Why KinematicBody2D wont move a RigidBody2D

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:bust_in_silhouette: Asked By bruno_ortiz
:warning: Old Version Published before Godot 3 was released.

Hello, i am trying to build a simple space shooter game, but i am stumbling in some aspects of the physics engine.

Actually i have a spaceship and a asteroid. I made the ship kinematic and the asteroid a RigidBody2D, because i wanted it to rotate when they collided. But when i move the spaceship using:

	move(velocity * delta)

The asteroid wont move at all. But when i move the ship using:

    set_pos(get_pos() + velocity * delta)

The asteroid behaves like expected and moves/rotates.

I am having a hard time trying to understand that. If someone could help me i would be really thankful.

:bust_in_silhouette: Reply From: Gokudomatic2

Just my guess, but I think the kinematicbody doesn’t actually do anything within the physics engine by itself. You have access to the physics engine methods to make it move, but if you want it to push other bodies, you have to code it. You have to manage everything, including going through all colliding objects and give them an impulse. Quite a lot of work for less performance if you want to do the same thing as the rigidbody. Are you sure you don’t want to use the rigidbody?

Humm, you are right, i’ll rethink my implementation, and do some tests with a rigidbody.

bruno_ortiz | 2016-07-08 02:14

:bust_in_silhouette: Reply From: Warlaan

If I understand you correctly your problem is that the kinematic body you move doesn’t push the asteroid aside when using the move()-method?
When you read the documentation for that method you will find that move doesn’t move a kinematic body all the way to the destination but only as far as possible without colliding into anything. That means that if you use the move-command the kinematic body will stop moving before it hits anything and therefore can’t push anything aside.

humm, ok i read that, and thought that could be it, but i was intrigued that the “setPos” method moved the asteroid.

bruno_ortiz | 2016-07-08 02:13

:bust_in_silhouette: Reply From: Max Aller

Godot newbie here (so I don’t know if this is a bad idea or if there’s a better way), but this worked for me:

func _fixed_process(delta):
	# ...calculate desired velocity, e.g. due to keypresses
    # ...then move the character

	motion = move(motion)
	if (is_colliding()):
		# If bumping into a rigid body, push it!
		var collider = get_collider()
		if (collider extends RigidBody2D):
			collider.apply_impulse(motion, collider.get_pos() - get_collision_pos())

    # etc

At the moment I’m using this to make a user-controller KinematicBody2D push around a circular RigidBody2D (platformer).

A non-programatically solution is to add another KinematicBody as child of the one using move with similar (or the same) shape as the parent, and different layer/masks, of course, make the rigid ignore the KB that uses “move” (mask).

eons | 2017-05-14 12:57