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

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: • Generate new particles with initial attributes. • Kill off particles destined to die. • Modify particle attributes. • 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

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