Physics Engines for Games

1
Physics Engines for Games

• Topics in Computer Science
• Math Physics of Games
• Fall 2009

2
Physics in a Game

• A physics engine is a computer program that
  simulates physics models, using variables such as
  
• mass
• velocity
• friction
• wind resistance
• It can simulate and predict effects under
  different conditions that would approximate what
  happens in real life or in a fantasy world. Its
  main uses are in scientific simulation and in
  video games.

3
Classes of Physics Engines

• There are generally two classes of physics
  engines, real-time and high-precision.
• High-precision physics engines require more
  processing power to calculate very precise
  physics and are usually used by scientists and
  computer animated movies.

4
Pretty Good Physics

• Computer games use physics engines to improve
  realism.
• In video games, or other forms of interactive
  computing, the physics engine simplifies its
  calculations and lowers its accuracy so that they
  can be performed in time for the game to respond
  at an appropriate rate for gameplay.
• This is referred to as real-time physics.

5
Core Components

• Physics engines have two core components,
• a collision detection system
• the dynamics simulation responsible for resolving
  the forces affecting the simulated objects.
• Modern physics engines may also contain
• fluid simulations
• animation control systems
• asset integration tools.

6
Scientific Engines - ENIAC

• 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.

7
Scientific Engines - Supercomputers

• Physics engines have been commonly used on
  supercomputers since the 1980s to simulate the
  flowing of atmospheric air and water, in order to
  predict weather patterns.
• This is known as computational fluid dynamics
  modeling, where particles of air are assigned a
  force vector, and these combined forces are
  calculated across vast regions of space to show
  how the overall weather patterns will circulate.
• Due to the requirements of speed and high
  precision, special computer processors known as
  vector processors were developed to accelerate
  the calculations.

8
Simulations of Physical Events

• Generally weather prediction is still an
  inaccurate science because the resolution of the
  simulated atmosphere is not detailed enough to
  match real-world conditions, and small
  fluctuations not modeled in the simulation can
  drastically change the predicted results after
  several days.
• Chaos Theory
• Initial State is the elusive component to weather
  simulation models.
• Similar fluid dynamic modeling is also commonly
  used for designing new types of aircraft and
  watercraft, and can provide engineers the
  information that used to be obtained solely from
  wind tunnel testing.

9
Michelin Man

• 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.
• Electronics manufacturers use fluid dynamic
  modeling to examine how cooling air will flow
  through the computer case, to locate thermal
  hotspots that may need additional cooling.

10
Game Engines

• In most computer games, speed of simulation is
  more important than accuracy of simulation.
• Typically most 3D objects in a game are
  represented by two separate meshes or shapes.
• One of these meshes is a highly complex and
  detailed shape which the player sees in the game,
  for example a vase with elegant curved and
  looping handles.
• For purposes of speed, a second highly simplified
  invisible mesh is used to represent the object to
  the physics engine.

11
Pretty Good Physics and Fidelity

• To the physics engine, the object may be
  processed as nothing more than a simple tall
  cylinder. It is therefore impossible to insert a
  rod or fire a projectile through the handle holes
  on a vase, because the physics engine does not
  know the handles exist and only processes the
  rough cylindrical shape.

12
Pretty Good Physics Fidelity

• 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.

13
Real Physics and Limitations for Games

• 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 unnecessary and a waste of the
  limited CPU power.
• The primary limit of physics engine realism is
  the precision of the numbers representing the
  position of an object and the forces acting on
  that object.
• Another unusual aspect of physics precision
  involves the frame rate, or the number of moments
  in time per second when physics is calculated.

14
Advanced Physics in Animation

• Physics-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 physics
  now that it is possible.
• 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.

15
Tokamak Open Source Physics Engine

• http//www.tokamakphysics.com
• Written in C
• We will update this to C for XNA
• Todays job is to figure what is in it.
• Download and look around.