We have just pushed an update to the particle system. You can find out more here: https://github.com/highfidelity/hifi/pull/5561
This PR modifies and adds to the particle class in the following ways
The emitStrength and emitDirection properties have been eliminated. In their place is an emitVelocty property, which allows for a more concise declaration of both the direction and strength (or speed) of the particles
The localGravity property has been removed and replaced with emitAcceleration- this is basically the same thing as localGravity, but wheras localGravity was a float and only acted on the particles along the Y-Axis, emitAcceleration is a Vec3 and acts with any arbitrary direction and strength.
Particles, once they spawn from the emitter, are simulated and moved based on their initial starting position, and do not move with the particle entity. This allows for cool trailing effects such as meteor tails, rave sticks, magic wand spell casting, and so forth. Soon a flag will be added to allow particle emitter entities to have their particles move with them.
velocitySpread and accelerationSpread properties have been added, which allows the user to specify an offset range to modify the respective emitVelocity and emitAcceleration properties. This allows for more dynamic effects such as a snowy blizzard