Written by Jason Cisarano  on August 12, 2008

Specials: Particle effects make it pretty.

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Also visible in this illustration are many of the basic properties common to particle effects. Remember that for our purposes here, a particle is a visible sprite with a series of associated properties. These steam particles begin their lives at an emitter positioned at the end of the pipe at the left, creating the illusion that the steam is coming out of the pipe itself. The emitter also defines the properties associated with each particleЧthe most basic being its location in the world (using some form of XYZ coordinate system) and its rate of movement in each of the three dimensions. By making these particles move quickly away from the pipe and upward somewhat more slowly, the designer has made the steam seem to shoot out of the pipe quickly before dispersing upwards. The emitter also defines the size and rotation of each particle according to the preset parameters. Notice that all of the particles don't follow the exact same pathЧthe particles follow a generally cone-shaped pattern narrower at the left than the right. That's controlled chaos in action. The designer hasn't specified the path of any individual particle, instead he has set parameters that the emitter will use to create each particle.

Another common parameter for a particle is its lifespan. In the view of the pipe on the right side, the particles have been given a lifespan that has them fading out after just a few seconds of life. Like the other parameters, variation is allowed, so some particles will fade out sooner than others to avoid a clear line where all particles simply disappear. Also visible on the right is a fade-in effect at the end of the pipe. The Call of Duty 2 screenshot clearly shows particle lifetime: as the smoke rises above the house, it becomes less dense as the particles fade out and spread across the sky.



As the old adage goes, Уwhere there's smoke, there's fire,Ф and the same thing is true for particle systems. Designers often combine particle systems for complex and convincing effects. For instance, these two explosions from Call of Duty 2 both use several kinds of particles to create their effects. The one on the left uses some kind of smoke particle stretched out to show the force of the blast combined with a flame effect to represent the explosion itself. The one on the right uses smoke and flying soil to great dramatic effect.



One major new development in particle systems is what's called Уsoft particles.Ф The problem with using flat sprites to represent things like smoke is that the sprites don't interact well with the world. In the Unreal example above, a few particles at the right edge of the left picture are cut in half as they move into the wall. The current solution to this problem is to give the particles depth and to fade them out gradually as they approach the line where they intersect the 3D objects in the game world. Different parts of the sprite will have different opacities depending on how close they are to intersecting world geometry. The next pair of screenshots, from a Microsoft DX10 tech demo, show the differences between a traditional billboard particle system on the left and soft particle system with depth on the right. The right side doesn't have the harsh lines visible on the leftЧthe equipment fades uniformly into the dust cloud surrounding them.



In the end, particles effects are all about looking goodЧthey rarely effect the gameplay itself, with smoke screens in shooters like Call of Duty as a primary exception. Take a look at the accompanying Bioshock screenshot. It's got sparks, fire, smoke and more, but despite the high cost in CPU/GPU time, none of them really has any effect on whether the player or the Big Daddy will survive this encounter. But without them, the game experience wouldn't be the same. Particles can represent water splashing from a fountain and they can also create the glowing spell effects common in all sorts of RPGs. The possibilities are endless.