Autodesk 62205-051408-9001 User Guide - Page 362

14 Creating Particle Effects Note: The only properties that determine how the particles behave over time are the over-life properties described in Over-Life Behavior Properties. Life-Life is the length of time that a particle is alive. A higher value means the particles live longer. (You cannot control exactly how many frames each particle lives.) Note: A particle can be alive but not visible. Visibility, Life Opacity, and Size are three settings that can cause a live particle to be invisible. Number-Controls how many particles are created. Higher values mean more particles are created. There is a tendency to want to use a large number of particles when creating an emitter. Often, the effect is better with fewer particles. For example, the Simple Explosion library emitter needs fewer than 100 particles. In most cases, it is not the number of particles that creates the effect you want, but the correct choice of particle shape and size. For example, if you want billowing smoke, use a particle shape that looks like a puff of smoke, make it big enough to see, and then use as few of them as possible. In addition to Number, several variables influence the number of particles created per frame: the shape of the emitter (for example, point or line), the Number Variation value, the Single Particle property, emitter Number, and the frame rate of the footage that is input into the Particles operator. Size-Determines how big the particles appear. Remember that since the Size property is a relative value, you are not setting the size as a number of pixels. Velocity-Controls how fast the particles move away from the emitter. Weight-Weight values can be negative or positive. A particle with zero weight is weightless. A particle with a positive weight falls (moves toward the bottom of the image) and a particle with a negative weight rises (moves toward the top of the image). Spin-Spin values can be negative or positive. A positive spin means that the particles rotate clockwise. A negative spin means a counterclockwise rotation. A value of 0 means no spin. Motion Randomness-Determines how randomly a particle moves. Note that this property overrides the Velocity and Weight settings; the random motion may cause particles to reverse direction. Bounce-Applies only when a particle hits a deflector. Higher values mean more bounce. Note that when a particle has a zero bounce value, it may be able to pass through a deflector. For example, if a particle has a positive weight (so it falls) and a zero bounce, and it comes to rest on a deflector, it may eventually sink through the deflector. A very small Bounce value prevents this problem. Visibility-Determines the transparency of the particles. A value of 0 means invisible and 100 means completely opaque. Some particle types look best completely visible (thick smoke, for example), but others look best with some transparency. 346