Transcript Character Customization

Particle System Design
The Challenge
• Particle systems vary so much
• But want some code reuse
• Option 1: parameterization
• Option 2: inheritance
• Option 3: subroutine library
• Let’s look at actual systems
Look for Custom Code:
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Motion
Rendering
Orientation
Interparticle Force
Color
Spawning
Emitters
Any of these could use custom code
Parameterization
• Simply not general enough
Inheritance
• Inheritance is usually
helpful 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 doesn’t work that way.
Subroutine Library
• Each system is its
own module, however,
• Library provides:
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routines for rendering
routines for calculating motion
routines for spawning
etc
• Let’s design this library
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
Soft Trails
• For each point along trail
generate two “side-points.”
Soft Trails
• For each point along trail
generate two “side-points.”
Soft Trails
• For each point along trail
generate 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) < 0) { v2= -v2; }
v = normalize(average(v1,v2))
sidepoint1 = P + v
sidepoint2 = P – v
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?
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;
};
Why closed form II
• Can use prerecorded paths.
• float positionX[1024];
• float positionY[1024];
• Can add several prerecorded paths
• multiple circles
• wiggle + parabola
• Can transform prerecorded path
• smoke example
• Library should contain many paths.
Why Timestepped
• Particles that “bounce.”
• or, interact with world in other ways.
• simply can’t do this in closed form.
• Decouple frame rate from renderer.
ps.elapsed = GetClock() – ps.createtime
timesteps = (int)(ps.elapsed / ps.framerate)
while (timesteps > ps.timesteps) {
ps.update();
ps.timesteps += 1;
}
• This should be a class in your library.
Data Structures
• Every particle system
needs 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;
...