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Particle Systems

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Object Representations

• So far we have represented (rendered) objects with– Lines– Polygons (triangles)– Curves

• These techniques won’t work for irregular objects

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Clouds

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Clouds

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Fire

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Smoke

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Water

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Water

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Explosions

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Explosions

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Irregular Objects

• These objects are “fuzzy”– They do not have well defined boundaries– They are not smooth– Their surface texture is irregular– Their shape changes over time– Their motion is cannot be described by simple

transformations

• To represent such objects we use particle systems

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Particle Systems

• Objects are represented by clouds (groups) of individual objects (particles)– The particles themselves may be polygons

• The particles are not static– Their motion is unique with respect to other particles– New particles are born– Old particles die

• The system is not deterministic– Stochastic processes are used to create and change the

particles

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Stochastic Process

• A statistical process involving a number of random variables depending on a variable parameter (which is usually time)

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Particle Systems

• Advantages over polygonal descriptions– Much simpler primitive (single point in space)– Easy to motion blur – Doesn’t require a lot of human design time since

they are based on procedures and random numbers– Can dynamically adjust the level of detail– Easy to change form over time

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Particle Systems

• A collection of many small objects that, when taken together, represent a larger object

Polygon Representation Particle Representation

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Particle Systems

• The particles act as a system

• But, each particle has its own attributes and behaviors

• It’s the individuality of each particle that makes the system versatile

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Particle Systems

• Since we’re dealing with animated sequences, time is the primary axis

• Over time…– New particles are generated

– Each new particle has it’s own attributes

– Old particles die

– Surviving particles move according to dynamic attributes

– Each time quanta is rendered creating an animated sequence

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Particle Generation

• The number of particles generated at any given time frame can be controlled by a density function

• This provides a level-of-detail control– Smaller objects may require less particles

– Larger objects may require more particles

– This feature can be used to reduce processing time

• Particle density can also be a function of time– Increase/decrease the mean number of particles per object

as a function of frame number

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Particle Generation

• Specify– Initial number of particles per object– Function for specifying the number of particles per

object over time

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Particle Attributes

• Each particle has its own set of attributes– Position point– Velocity vector– Acceleration vector– Size– Color – Transparency (alpha)– Shape (pixels, triangles, polygons)– Life time

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Particle Attributes

• All of the attributes can also be specified as an initial value and a function over time– Velocity affects position (derivative of position)– Acceleration affects velocity (derivative of

velocity)– Shape can change– Color can change– Lifetime causes a particle to fade out over time

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Particle Extinction

• Each particle has a life expectancy measured in units of frames (time)– Particles can die gracefully according to schedule– Particles can die suddenly based on external

factors

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Particle Dynamics

• Each particle can move independently– Simple vector arithmetic is used to move a particle

• Position = Position + velocity : (speed)

• Velocity = Velocity + acceleration : (gravity)

• Acceleration = Acceleration + external forces (wind, friction, etc.)

• Each particle can change its color independently– Based on initial color, final color, and alpha

blending function

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Particle Dynamics

• Each color can change its size independently– Grow or shrink over time– Can also use an “alpha-blend” type of function

• Each color can change its alpha independently– Can start out opaque and turn transparent

• Particles can collide with one another causing path changes

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Particle Rendering

• Each particle can have its own rendering algorithm– Can move without leaving a trace of where it has been– Persistence: can move leaving a trail of where it has been

• Leads to an affect called motion blurring

– Use of alpha to blend overlapping particles

• Can be rendered as– Pixels– Circles – Triangles– Polygons– All can be shaded using techniques previously discussed

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Demonstration Programs

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Homework

• Create a particle system demonstration– Incorporate any effects you like

• Particle physics

• Color fading

• Curve generation

• etc.