Dr' Claude C' Chibelushi

1
Fac. of Comp., Eng. Tech. Staffordshire
University
Programming Physics Engines for Games
Introduction
Dr. Claude C. Chibelushi
2
Outline

• Introduction
• Dynamics / Behaviour Modelling Examples
• Software Architecture
• Basic Physics Concepts
• Newtons Laws of Motion
• Summary

3
Introduction

• Many computer / video games, and virtual reality
  (VR) applications require
• illusion of realism (believability)
• available techniques
• cover variety of fields
• graphics, audio, physics, artificial intelligence
  (AI), haptics,
• evolve rapidly
• advances in computing power hence room for
  better physics-based and AI-based simulation

4
Introduction

• Simulation of real-world physics or intelligence
  enriches game or VR content
• physics and AI models enable believable rendering
  of movement and behaviour
• e.g. simulation of mechanical equipment (tanks,
  airplanes, ), simulation of football tactics
• believable virtual world enhances player / user
  immersion

5
Introduction

• Motion / behaviour simulation
• often uses approximate models of reality
• within budgetary and technological constraints
• development costs and deadlines
• hardware platforms processor, memory, I/O
  graphics pipeline
• hence programming believable virtual worlds often
  requires
• simplifications, optimisations, tricks
• believable is often not equivalent to perfectly
  realistic

6
Dynamics / Behaviour Modelling Examples

• Car racing game, flight simulator
• Continuous update of car / aircraft position
• based on current acceleration, speed, load, state
  of road surface (icy, wet / dry, gravel, ...),
  air viscosity, ...
• requires physics models

7
Dynamics / Behaviour Modelling Examples

• Tennis game
• Estimate ball trajectory for given conditions
• strike power, racket orientation, wind speed and
  direction, ...
• requires physics models

8
Dynamics / Behaviour Modelling Examples

• Virtual table tennis
• Possible scenarios computer as one or both
  players
• ball trajectory calculation
• requires physics models
• simulation of sound for ball bounce / hit /
  flight
• requires physics models
• strategy for defence / offence by computer
• requires AI models for virtual player
• etc ...

9
Software Architecture

• Simulation of interactive animated virtual world
  is woven around continuous loop
• main loop steps
• read user input
• perform some flow-control logic
• update state of virtual world (graphics entities
  and associated audio)
• output graphics, audio, vibrations, ...
• loop time determines frame rate

10
Software Architecture

• Simplified simulation loop
• initialisation()
• repeat
• input()
• updateObjectState()
• graphicsAndSound()
• frameRateSync()
• cleanup()

Handle input events read game pad, mouse,
Apply object behaviour / motion control using
model based on AI, physics,
Apply graphics visibility determination, detail
mapping, Output 3D sound
May need to delay,
11
Software Architecture

• Typical components of game or VR program
• input event-handling unit
• flow-control logic unit
• dynamic / behaviour unit (physics / AI)
• world data unit
• output units
• graphics unit, audio unit,

12
Software Architecture

• Block-diagram of typical game or VR program

13
Basic Physics Concepts

• Branches of mechanics

Mechanics
14
Basic Physics Concepts

• Mechanics
• Study of motion of matter, and of forces that
  cause the motion
• based on concepts of matter, time, space, force,
  and energy
• many sub-fields e.g.
• statics study of bodies at rest
• dynamics study of bodies in motion
• forces that cause motion explicitly taken into
  account?
• no kinematics
• yes kinetics

15
Basic Physics Concepts

• Body models
• Particle model suitable for
• body of negligible dimensions relative to motion
  path
• body whose dimensions are not relevant to problem
  
• i.e. no, or unimportant, angular motion
• e.g. car, airplane, rocket can be modelled as
  single particle (concentrated at centre of mass)

16
Basic Physics Concepts

• Body models
• Rigid body model suitable for
• solid body with negligible or no deformation,
  regardless of external force(s) acting on it
• Deformable body model suitable for
• continuous solid or fluid, which can deform under
  action of external force(s)

17
Basic Physics Concepts

• Classical mechanics (a.k.a "Newtonian mechanics")
  
• based on Newtons laws of motion
• three laws which address why / how objects move
• suitable for
• motion that is not too fast (speed much less than
  speed of light)
• size that is not too small (diameter much greater
  than atom)

18
Basic Physics Concepts

• Some important dynamics properties
• Position location relative to frame of reference
  (origin of coordinate system) at any instant
• displacement change in position during some time
  interval
• Velocity rate of change of position
• speed magnitude of velocity
• distance travelled divided by corresponding
  duration
• Acceleration rate of change of velocity

19
Basic Physics Concepts
Some important dynamics properties
flight path
time t2 pos. s2 vel. v2 acc. a2
time t1 pos. s1 vel. v1 acc. a1
20
Basic Physics Concepts

• Some important dynamics properties
• Average velocity or acceleration during time
  interval ?t t2 t1
• for displacement ?s s2 s1
• average velocity v ?s / ?t
• what if path is not straight?
• for velocity change ?v v2 v1
• average acceleration a ?v / ?t
• Instantaneous velocity or acceleration
• velocity or acceleration at specific instant
• i.e. rate of change measured over extremely small
  time interval

21
Basic Physics Concepts

• Some important dynamics properties
• Force that which causes acceleration
• Broad categories
• contact forces
• e.g. collision, friction
• field forces (action at distance)
• e.g. force due to gravity field (gravitational
  attraction), magnetic field

22
Basic Physics Concepts

• Some important dynamics properties

Examples of contact forces
23
Newtons Laws of Motion

• Law 1 (also called law of inertia)
• Body subjected to no net external force remains
  at rest, or moves at constant velocity (in a
  straight line)
• no force hence no acceleration

• No net external force
• either v 0
• or v is constant

24
Newtons Laws of Motion

• Law 2
• Acceleration of object is proportional to net
  force acting on it
• constant of proportionality is object mass
• Fnet SF m a

25
Newtons Laws of Motion

• Law 3
• For every action there is an equal and opposite
  reaction
• action-reaction force pair FA,B - FB,A

Truck collision
26
Newtons Laws of Motion

• Law 3

Free-body diagrams show all forces acting on
given free body (or group of bodies)
27
Suggested Reading

• L. Bishop, D. Eberly, T. Whitted, M. Finch, M.
  Shantz, Designing a PC Game Engine, IEEE Computer
  Graphics and Applications, Vol. 18, No. 1, pp.
  46-53, 1998.
• Relevant parts of Ch. 1 2, D.M. Bourg, Physics
  for Game Developers, OReilly Associates, 2002.

28
Summary

• Believable simulation of real-world motion /
  behaviour often based on
• AI or physics-based rendering to produce
  immersive virtual world
• Most game / VR applications revolve around
  continuous simulation loop
• Typical game or VR program has units for
• input event-handling, flow control logic,
  dynamics / behaviour, output to user

29
Summary

• Mechanics study of motion and of forces that
  cause the motion
• many sub-fields kinematics, kinetics, ...
• Some important dynamics properties
• position / displacement, speed / velocity,
  acceleration, force
• Newtons laws of motion
• three laws about why / how objects move