Unity To Godot (Placing 3D Object On Mouse Click)

Designed with a beginner-friendly approach, this guide is especially curated for developers experienced with Unity who are seeking to smoothly transition their skills to the Godot engine. In this informative article, we will walk you through the process of setting up a basic 3D scene in the Godot game engine. Our focus will be on implementing a feature that allows the spawning of a 3D box at the precise position where the mouse is clicked.

Comparing Godot and Unity

After creating a new project in Godot, you'll find yourself in the following window:

• The left side of the screen, called Scene, is similar to Unity's Hierarchy, where scene objects are organized.
• On the right side, you have the Inspector, which is analogous to Unity's Inspector.
• All project assets can be accessed through the FileSystem tab, which is akin to Unity's Project tab.
• You can switch between 3D and 2D scene views using a button at the top of the window.
• Godot comes with its own scripting language and editor, eliminating the need to install additional software for script creation and editing.

In Unity, project-related assets were stored inside an Assets folder, whereas in Godot, everything is contained within the project folder.

Setting Up 3D scene

• Let's start by creating a 3D scene from the Scene tab. Clicking on 3D Scene will generate a new empty node named Node3D, which serves as the foundation for all 3D-related nodes in the scene.

Everything in godot is a node

• You can add new nodes by clicking the plus icon in the Scene tab or by pressing Command+A (Ctrl+A on Windows). This action opens a comprehensive list of available nodes in Godot. You can either scroll through the list or use the search function to find the desired node. For our scene, we'll add a 3D camera and a directional light.
• You can adjust the light's direction by modifying the rotation of the DirectionalLight3D node in the Inspector. The rotation property is located under the Transform component, similar to Unity.

Creating a floor/plane

Creating a floor in Godot differs from Unity's approach. Here's how to do it:

• Add a StaticBody3D as the floor, as it represents a static object.
• Attach CollisionShape3D as a child of StaticBody3D to detect collisions.
• In the Inspector tab, select the desired shape for collision detection. In our case, we'll choose BoxShape3D.
• Add a MeshInstance3D to give a visual representation of the floor.
• In the Inspector tab, select BoxShape3D for the shape of the mesh.
• Adjust the size of the box by clicking the box icon in the Inspector panel and selecting your desired size.

Make sure to match the shape of the box in the CollisionShape3D to that of the mesh and collider.

Creating 3D Box

Now, let's create a 3D object and save it as a scene (similar to Unity's prefab) so that we can instantiate it on a mouse click:

• Add a new node called MeshInstance3D and rename it to BoxScene.
• Select BoxMesh as the mesh in the Inspector panel.
• Save BoxScene as a scene by right-clicking it and selecting Save Branch as Scene.
• After saving, delete the BoxScene node from the scene.

Adding Script To The Node

Scripts can be added by selecting the floor node and then clicking on the Add Script icon in the Scene tab. Alternatively, you can create a new script in the Inspector by specifying it in the Script field.

By default, there are two functions already present in the script:

extends StaticBody3D


# Called when the node enters the scene tree for the first time.
func _ready():
	pass # Replace with function body.


# Called every frame. 'delta' is the elapsed time since the previous frame.
func _process(delta):
	pass

• _ready is similar to Unity's Start function and runs at the start of the game.
• _process is akin to Unity's Update function and runs every frame. The delta argument represents the elapsed time since the previous frame, which can be used to normalize values, similar to Unity's Time.deltaTime.

Listening to Events In Script

You can listen to events by connecting an event to a script. Here's how:

1. Select the floor node.
2. Go to the Node tab on the side of the Inspector panel.
3. Choose input_event and then click Connect at the bottom of the panel.

You can name the function that will be called when that event occurs or use the suggested one. The newly created function will take parameters such as camera, event, position, normal, and shape_idx.

func _on_input_event(camera, event, position, normal, shape_idx):
	pass # Replace with function body.

This event contains various event information. You need to check if it's a mouse button event and if it's a mouse button press:

func _on_input_event(camera, event, position, normal, shape_idx):
	if(event is InputEventMouseButton):
		if(event.pressed):
			print("Pressed")

Spawning a 3DBox On Mouse Click

To spawn a 3D box on a mouse click, follow these steps:

• Preload the BoxScene in the script using the preload function.
• Instantiate the preloaded node, set its position (for now, to Vector3.ZERO), and add the instantiated node as a child:

extends StaticBody3D

var BoxScene = preload("res://BoxScene.tscn")
# Called when the node enters the scene tree for the first time.
func _ready():
	pass # Replace with function body.


# Called every frame. 'delta' is the elapsed time since the previous frame.
func _process(delta):
	pass


func _on_input_event(camera, event, position, normal, shape_idx):
	if(event is InputEventMouseButton):
		if(event.pressed):
			print("Pressed")
			var BoxInstance = BoxScene.instantiate()
			BoxInstance.position  = Vector3.ZERO
			add_child(BoxInstance)

Getting 3d Position from Mouse Click

To obtain the 3D position from a mouse click, use a raycast from the mouse position in the direction the camera is facing to find the raycast hit point on the floor:

func raycast_from_mouse(m_pos):
	var cam = get_viewport().get_camera_3d()
	var ray_start = cam.project_ray_origin(m_pos)
	var ray_end = ray_start + cam.project_ray_normal(m_pos) * 2000
	var world3d : World3D = get_world_3d()
	var space_state = world3d.direct_space_state
	
	if space_state == null:
		return
	
	var query = PhysicsRayQueryParameters3D.create(ray_start, ray_end)
	query.collide_with_areas = true
	
	return space_state.intersect_ray(query)["position"]

• First, we get the camera from the viewport
• Set the ray start point as the mouse pos
• Set ray end point in camera's normal direction multiplied by 2000
• PhysicsRayQueryParameters3D.create() creates the actual raycast and returns the query.

The returned position from raycast_from_mouse function is used to set the position of the spawned 3dBox.

Here is the full code

extends StaticBody3D

var BoxScene = preload("res://BoxScene.tscn")
# Called when the node enters the scene tree for the first time.
func _ready():
	pass # Replace with function body.


# Called every frame. 'delta' is the elapsed time since the previous frame.
func _process(delta):
	pass


func _on_input_event(camera, event, position, normal, shape_idx):
	if(event is InputEventMouseButton):
		if(event.pressed):
			print("Pressed")
			var BoxInstance = BoxScene.instantiate()
			BoxInstance.position  = raycast_from_mouse(event.position)
			BoxInstance.position.y = 1
			add_child(BoxInstance)

func raycast_from_mouse(m_pos):
	var cam = get_viewport().get_camera_3d()
	var ray_start = cam.project_ray_origin(m_pos)
	var ray_end = ray_start + cam.project_ray_normal(m_pos) * 10000
	var world3d : World3D = get_world_3d()
	var space_state = world3d.direct_space_state
	
	if space_state == null:
		return
	
	var query = PhysicsRayQueryParameters3D.create(ray_start, ray_end)
	query.collide_with_areas = true
	
	return space_state.intersect_ray(query)["position"]

Thank you for reading this article. Please let me know in comments if you would like more such articles on transitioning from Unity to Godot.