class_name WeatherParticles extends GPUParticles3D
## Particle system that follows the camera.

## How far directly in front of the camera to remain regardless of velocity.
@export var front_flat_distance: float = 1.0
## How many seconds ahead of the camera to remain when it moves.
@export var seconds_ahead: float = 4.0

## Last known position of the camera, for tracking velocity.
var _last_camera_posn := Vector3.ZERO

func _process(delta: float) -> void:
	var camera := get_viewport().get_camera_3d()
	if camera:
		# Track camera velocity.
		var camera_velocity := (
			camera.global_position - _last_camera_posn
		)/delta
		_last_camera_posn = camera.global_position
		# Reposition particle system.
		global_position = (
			camera.global_position +
			camera.basis.z*front_flat_distance +
			camera_velocity*seconds_ahead
		)