Particle Systems

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Particle Systems

• CS 536
• Rahul Malhotra

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Effects
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History Spacewar (1962)
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History Asteroids (1978)
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History Star Trek II (1983)
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Uses of Particle Systems

• Explosions
• Smoke
• Fog
• Weapon Effects
• Impact Effectsa

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Definition

• A particle system is a collection of a number of
  individual elements or particles.
• Particle systems control a set of particles that
  act autonomously but share some common
  attributes.

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Basic Particle System Physics

• Particle is a point in 3D space.
• Forces (e.g. gravity or wind) accelerate a
  particle.
• Acceleration changes velocity.
• Velocity changes position

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Attributes of a Particle

• Position
• Velocity
• Life Span
• Size
• Weight
• Representation
• Color
• Owner

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Attributes of a Particle System

• Particle List
• Position
• Emission Rate
• Forces
• Current State
• Blending
• Representation

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Methods of a particle system

• Initialize
• Update
• Render
• Move
• Get/Set force

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Implementation - Particle

• struct Particle
• Vector3 m_pos // current position
  of the particle
• Vector3 m_prevPos // last position of
  the particle
• Vector3 m_velocity // direction and
  speed
• Vector3 m_acceleration // acceleration
• float m_energy // determines how
  long the particle is alive
• float m_size // size of particle
• float m_sizeDelta // amount to change
  the size over time
• float m_weight // determines how
  gravity affects the particle
• float m_weightDelta // change over time
• float m_color4 // current color of
  the particle
• float m_colorDelta4 // how the color
  changes with time

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Implementation - Particle System

• class ParticleSystem
• public
• ParticleSystem(int maxParticles, Vector3
  origin)
• // abstract functions
• virtual void Update(float elapsedTime) 0
• virtual void Render() 0
• virtual int Emit(int numParticles)
• virtual void InitializeSystem()
• virtual void KillSystem()
• protected
• virtual void InitializeParticle(int index)
  0
• Particle m_particleList // particles for
  this emitter
• int m_maxParticles // maximum number
  of particles in total
• int m_numParticles // indicies of all
  free particles
• Vector3 m_origin // center of the
  particle system
• float m_accumulatedTime //track when
  waslast particle emitted
• Vector3 m_force // force (gravity,
  wind, etc.) acting on the particle system

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How to represent particles?

• Points
• Lines
• Texture-mapped quads
• Point Sprites

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Render Particles as Points

• glBegin( GL_POINTS )
• glVertex3f( m_position.x, m_position.y,
  m_position.z )
• glEnd()

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Render Partices as Lines

• glBegin(GL_LINES)
• glColor4f( r, g, b, 0.1f )
• glVertex3f(m_position.x, m_position.y,
  m_position.z)
• glColor4f( r, g, b, a )
• glVertex3f(m_position.x m_direction.x,
• m_position.y m_direction.y,
• m_position.z m_direction.z)
• glEnd()

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Render Particles as Quads

• glBegin(GL_TRIANGLE_FAN)
• if(textured)
• glTexCoord2f(0.0f, 0.0f)
• glVertex3f(pts0.x, pts0.y, pts0.z)
• if(textured)
• glTexCoord2f(1.0f, 0.0f)
• glVertex3f(pts1.x, pts1.y, pts1.z)
• if(textured)
• glTexCoord2f(1.0f, 1.0f)
• glVertex3f(pts2.x, pts2.y, pts2.z)
• if(textured)
• glTexCoord2f(0.0f, 1.0f)
• glVertex3f(pts3.x, pts3.y, pts3.z)
• glEnd()

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Render Particles as Point Sprites

• glTexEnvf(GL_POINT_SPRITE, GL_COORD_REPLACE,
  GL_TRUE)
• glEnable(GL_POINT_SPRITE)
• glBegin( GL_POINTS )
• glVertex3f( m_position.x, m_position.y,
  m_position.z )
• glEnd()
• glDisable(GL_POINT_SPRITE)

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Point Sprites vs Textured Quads

• Point Sprites dissappear suddenly
• Cannot rotate a point.
• Point sprites are not supported in older cards.
• Point sprite size is dependent on available
  OpenGL point sizes.

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Snowstorm - Demo
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Particle Systems API 2.0

• Free Particle System
• Much lighter than a full physics engine
• Simulations of groups of moving objects
  explosion, bounce, etc.
• Download from www.particlesystems.org
• Demo

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Advanced Topics

• Adding Scripting capability
• Particle Systems Manager
• Improving Particle Systems with the GPU

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Refernces

• More OPENGL Game Programming Dave Astle
• http//www.particlesystems.org/
• http//www.2ld.de/gdc2007/EverythingAboutParticleE
  ffectsSlides.pdf
• www.evl.uic.edu/aej/527/lecture05.html
• mit.edu/groups/el/projects/spacewar/
• http//www.mirwin.net/
• http//www.javaworld.com/javaworld/jw-05-2006/jw-0
  529-funandgames.html?page1