Advanced Games Development Physics Engines 1

1
Advanced Games DevelopmentPhysics Engines 1

• CO2301 Games Development 1
• Week 5

2
Todays Lecture

• Introduction
• Collision Bodies
• Physical Properties
• Relationship with Scene Models
• Practical Issues

3
Introduction

• Physics engines simulate Newtonian physics for
  models in a scene
• Usually a separate library
• Real-time simulation of movement and interaction

• Game provides the physical info about the world
• Static mass, centre of gravity, etc.
• Dynamic e.g. the forces acting on a model
• Relationships Joints, degrees of freedom, etc
• Physics engine simulates movement / collisions
  based on this information

4
Existing Physics Engines

• A physics engine is difficult to write
• Core component is a program called a solver
• Solving simultaneous Newtonian equations
• Needs precise maths and programming
• Common to use existing physics engines, e.g
• Havok Commercial engine popular for games
• Vortex Commercial engine more targeted for
  simulation
• ODE Fairly powerful freeware engine
• SPE Lightweight, free for non-commercial use
• Newton Freeware engine, easy to use
• We will look at Newton in the lab

5
Rigid Body Simulation

• Physics engines can simulate rigid or soft bodies
• Rigid body simulation more straight-forward
• Rigid bodies represented by their collision volume

• The collision volume is a simplification of the
  visual model
• e.g. a car may be modelled as a box
• Or character as a set of boxes and cylinders

6
Collision Primitives

• Can choose the collision volume from a set of
  simple primitives
• Box (or cuboid)
• Sphere (or ellipsoid)
• Cone
• Cylinder
• Rounded cylinder (capsule or chamfer)
• Each treated as a precise mathematical object
  rather than a polygonal mesh
• Improves the speed of the engine and produces
  smoother movement
• But simple primitives means inaccurate collision

7
Complex Collision Volumes

• What if no suitable collision volume for a model,
  or precise collisions required?
• Can use the boundary of the polygonal mesh of a
  model as its collision volume
• This is called a convex hull
• Increases time and memory required for the
  physics simulation
• But will increase accuracy of collision
• May be side-effects if the model lacks detail
• E.g. a polygonal cylinder will not roll as
  smoothly as a mathematical one

8
Physical Properties - Static

• Each body in the physics engine has some static
  physical properties
• Mass amount of matter
• Centre of gravity point of equilibrium of a
  model
• Moments of inertia how mass is spread around a
  model
• Elasticity bounciness
• Friction several types, static, kinetic, rolling
• Used to apply Newtons laws of motion
• And to calculate the effect of interactions with
  other models

9
Physical Properties - Dynamic

• Physics engine bodies also have a further set of
  dynamic properties (current state)
• Position and orientation
• Linear velocity current movement
• Angular velocity current spin
• Forces e.g. gravity, buoyancy, wind
• Torques rotational forces, e.g. engine force
  spinning an axle
• Damping Linear and angular slowing of
  velocities, often used for numerical stability
• Initial state is set by the game, then state is
  dynamically updated by the physics engine

10
Physical Properties - Relationships

• Bodies can be connected together
• These connections usually represent joints
• E.g. hinges, ball/socket joint, sliding joints
  etc.
• A joint is defined by
• The models involved in the joint
• Degrees of freedom linear and angular, choice of
  these determines the joint type
• Stiffness and springiness
• Several joints can be used for more complexity
• E.g. Chains, machines, rag-dolls, etc.
• Rag-doll set of primitives connected like a
  human body

11
Physics Simulation Initialisation

• Create a rigid body in the physics engine for
  each model in our scene
• Choosing a suitable collision volume
• Define the static properties for each body
• E.g. Mass,
• Define any joints connecting bodies together
• Initialise the dynamic properties
• E.g. Initial position and velocity
• Ensure this matches the model position in 3D
  engine

12
Physics Simulation Update

• Physics simulation runs in the game loop on a
  tick
• Same as scene update tick, as it affects scene
  models
• E.g. might aim to update physics at 50fps (every
  0.02s)
• Each tick, a new state (position, velocity etc.)
  is calculated for each body
• To do this, the physics engine needs to know the
  current forces / torques on the models
• This information is provided by the game
• The new positions of the physics bodies are
  copied to their scene models equivalents

13
Relationship with Scene Models

• Implies a change when working with models that
  are in the physics simulation
• No need to move or rotate these models
• Although we may occasionally reposition or reset
  them
• Instead define the forces/torques acting on them
• Get their positions each frame from their physics
  bodies
• A significant shift from previous projects
• Two views of the same data 3D model physics
  body
• Can be difficult to have precise control over a
  model

14
Practical Issues

• Physics engines are often numerically unstable
• Results can be inaccurate/difficult to control
• Settings must be tweaked carefully to achieve
  efficient and stable results
• In particular
• Damp all velocities so movement is easily
  stabilised
• Dont calculate physics for stationary bodies
• Freeze models when they are moving very slowly,
  i.e. stop them
• Limit size of physics world to maximise accuracy