PinJoint2D physics messed up by adding GPUParticles2D to Node B's RigidBody2D

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1

Godot Version

v4.6.stable.official [89cea1439]

Question

I set up my 2D character like this: a CharacterBody2D (Node A) which can be moved by the player and a Rigidbody2D (Node B) for a head which follows the body through a PinJoint2D. After some tweaking everything worked fine.

Then I added a GPUParticles2D to the Rigidbody2D (Node B) and it messed up the PinJoint2D’s physics. If I reparent the particle effect to the CharacterBody2D the issue disappears but then the effect won’t follow the head. Is this a bug? Any help would be appreciated.

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1 Like
2

What do you mean by messed up physics? GPUParticles2D has nothing to do with physics so it can’t affect them.

3

That’s what I’d think. And maybe still think but the GPUParticles2D’s parenting is the only difference. I made two videos which I’m only able to link as a new user:

How the pinjoint should work: Dropbox

How it is when Node B gets a GPUParticles2D: Dropbox

Hope this clears up at least what the issue looks like.

4

Uhm, you are right… Just accessing the RigidBody2D.global_transform inside the _physics_process() method in the CharacterBody2D messes up the RigidBody2D (GPUParticles2D does this internally) but only before calling move_and_slide() :confused:. Like:

extends CharacterBody2D

@onready var rigid_body_2d: RigidBody2D = %RigidBody2D

func _physics_process(delta: float) -> void:
	rigid_body_2d.global_transform
	# ...
	move_and_slide()

but doing this does not:

extends CharacterBody2D

@onready var rigid_body_2d: RigidBody2D = %RigidBody2D

func _physics_process(delta: float) -> void:
	# ...
	move_and_slide()
	rigid_body_2d.global_transform

There’s a note in the RigidBody2D documentation about accessing the global transform if the node is a child of another moving node:

Note: Changing the 2D transform or linear_velocity of a RigidBody2D very often may lead to some unpredictable behaviors. This also happens when a RigidBody2D is the descendant of a constantly moving node, like another RigidBody2D, as that will cause its global transform to be set whenever its ancestor moves.

I’m not sure if this is technically a bug or just a limitation but try checking the issue tracker if there’s any issue related to this or open a new one.

As a workaround you can re-parent the GPUParticles2D node to the CharacterBody2D node and do:

extends CharacterBody2D

@onready var gpu_particles_2d: GPUParticles2D = %GPUParticles2D
@onready var rigid_body_2d: RigidBody2D = %RigidBody2D

func _physics_process(delta: float) -> void:
	# ...
	move_and_slide()
	
	gpu_particles_2d.global_transform = rigid_body_2d.global_transform
5

Thanks for the quick and thorough reply!

Not sure why this happens but if there’s a note about it in the documentation maybe someday someone will fix the cause of these “unpredictable behaviors”.

Your suggestion worked like a charm, thanks again!