5.7 KiB
Ray-casting
Introduction
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.
Pandemonium 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
RayCast
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.
Space
In the physics world, Pandemonium stores all the low level collision and
physics information in a space. The current 2d space (for 2D Physics)
can be obtained by accessing
CanvasItem.get_world_2d().space.
For 3D, it's Spatial.get_world().space.
The resulting space RID can be used in
PhysicsServer and
Physics2DServer respectively for 3D and 2D.
Accessing space
Pandemonium 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
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
Physics2DDirectSpaceState
and PhysicsDirectSpaceState
must be used.
Use the following code in 2D:
gdscript GDscript
func _physics_process(delta):
var space_rid = get_world_2d().space
var space_state = Physics2DServer.space_get_direct_state(space_rid)
Or more directly:
gdscript GDScript
func _physics_process(delta):
var space_state = get_world_2d().direct_space_state
And in 3D:
gdscript GDScript
func _physics_process(delta):
var space_state = get_world().direct_space_state
Raycast query
For performing a 2D raycast query, the method
Physics2DDirectSpaceState.intersect_ray()
may be used. For example:
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))
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:
gdscript GDScript
if result:
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.
Collision exceptions
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:
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])
The exceptions array can contain objects or RIDs.
Collision Mask
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 the same mask as the parent body, use the collision_mask
member variable:
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)
See doc_physics_introduction_collision_layer_code_example for details on how to set the collision mask.
3D ray casting from screen
Casting a ray from screen to 3D physics space is useful for object
picking. There is not much need to do this because
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 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:
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
Remember that during input(), the space may be locked, so in practice
this query should be run in physics_process().