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Defines the behaviour of particlesA rule is used to define behaviours for your particles.
There is two type of rules: event based and continuous.
They can be used, for example, to emit a trail of particles behind some other particles. They can also be used to explode a particle into hundreds of others, with different colors and behaviours themselves.
Rules can also be used to define the physics of a particle, to make it bounce off objects in the scene, or to destroy it upon impact.
A very important concept in particles rules is the particle's Group. When created, a particle is in the Group . You can increase that Group ID using a Change Group Particle Action (in case of the Emit Particle Action, a Change Group Particle Action will automatically be added to increase the particles Group ID).
If you want to make a particle split once, when hitting a surface, the rule should be applied only on the first Group. Then, you should increase the Group when splitting to ensure it doesn't happen again.
Particles Group ID will always be indicated in the Element Tree inside of the particle Rule name with a [n] annotation. Particles Group Change of ID will be annotated with a [+n] or a [-n] mark, so you shouldn't be lost. At least we hope.
Here is a little schematic:
Here is some tutorials to help you get the particle rules concept :
Watch directly on youtube
Go see the Group 3d Mask part of the documentations for further details on particles Groups.
Event Particle Rule - Event based particle rule category
Continuous Particle Rule - Continuous particle rule category