doc:Manual/Game_Engine/Logic/Object_type/Rigid_body |title=Doc:Manual/Game Engine/Logic/Object type/Rigid body - BlenderWiki |publisher=Wiki.blender.org |date=2009-11-20 |access-date=2010-08-16 |archive-date=2011-10-01 |archive-url=https://web.archive.org/web/20111001101647/http://wiki.blender.org/index.php/Doc:Manual/Game_Engine/Logic/Object_type/Rigid_body |url-status=dead }}</ref>

==== Paradigms ====
Physics engines for video games typically have two core components, a [[collision detection]]/[[collision response]] system, and the [[Dynamical simulation|dynamics simulation]] component responsible for solving the forces affecting the simulated objects. Modern physics engines may also contain [[Computational fluid dynamics|fluid simulations]], animation [[Motion control|control systems]] and [[COLLADA|asset integration]] tools. There are three major paradigms for the physical simulation of solids:<ref>{{cite book |last1=Erleben |first1=Kenny |last2=Sporring |first2=Jon |last3=Henriksen |first3=Knud |last4=Dohlmann |first4=Henrik |date=2005 |title=Physics-Based Animation}}</ref>
* Penalty methods, where interactions are commonly modelled as [[Soft body dynamics#Spring/mass models|mass-spring]] systems. This type of engine is popular for deformable, or [[Soft body dynamics|soft-body physics]].
* Constraint based methods, where [[Constraint (mathematics)|constraint equations]] are solved that estimate physical laws.
* Impulse based methods, where [[impulse (physics)|impulse]]s are applied to object interactions. However, this is actually just a special case of a constraint based method combined with an iterative solver that propagates impulses throughout the system.

Finally, hybrid methods are possible that combine aspects of the above paradigms.