class_name MarineSnow
extends GPUParticles3D

## Suspended particles (marine snow, sediment) around the viewer. The emitter is a
## world-space box re-centred on the active camera every frame, so flakes keep their
## own slow drift while the submarine moves through them instead of travelling with
## it. Sinks a few centimetres per second with a little turbulence, as detritus does.

const BOX_EXTENTS_M := Vector3(16.0, 10.0, 16.0)
const PARTICLE_COUNT := 2400
const LIFETIME_S := 16.0
const SINK_SPEED_M_S := 0.035
const DRIFT_SPEED_M_S := 0.06
const FLAKE_SIZE_M := Vector2(0.05, 0.16)

const SNOW_SHADER = preload("res://game/world/sites/marine_snow.gdshader")


func _ready() -> void:
	local_coords = false
	amount = PARTICLE_COUNT
	lifetime = LIFETIME_S
	preprocess = LIFETIME_S
	fixed_fps = 30
	visibility_aabb = AABB(-BOX_EXTENTS_M * 1.5, BOX_EXTENTS_M * 3.0)
	process_material = _process_material()
	var quad := QuadMesh.new()
	quad.size = Vector2.ONE
	var material := ShaderMaterial.new()
	material.shader = SNOW_SHADER
	quad.material = material
	draw_pass_1 = quad
	emitting = true


## A jump larger than the box (spawn, teleport, first camera) would leave the viewer in
## empty water for a whole lifetime: the emitter restarts and preprocesses in place.
func _process(_delta: float) -> void:
	var camera := get_viewport().get_camera_3d() if is_inside_tree() else null
	if camera == null:
		return
	var jumped := global_position.distance_to(camera.global_position) > BOX_EXTENTS_M.y
	global_position = camera.global_position
	if jumped:
		restart()


func _process_material() -> ParticleProcessMaterial:
	var material := ParticleProcessMaterial.new()
	material.emission_shape = ParticleProcessMaterial.EMISSION_SHAPE_BOX
	material.emission_box_extents = BOX_EXTENTS_M
	material.direction = Vector3(1.0, 0.0, 0.3)
	material.spread = 180.0
	material.initial_velocity_min = DRIFT_SPEED_M_S * 0.3
	material.initial_velocity_max = DRIFT_SPEED_M_S
	material.gravity = Vector3(0.0, -SINK_SPEED_M_S, 0.0)
	material.turbulence_enabled = true
	material.turbulence_noise_strength = 0.4
	material.turbulence_noise_scale = 6.0
	material.turbulence_noise_speed_random = 0.2
	material.turbulence_influence_min = 0.02
	material.turbulence_influence_max = 0.06
	material.scale_min = FLAKE_SIZE_M.x
	material.scale_max = FLAKE_SIZE_M.y
	# Fade in and out through alpha so flakes never pop at the box edge or at respawn.
	var alpha := Gradient.new()
	alpha.set_offset(0, 0.0)
	alpha.set_color(0, Color(1.0, 1.0, 1.0, 0.0))
	alpha.add_point(0.15, Color(1.0, 1.0, 1.0, 1.0))
	alpha.add_point(0.85, Color(1.0, 1.0, 1.0, 1.0))
	alpha.set_offset(alpha.get_point_count() - 1, 1.0)
	alpha.set_color(alpha.get_point_count() - 1, Color(1.0, 1.0, 1.0, 0.0))
	var ramp := GradientTexture1D.new()
	ramp.gradient = alpha
	material.color_ramp = ramp
	return material