RigidBodyParticles2D

A Godot 3.0 addon that facilitates simple rigid body based particle systems.

Instructions

1. Copy the addons/ directory into your project.
2. Create a new scene that has a RigidBody2D as its root node (Don't forget to setup a CollisionShape2D and the collision layer/mask). You can add whatever you like below this node; for example a Sprite2D. This scene represents an individual particle, and will be instanced each time a particle is emitted.
3. In another new or existing scene use 'Instance Child Scene' to add addons/RigidBodyParticles2D/RigidBodyParticles2D.tscn as a node. This will be the particle emitter.
4. On the newly instanced emitter node, set the particle_scene property ("Particle Scene" in the inspector) to the scene you created in step 2.
5. Tune other various properties on the emitter node, play the scene, rinse and repeat to your liking.

Description

This addon makes it possible to create simple particle systems that emit RigidBody2D based scenes, which Godot's Particles2D node cannot. This lets particles interact with the environment like bouncing off other physics bodies, or use collision detection to apply affects like damage.

And because the particles are user created scenes, you have more control over their behaviors. You can attach any other nodes to the parent RigidBody2D like a Sprite2D, Light2D or even a Particles2D.

Using custom scripts you can modify particles during over their lifetime, like rotating them so they're oriented along the path they are traveling, stretching their trails in line with their velocity, or changing the lighting intensity as they near end of life.

Note that emitting many RigidBody2D instances in rapid succession can have an adverse affect on performance. For this reason, you should keep the particle scenes small and efficient, and the number of instanced particles relatively low.

Randomness

Randomness in RigidBodyParticles2D differs from Particles2D in that the resultant value can be either higher or lower than the specified base value. The general calculation is:

    rand_param_value = param_value + param_value * ( 2 * randf() - 1 ) * param_random

For instance if the base impulse parameter is set to 100 and impulse_random is set to 0.5, then the resultant randomized value can range from 50 to 150 (i.e., 100 +/- 100 * 0.5 * randf()).

Signals

• shot_started - Emitted each time a "shot" (set of particles) starts.

• shot_ended - Emitted each time a "shot" (set of particles) ends.

Properties

• emitting - Enable emitting if true. Disable emitting if false.

• amount - Number of particles to emit for each "shot".

• amount_random - Randomness parameter for amount. The valid range is [0,1]. See note about randomness in the description above.

• particle_scene - The PackedScene that gets instanced and emitted for each particle.

• one_shot - Emit only one "shot" (set) of particles if true. Repeatedly emit particle "shots" if false.

• explosiveness - Controls the delay between each particle within a single "shot" of particles. The valid range is [0,1]. An explosiveness of 0 means that particles are emitted with even spacing over the lifetime of the "shot". An explosiveness of 1 means that all particles are emitted at once at the start of the lifetime of a "shot".

• tracker_name - This String property indicates the name of the tracker node that is attached to each instanced particle. The lifetime property of the tracker is set to the lifetime of the particle, and is useful for setting up Tweens that vary over the life of a particle. For example, in a script attached to your particle scene you can access the particle's lifetime as:

    onready var tracker  = get_node( get_parent().tracker_name )

    function _ready():
    	## fade light over duration of particle's existence
    	$Tween.interpolate_property($Light2D, "energy", 0.7, 0.4, tracker.lifetime,
    		Tween.TRANS_LINEAR, Tween.EASE_OUT_IN)
    	$Tween.start()

• emission_shape - Specify a Shape2D to be used as the area where particles are emitted. This can be a CircleShape2D, RectangleShape2D, CapsuleShape2D or SegmentShape2D. If a shape is not specified it defaults to a point emitter. If an invalid shape is specifed, an error is printed and a point emitter is used.

• lifetime - Specifies the lifetime of a particle and the delay between "shots" in seconds.

• lifetime_random - Randomness parameter for lifetime. The valid range is [0,1]. See note about randomness in the description above.

• impulse - Specifies the initial impulse magnitude (not direction) applied to the RigidBody2D particle as it is emitted.

• impulse_random - Randomness parameter for impulse. The valid range is [0,1]. See note about randomness in the description above.

• impulse_angle_degrees - Specifies the impulse angle in degrees. Valid range is [-360,360].

• impulse_spread_degrees - Controls the spread of the angle of impulse for each particle. Each particle's initial angle will range from [impulse_angle_degrees - impulse_spread_degrees, impulse_angle_degrees + impulse_spread_degrees]. Valid range is [-360,360].

• force - Specifies the applied_force magnitude (not direction) applied to the RigidBody2D particle as it is emitted.

• force_random - Randomness parameter for force. The valid range is [0,1]. See note about randomness in the description above.

• force_angle_degrees - Specifies the force angle in degrees. Valid range is [-360,360].

• force_spread_degrees - Controls the spread of the angle of force for each particle. Each particle's initial angle will range from [force_angle_degrees - force_spread_degrees, force_angle_degrees + force_spread_degrees]. Valid range is [-360,360].

• initial_rotation_degrees - Sets the initial rotation of the particle when emitted. Valid range is [-360,360].

• initial_rotation_degrees_random - Randomness parameter for initial_rotation_degrees. The valid range is [0,1]. See note about randomness in the description above.

Examples

• examples/simple - This simple example shows a very simple scene RigidBody2D particles with a Sprite2D attached. The particles are emitted and bounce around in the physics simulation.

• examples/sparks - This more complex example uses a particle scene that includes multiple Sprite2D nodes, a Tween and a Light2D node. Additionally it uses a custom script to orient the tail of the spark along the path it is traveling, stretch the tail in line with its velocity, and vary the color of the sprite and light over the lifetime of the particle.

Acknowledgements:

• Spark sprites from Kenney's Particle Pack (https://kenney.nl/assets/particle-pack).

CHANGES

2019-11-16 - fix issue with particle bursts at the end of a "shot" when the emitter fell behind due to timer lag

TODO

• fix initial tail of spark so it never goes behind origin
• add support for arrays of particle scenes with multiple selection methods (random, random weighted, round-robin)
• add delay and delay randomness to impulse and force application
• add force duration and duration randomness to force