Particle System Design

1
Particle System Design
2
The Challenge

• Particle systems vary so much
• But want some code reuse
• Option 1 parameterization
• Option 2 inheritance
• Option 3 subroutine library
• Lets look at actual systems

3
Look for Custom Code

• Motion
• Rendering
• Orientation
• Interparticle Force
• Color
• Spawning
• Emitters
• Any of these could use custom code

4
Parameterization

• Simply not general enough

5
Inheritance

• Inheritance is usuallyhelpful for code reuse.
• But in the case of particles,
• want to inherit motion from here,
• want to inherit rendering from there,
• want to inherit spawning from that,
• Inheritance doesnt work that way.

6
Subroutine Library

• Each system is itsown module, however,
• Library provides
• routines for rendering
• routines for calculating motion
• routines for spawning
• etc
• Lets design this library

7
Rendering Routines

• Render camera-aligned quads
• pos, size, color, tex, theta, blend
• Render soft trails
• ie, for jet engine exhaust
• or, for sparks from a grinder
• Render each particle as 3D model

8
Soft Trails

• For each point along trailgenerate two
  side-points.

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Soft Trails

• For each point along trailgenerate two
  side-points.

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Soft Trails

• For each point along trailgenerate two
  side-points.

for each point P eye vector from cam to P
lineseg1 one of two segments connected to P
lineseg2 two of two segments connected to P
v1 cross(lineseg1,eye) v2
cross(lineseg2,eye) if (dot(v1,v2) lt 0) v2
-v2 v normalize(average(v1,v2))
sidepoint1 P v sidepoint2 P v
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Particle Motion

• Two ways to do it
• Option 1 Timestep simulation.
• Calculate acceleration
• Position Velocity
• Velocity Acceleration
• Option 2 Closed form equations.
• Particle.X f(t)
• Particle.Y g(t)
• Why not always use timesteps?

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Why closed form I

• Can be hardware accelerated.
• Example simple fountain.

struct vertex float3 startpos float3
initialvelocity float lifespan vertex
shader globals float3 gravity float
currentTime
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Why closed form II

• Can use prerecorded paths.
• float positionX1024
• float positionY1024
• Can add several prerecorded paths
• multiple circles
• wiggle parabola
• Can transform prerecorded path
• smoke example
• Library should contain many paths.

14
Why Timestepped

• Particles that bounce.
• or, interact with world in other ways.
• simply cant do this in closed form.
• Decouple frame rate from renderer.
• This should be a class in your library.

ps.elapsed GetClock() ps.createtime timesteps
(int)(ps.elapsed / ps.framerate) while
(timesteps gt ps.timesteps) ps.update() ps.tim
esteps 1
15
Data Structures

• Every particle systemneeds some form of particle
  list.
• But, it seems every system has different particle
  attributes.
• pos, vel, accel, theta, direction, age,history,
  color, rand seed, opacity, lifespan,size,
  texture, 3D mesh, quat, etc, etc...
• My solution

class particlearray int array_size int
array_fill float3 pos float3 vel
float3 accel ...