Particle controllers affect the behavior of Particles. They are bound to ParticleGroups and typically invoked by their group's update() method.

A controller is a simple callable object (i.e., function-like) with the following signature: my_controller(time_delta, group)

When invoked, the controller is free to manipulate the group and its particles anyway it likes. Typically, a controller will iterate the group's particles and modify them in some way. For example, here is a controller that changes the direction of a particles movement based on time, making them travel in a meandering path:

``` from lepton import ParticleGroup from math import sin, cos

def meanderer(td, group): for particle in group: particle.position.x += sin((particle.age + particle.velocity.x) / 10) * 5 particle.position.y += cos((particle.age + particle.velocity.y) / 10) * 5

group = ParticleGroup(controllers=[meanderer]) ```

That's it! The simplest controllers can be functions, like the above. If the controller needs to store configuration state, you can simply define it as a class with an __call__ method.

Note that a single controller object may be bound to multiple groups (or bound to a system, which is effectively the same thing). Controller code should not assume that it will always be called with the same group object argument. If possible, controllers should avoid having group-specific state, but if that is necessary, it will need to be implemented such that the controller can be bound to many groups.

Built-in Controllers

The following controllers come with Lepton:

• LifetimeController -- Kills particles older than a specified age.
• GravityController -- Imparts a fixed acceleration to all particles in a group
• MovementController -- Updates the position of particles based on their velocity.
• ColorBlenderController -- Modulates the color of particles over time.
• FaderController -- Fades the alpha component of particles in and out over time.
• CollectorController -- Kills particles as the enter or exit a Domain.
• BounceController -- Deflects particles off the surface of a Domain.
• MagnetController -- Attracts or repels particles to or from a Domain.
• DragController -- Simulates drag forces for particles moving through a fluid (i.e., air or water).