Add a volumetric fog demo

This is an example of Godot's volumetric fog capabilities
with the Vulkan renderer.

Showcased features are:

- Positive/negative density volumes that affect albedo (incoming light)
  and emission.
- Box/ellipsoid shapes, height falloff, and density modulation using a
  3D texture.
- Temporal reprojection for greater stability and avoiding flickering.
  - The difference is demonstrated with a moving fog volume.
- Global density adjustment. With FogVolume nodes that have a
  positive density, it's possible to apply volumetric fog only in
  specific areas.

Author: Calinou

3d/volumetric_fog/README.md

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+# Volumetric Fog
+
+This is an example of Godot's volumetric fog capabilities with the Vulkan renderer.
+
+Showcased features are:
+
+- Positive/negative density volumes that affect albedo (incoming light) and emission.
+- Box/ellipsoid shapes, height falloff, and density modulation using a 3D texture.
+- Temporal reprojection for greater stability and avoiding flickering.
+  - The difference is demonstrated with a moving fog volume.
+- Global density adjustment. With FogVolume nodes that have a positive density,
+  it's possible to apply volumetric fog only in specific areas.
+
+Language: GDScript
+
+Renderer: Vulkan
+
+## Screenshots
+
+![Screenshot](screenshots/volumetric_fog.png)

3d/volumetric_fog/camera.gd

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+extends Camera3D
+
+const MOUSE_SENSITIVITY = 0.002
+const MOVE_SPEED = 0.6
+
+var rot = Vector3()
+var velocity = Vector3()
+
+@onready var label = $Label
+
+
+func _ready():
+	Input.set_mouse_mode(Input.MOUSE_MODE_CAPTURED)
+	update_label()
+
+
+func _process(delta):
+	var motion = Vector3(
+			Input.get_action_strength("move_right") - Input.get_action_strength("move_left"),
+			0,
+			Input.get_action_strength("move_back") - Input.get_action_strength("move_forward")
+	)
+
+	# Normalize motion to prevent diagonal movement from being
+	# `sqrt(2)` times faster than straight movement.
+	motion = motion.normalized()
+
+	velocity += MOVE_SPEED * delta * (transform.basis * motion)
+	velocity *= 0.85
+	position += velocity
+
+
+func _input(event):
+	# Mouse look (only if the mouse is captured).
+	if event is InputEventMouseMotion and Input.get_mouse_mode() == Input.MOUSE_MODE_CAPTURED:
+		# Horizontal mouse look.
+		rot.y -= event.relative.x * MOUSE_SENSITIVITY
+		# Vertical mouse look.
+		rot.x = clamp(rot.x - event.relative.y * MOUSE_SENSITIVITY, -1.57, 1.57)
+		transform.basis = Basis(rot, 0)
+
+	if event.is_action_pressed("toggle_mouse_capture"):
+		if Input.get_mouse_mode() == Input.MOUSE_MODE_CAPTURED:
+			Input.set_mouse_mode(Input.MOUSE_MODE_VISIBLE)
+		else:
+			Input.set_mouse_mode(Input.MOUSE_MODE_CAPTURED)
+			
+	if event.is_action_pressed("toggle_temporal_reprojection"):
+		get_world_3d().environment.volumetric_fog_temporal_reprojection_enabled = not get_world_3d().environment.volumetric_fog_temporal_reprojection_enabled
+		update_label()
+	elif event.is_action_pressed("increase_temporal_reprojection"):
+		get_world_3d().environment.volumetric_fog_temporal_reprojection_amount = clamp(get_world_3d().environment.volumetric_fog_temporal_reprojection_amount + 0.01, 0.0, 0.99)
+		update_label()
+	elif event.is_action_pressed("decrease_temporal_reprojection"):
+		get_world_3d().environment.volumetric_fog_temporal_reprojection_amount = clamp(get_world_3d().environment.volumetric_fog_temporal_reprojection_amount - 0.01, 0.0, 0.99)
+		update_label()
+	elif event.is_action_pressed("increase_fog_density"):
+		get_world_3d().environment.volumetric_fog_density = clamp(get_world_3d().environment.volumetric_fog_density + 0.01, 0.0, 1.0)
+		update_label()
+	elif event.is_action_pressed("decrease_fog_density"):
+		get_world_3d().environment.volumetric_fog_density = clamp(get_world_3d().environment.volumetric_fog_density - 0.01, 0.0, 1.0)
+		update_label()
+
+
+func update_label():
+	if get_world_3d().environment.volumetric_fog_temporal_reprojection_enabled:
+		label.text = "Fog density: %.2f\nTemporal reprojection: Enabled\nTemporal reprojection strength: %.3f" % [get_world_3d().environment.volumetric_fog_density, get_world_3d().environment.volumetric_fog_temporal_reprojection_amount]
+	else:
+		label.text = "Fog density: %.2f\nTemporal reprojection: Disabled" % get_world_3d().environment.volumetric_fog_density

3d/volumetric_fog/icon_outlined.png

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+[remap]
+
+importer="3d_texture"
+type="StreamTexture3D"
+uid="uid://bpvy0e34otmhk"
+path="res://.godot/imported/icon_outlined.png-be09dc71a0cb04701900baffec3ab610.stex3d"
+metadata={
+"vram_texture": false
+}
+
+[deps]
+
+source_file="res://icon_outlined.png"
+dest_files=["res://.godot/imported/icon_outlined.png-be09dc71a0cb04701900baffec3ab610.stex3d"]
+
+[params]
+
+compress/mode=0
+compress/lossy_quality=0.7
+compress/hdr_compression=1
+compress/bptc_ldr=0
+compress/channel_pack=0
+mipmaps/generate=false
+mipmaps/limit=-1
+slices/horizontal=1
+slices/vertical=1

3d/volumetric_fog/icon_outlined.png.import

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+; Engine configuration file.
+; It's best edited using the editor UI and not directly,
+; since the parameters that go here are not all obvious.
+;
+; Format:
+;   [section] ; section goes between []
+;   param=value ; assign values to parameters
+
+config_version=4
+
+[application]
+
+config/name="Volumetric Fog"
+run/main_scene="res://volumetric_fog.tscn"
+
+[display]
+
+window/dpi/allow_hidpi=true
+window/stretch/mode="canvas_items"
+window/stretch/aspect="expand"
+
+[input]
+
+toggle_temporal_reprojection={
+"deadzone": 0.5,
+"events": [Object(InputEventKey,"resource_local_to_scene":false,"resource_name":"","device":0,"window_id":0,"store_command":true,"alt_pressed":false,"shift_pressed":false,"meta_pressed":false,"command_pressed":false,"pressed":false,"keycode":0,"physical_keycode":32,"unicode":0,"echo":false,"script":null)
+]
+}
+increase_temporal_reprojection={
+"deadzone": 0.5,
+"events": [Object(InputEventMouseButton,"resource_local_to_scene":false,"resource_name":"","device":0,"window_id":0,"store_command":true,"alt_pressed":false,"shift_pressed":false,"meta_pressed":false,"command_pressed":false,"button_mask":0,"position":Vector2(0, 0),"global_position":Vector2(0, 0),"factor":1.0,"button_index":4,"pressed":false,"double_click":false,"script":null)
+, Object(InputEventKey,"resource_local_to_scene":false,"resource_name":"","device":0,"window_id":0,"store_command":true,"alt_pressed":false,"shift_pressed":false,"meta_pressed":false,"command_pressed":false,"pressed":false,"keycode":0,"physical_keycode":16777232,"unicode":0,"echo":false,"script":null)
+]
+}
+decrease_temporal_reprojection={
+"deadzone": 0.5,
+"events": [Object(InputEventMouseButton,"resource_local_to_scene":false,"resource_name":"","device":0,"window_id":0,"store_command":true,"alt_pressed":false,"shift_pressed":false,"meta_pressed":false,"command_pressed":false,"button_mask":0,"position":Vector2(0, 0),"global_position":Vector2(0, 0),"factor":1.0,"button_index":5,"pressed":false,"double_click":false,"script":null)
+, Object(InputEventKey,"resource_local_to_scene":false,"resource_name":"","device":0,"window_id":0,"store_command":true,"alt_pressed":false,"shift_pressed":false,"meta_pressed":false,"command_pressed":false,"pressed":false,"keycode":0,"physical_keycode":16777234,"unicode":0,"echo":false,"script":null)
+]
+}
+increase_fog_density={
+"deadzone": 0.5,
+"events": [Object(InputEventKey,"resource_local_to_scene":false,"resource_name":"","device":0,"window_id":0,"store_command":true,"alt_pressed":false,"shift_pressed":false,"meta_pressed":false,"command_pressed":false,"pressed":false,"keycode":0,"physical_keycode":16777233,"unicode":0,"echo":false,"script":null)
+]
+}
+decrease_fog_density={
+"deadzone": 0.5,
+"events": [Object(InputEventKey,"resource_local_to_scene":false,"resource_name":"","device":0,"window_id":0,"store_command":true,"alt_pressed":false,"shift_pressed":false,"meta_pressed":false,"command_pressed":false,"pressed":false,"keycode":0,"physical_keycode":16777231,"unicode":0,"echo":false,"script":null)
+]
+}
+move_forward={
+"deadzone": 0.5,
+"events": [Object(InputEventKey,"resource_local_to_scene":false,"resource_name":"","device":0,"window_id":0,"store_command":true,"alt_pressed":false,"shift_pressed":false,"meta_pressed":false,"command_pressed":false,"pressed":false,"keycode":0,"physical_keycode":87,"unicode":0,"echo":false,"script":null)
+]
+}
+move_back={
+"deadzone": 0.5,
+"events": [Object(InputEventKey,"resource_local_to_scene":false,"resource_name":"","device":0,"window_id":0,"store_command":true,"alt_pressed":false,"shift_pressed":false,"meta_pressed":false,"command_pressed":false,"pressed":false,"keycode":0,"physical_keycode":83,"unicode":0,"echo":false,"script":null)
+]
+}
+move_right={
+"deadzone": 0.5,
+"events": [Object(InputEventKey,"resource_local_to_scene":false,"resource_name":"","device":0,"window_id":0,"store_command":true,"alt_pressed":false,"shift_pressed":false,"meta_pressed":false,"command_pressed":false,"pressed":false,"keycode":0,"physical_keycode":68,"unicode":0,"echo":false,"script":null)
+]
+}
+move_left={
+"deadzone": 0.5,
+"events": [Object(InputEventKey,"resource_local_to_scene":false,"resource_name":"","device":0,"window_id":0,"store_command":true,"alt_pressed":false,"shift_pressed":false,"meta_pressed":false,"command_pressed":false,"pressed":false,"keycode":0,"physical_keycode":65,"unicode":0,"echo":false,"script":null)
+]
+}
+toggle_mouse_capture={
+"deadzone": 0.5,
+"events": [Object(InputEventKey,"resource_local_to_scene":false,"resource_name":"","device":0,"window_id":0,"store_command":true,"alt_pressed":false,"shift_pressed":false,"meta_pressed":false,"command_pressed":false,"pressed":false,"keycode":0,"physical_keycode":16777217,"unicode":0,"echo":false,"script":null)
+]
+}
+
+[rendering]
+
+anti_aliasing/quality/screen_space_aa=1
+anti_aliasing/quality/use_debanding=true