Particle systems are a technique for rendering special effects that are typically very fluid and organic. They are common in games, generally being used for smoke, fire, sparks, and splashes of water. For example, the explosions in Spacewar are particle systems.

A particle system consists of any number of small particles. Each particle has its own physical properties, typically including position, velocity, and acceleration. More complex particle systems may include even more properties. Particles are created and initialized with some initial properties determined by the overall particle system, but once the system has begun, the particles all act independently of one another. Particles are typically drawn as 2D alpha blended sprites. Once many of these independently updating particles are drawn on top of one another, the particle system has the appearance of a chaotic and natural system.

This sample's particle systems are based on the Spacewar particle systems. However, they also demonstrate the use of up-front allocations to avoid garbage collections. Also, the particle systems have been changed from Spacewar's SceneItem to a DrawableGameComponent, so they can be easily plugged into any XNA Framework game.

Sample Controls

The ParticleSampleGame class is the main Game for the sample. It demonstrates two special effects using particle systems: an explosion and a rising column of smoke. The explosion is done using two separate particle systems: one for the fiery core, and one for the smoke that remains afterwards. The smoke plume is one particle system. To use these effects, ParticleSampleGame creates and maintains three particle systems: ExplosionParticleSystem, ExplosionSmokeParticleSystem, and SmokePlumeParticleSystem. The ParticleSampleGame class has a state that corresponds to what effect is being demonstrated, and it keeps timers, so that it knows when to start a new effect.

The most interesting class in this sample is ParticleSystem, an abstract class that contains the core functionality for a particle system. The AddParticles method of ParticleSystem is called to add a new effect to the scene. AddParticles uses several constants to initialize the particles. The constants, which should all be set by subclasses of ParticleSystem, give each particle system its unique look. Subclasses can also override several virtual methods in ParticleSystem to get more flexible control over particles as they are created.

One key thing to note is that all of the particles are allocated when the program starts. Particles are then reused as necessary, and are never instantiated during run time. This behavior avoids having any unnecessary garbage collections at run time, and keeps your game running smoothly. However, it does have one negative side effect: it is possible that when AddParticles is called, all particles are currently busy. In this situation, AddParticles adds as many as it can, and then stops, which results in a desired effect never showing up. This issue is addressed by a parameter to the ParticleSystem constructor, an integer that specifies the maximum number of effects desired. It is up to the developer who is using the ParticleSystem to pick the lowest number possible that will still yield good results.