Billboard particles always face the camera. That means that no matter what angle you look at the particle system, the particle’s surface is always oriented towards the viewer. Other parameters, such as rotation, scale, and position, may affect what the surface appears like, but the orientation towards the camera always stays the same with billboard particles. They’re one of the most common particle types where rendering speed is a concern because all they are is a rectangular polygon capable of a transparent texture. A very large amount of billboard particles can be rendered on-screen in real time with any modern GPU.
Billboard particles work best in situations where the “other side” of a particle is unimportant. That is, if a user sees the effect head-on or very far away, where the camera won’t go, it’s much easier to use billboard particles and approximate depth as necessary. Smokestacks, far-away fires, snow, rain, and some varieties of sparks are best done using billboard particles.
The theoretical concepts behind metaballs are beyond the scope of this overview, but they can be generally described as the best high-speed simulation available for liquids and similar substances, such as the “lava” inside lava lamps. The way they work is fairly straightforward. Each particle is a round sphere, until it comes in contact with another particle. At a certain distance apart from each other, two particles will start forming a bridge and eventually join in the shape of a larger particle. This effect is very common in nature, especially in the behavior of liquid drops, which makes metaballs useful in modeling fluids. Metaballs can also be used to create 3D models of objects, especially organic objects like animals or humans, as seen below.
Particle age is an important concept because it addresses what happens to the particle’s appearance as it moves through its lifespan. In real life, a fire has many stages: flames, soot, smoke. Sometimes the smoke is black, sometimes it’s white, sometimes it transitions from one to another due to a substance in the fire. These changes can be modeled using particle age-dependent materials and settings. Emitters usually provide a few age-dependent settings, such as size and speed, but they can’t change whether a particle looks like a flame or like black smoke. That’s where using age-dependent materials comes in. Particles can change not only color, but apparent shape, and they can even be made to look like an animation, such as a man walking. In branding shots and advertisements, where graphics are used for symbolism rather than realism, such versatility can come in very handy.
The particular name for these objects varies from package to package. You’ll see them called “forces” in 3ds max or “fields” in Maya. The general idea behind effectors is that they affect the particle system somehow. Normally, the intent is to simulate real-world constraints on an otherwise ideal system. Therefore, the standard package of effectors includes things like gravity, wind, and a suite of deflectors and attractors. Particle effectors used in combination with proper emitter settings and proper lighting can create very convincing (or alternatively, very fantastic) effects.