One of the most common tasks in game development is casting a ray (or custom shaped object) and checking what it hits. This enables complex behaviors, AI, etc. to take place. This tutorial will explain how to do this in 2D and 3D.
Godot stores all the low level game information in servers, while the scene is just a frontend. As such, ray casting is generally a lower-level task. For simple raycasts, node such as :ref:`RayCast <class_RayCast>` and :ref:`RayCast2D <class_RayCast2D>` will work, as they will return every frame what the result of a raycast is.
Many times, though, ray-casting needs to be a more interactive process so a way to do this by code must exist.
In the physics world, Godot stores all the low level collision and physics information in a space. The current 2d space (for 2D Physics) can be obtained by accessing :ref:`CanvasItem.get_world_2d().space <class_CanvasItem_get_world_2d>`. For 3D, it's :ref:`Spatial.get_world().space <class_Spatial_get_world>`.
The resulting space :ref:`RID <class_RID>` can be used in :ref:`PhysicsServer <class_PhysicsServer>` and :ref:`Physics2DServer <class_Physics2DServer>` respectively for 3D and 2D.
Godot physics runs by default in the same thread as game logic, but may be set to run on a separate thread to work more efficiently. Due to this, the only time accessing space is safe is during the :ref:`Node._physics_process() <class_Node__physics_process>` callback. Accessing it from outside this function may result in an error due to space being locked.
To perform queries into physics space, the :ref:`Physics2DDirectSpaceState <class_Physics2DDirectSpaceState>` and :ref:`PhysicsDirectSpaceState <class_PhysicsDirectSpaceState>` must be used.
Use the following code in 2D:
.. tabs::
.. code-tab:: gdscript GDscript
func _physics_process(delta):
var space_rid = get_world_2d().space
var space_state = Physics2DServer.space_get_direct_state(space_rid)
.. code-tab:: csharp
public override void _PhysicsProcess(float delta)
{
var spaceRid = GetWorld2d().Space;
var spaceState = Physics2DServer.SpaceGetDirectState(spaceRid);
}
Or more directly:
.. tabs::
.. code-tab:: gdscript GDScript
func _physics_process(delta):
var space_state = get_world_2d().direct_space_state
.. code-tab:: csharp
public override void _PhysicsProcess(float delta)
{
var spaceState = GetWorld2d().DirectSpaceState;
}
And in 3D:
.. tabs::
.. code-tab:: gdscript GDScript
func _physics_process(delta):
var space_state = get_world().direct_space_state
.. code-tab:: csharp
public override void _PhysicsProcess(float delta)
{
var spaceState = GetWorld().DirectSpaceState;
}
For performing a 2D raycast query, the method :ref:`Physics2DDirectSpaceState.intersect_ray() <class_Physics2DDirectSpaceState_intersect_ray>` may be used. For example:
.. tabs::
.. code-tab:: gdscript GDScript
func _physics_process(delta):
var space_state = get_world_2d().direct_space_state
# use global coordinates, not local to node
var result = space_state.intersect_ray(Vector2(0, 0), Vector2(50, 100))
.. code-tab:: csharp
public override void _PhysicsProcess(float delta)
{
var spaceState = GetWorld2d().DirectSpaceState;
// use global coordinates, not local to node
var result = spaceState.IntersectRay(new Vector2(), new Vector2(50, 100));
}
The result is a dictionary. If the ray didn't hit anything, the dictionary will be empty. If it did hit something it will contain collision information:
.. tabs::
.. code-tab:: gdscript GDScript
if result:
print("Hit at point: ", result.position)
.. code-tab:: csharp
if (result.Count > 0)
GD.Print("Hit at point: ", result["position"]);
The result dictionary when a collision occurs contains the following
data:
{
position: Vector2 # point in world space for collision
normal: Vector2 # normal in world space for collision
collider: Object # Object collided or null (if unassociated)
collider_id: ObjectID # Object it collided against
rid: RID # RID it collided against
shape: int # shape index of collider
metadata: Variant() # metadata of collider
}
The data is similar in 3D space, using Vector3 coordinates.
A common use case for ray casting is to enable a character to gather data about the world around it. One problem with this is that the same character has a collider, so the ray will only detect its parent's collider, as shown in the following image:
To avoid self-intersection, the intersect_ray() function can take an
optional third parameter which is an array of exceptions. This is an
example of how to use it from a KinematicBody2D or any other
collision object node:
.. tabs::
.. code-tab:: gdscript GDScript
extends KinematicBody2D
func _physics_process(delta):
var space_state = get_world_2d().direct_space_state
var result = space_state.intersect_ray(global_position, enemy_position, [self])
.. code-tab:: csharp
class Body : KinematicBody2D
{
public override void _PhysicsProcess(float delta)
{
var spaceState = GetWorld2d().DirectSpaceState;
var result = spaceState.IntersectRay(globalPosition, enemyPosition, new object[] { this });
}
}
The exceptions array can contain objects or RIDs.
While the exceptions method works fine for excluding the parent body, it becomes very inconvenient if you need a large and/or dynamic list of exceptions. In this case, it is much more efficient to use the collision layer/mask system.
The optional fourth argument for intersect_ray() is a collision mask. For
example, to use same mask as the parent body, use the collision_mask
member variable:
.. tabs::
.. code-tab:: gdscript GDScript
extends KinematicBody2D
func _physics_process(delta):
var space_state = get_world().direct_space_state
var result = space_state.intersect_ray(global_position, enemy_position,
[self], collision_mask)
.. code-tab:: csharp
class Body : KinematicBody2D
{
public override void _PhysicsProcess(float delta)
{
var spaceState = GetWorld2d().DirectSpaceState;
var result = spaceState.IntersectRay(globalPosition, enemyPosition,
new object[] { this }, CollisionMask);
}
}
Casting a ray from screen to 3D physics space is useful for object picking. There is not much of a need to do this because :ref:`CollisionObject <class_CollisionObject>` has an "input_event" signal that will let you know when it was clicked, but in case there is any desire to do it manually, here's how.
To cast a ray from the screen, you need a :ref:`Camera <class_Camera>`
node. A Camera can be in two projection modes: perspective and
orthogonal. Because of this, both the ray origin and direction must be
obtained. This is because origin changes in orthogonal mode, while
normal changes in perspective mode:
To obtain it using a camera, the following code can be used:
.. tabs::
.. code-tab:: gdscript GDScript
const ray_length = 1000
func _input(event):
if event is InputEventMouseButton and event.pressed and event.button_index == 1:
var camera = $Camera
var from = camera.project_ray_origin(event.position)
var to = from + camera.project_ray_normal(event.position) * ray_length
.. code-tab:: csharp
private const float rayLength = 1000;
public override void _Input(InputEvent @event)
{
if (@event is InputEventMouseButton eventMouseButton && eventMouseButton.Pressed && eventMouseButton.ButtonIndex == 1)
{
var camera = (Camera)GetNode("Camera");
var from = camera.ProjectRayOrigin(eventMouseButton.Position);
var to = from + camera.ProjectRayNormal(eventMouseButton.Position) * rayLength;
}
}
Remember that during _input(), the space may be locked, so in practice
this query should be run in _physics_process().