Some objects are difficult to represent as a set of surface primitives, even taking advantage of transparency and texture mapping techniques. These include objects that have poorly defined or dynamic topologies, or have no solid surface. Natural phenomena that meet this criteria include smoke, clouds, fire, water, etc.
Particle systems can be used to represent these objects. A particle system is a large set of simple primitive objects which are processed as a group to represent an object. The characteristics of these objects, such as size, position, color, and the lifetime of the particle itself, can be changed dynamically. If these parameters of the particles are coordinated, the collection of particles can represent an object.