Particle Systems(Motion Machines of 2D Objects with Textures)

Matthew K. Bowles

Advanced Computer Graphics

Spring 2004

Particle Systems

• What are particle systems used for?

• Why do we care?

Particle Systems

• Properties:– Evolves (not static).– Procedural (state machines).– Non-deterministic (randomness).– Simple (computationally efficient).– LOD is easy (particle count & size).– Good at complex objects (grass).– Good at amorphous objects (fire).– Good at complex behaviour (explosions).

Particle Systems

• Process:1. Generate new particles with initial

attributes.

2. Kill off particles destined to die.

3. Modify particle attributes.

4. Render remaining particles.

Particle Motion

• State Machines

• Two primary methods for modeling transitions.– Age-

• State transitions due to the temporal plane

– Collision Planes-• State transitions due to the spatial plane

Particle Systems

• Variance:– For all particle attributes we want to provide

some randomness in order to make the system seem more natural.

– Result = Mean + Variance * Rand()

Particle Motion

• Position(x, y, z):– Position = Position + Velocity * Delta_Time

• High variance = Rain & Star field

• Med variance = Fire & Grass

• Low variance = Fireworks & Fountains

– We only draw living particles.

Particle Motion

• Velocity(vx, vy, vz):– Velocity = Velocity – Acceleration *

Delta_Time• High variance = Fireworks & Fountains

• Med variance = Fire & Grass

• Low variance = Rain & Starfield

– The motion is dependent on the relative velocity of each component.

Particle Motion

• Acceleration(ax, ay, az):– Models the sum of the forces on a particle

(typically constant over an entire set of particles). Might change as a function of the current state.

– X and Z often differentiated from Y (i.e., most 3D models maintain Y as the dimension with Earth’s gravity).

Particle Motion

• What about using a direction vector and a velocity magnitude for modeling motion? (d,|m|)– More useful for 3D objects, which must

maintain an orientation.– Particles have no orientation.

Particle Motion

• What about using global accelerations?– Pros – Uses less space and easier to maintain.– Cons – Supports a limited set of particle sets.

Particle Objects

• Idea – – A particle’s shape is essentially the same, no matter

what side of the particle we are looking at.

• Result – – We render a particle as a 2D object.

• Problem – – What about the change in shape due to the camera

view?

Particle Objects

• Bill Boarding– Rendering a 2D object so that the surface of the object is

always perpendicular to the camera view vector.– v0 = vCenter + ((-vRight - vUp) * (particle_size / 2));– v1 = vCenter + (( vRight - vUp) * (particle_size / 2));– v2 = vCenter + (( vRight + vUp) * (particle_size /

2));– v3 = vCenter + ((-vRight + vUp) * (particle_size /

2));– Where vCenter is the location of our particle, and vRight and

vUp are taken from the model-view matrix.

Particle Objects

• Do we have to build a quad for a particle?

• Building the quad and executing the Bill Boarding algorithm takes some overhead processing.

Particle Objects

• Many graphic languages (DirectX and openGL) implement point sprites.

• Point sprites – Build the quad and execute the Bill Boarding

algorithm on the GPU, thus saving CPU processing time.

Particle Textures

• RGBA – – Red, Green, Blue, and Alpha…– What is alpha?

• Alpha used to model opacity (solid objects) and translucence (i.e., glass, water, etc…)

• Lower Alpha Greater Opacity• Higher Alpha Greater Translucence• Is Alpha all that we need?

Particle Textures

• We must enable blending and specify how to blend with a blending factor for source and destination.– Source Incoming Fragment Color (Rs,Gs,Bs,As)

– Destination Stored Pixel Color (Rd,Gd,Bd,Ad)

– Source Blending Factor (Sr, Sg, Sb, Sa)

– Destination Blending Factor (Dr, Dg, Db, Da),

– (RsSr+RdDr, GsSg+GdDg, BsSb+BdDb, AsSa+AdDa)

Particle Textures

• We model the particle as a quad (i.e., a square). However, most particles don’t look like squares… What’s the deal?

• In our particle texture we render some portions invisible. We do this by specifying a clear color and setting the invisible portions of the texture to the same RGB values of the clear color.

Particle Textures

• What about RGB?– We modulate the particle color with the particle

texture so that we can have a dynamic color variance between particles (i.e., we would like to use a limited amount of textures, so we can’t support to many different colors with just texture).

Particle Textures

• Other considerations?– Disable hidden-surface removal. Particles must

blend colors with the objects behind them. Therefore, we must consider all objects that are hidden by particles.

Particle Systems

• Advanced Work– Modeling complex physical phenomena using

real world physics.– Flocking (modeling particles as boids)

• Remember the limitations on motion and shape we stated earlier. We lied. There is a whole other world out there.

Particle System References

• William T. Reeves, Particle Systems - A Technique for Modeling a Class of Fuzzy Objects”, Computer Graphics 17:3 pp. 359-376, 1983 (SIGGRAPH 83).

• www.cs.otago.ac.nz/cosc455/ParticleSystems.pdf • www.opengl.org• www.codesampler.com• www.gametutorials.com• OpenGL Programming Guide (Addison-Wesley

Publishing Company) Second Edition 1997