How to ignore Gravity with Dynamic RigidBody?

[TarekAS]

Hi,

I'm making a flying camera that needs to have collision on and not be affected by gravity. Since the RigidBody::KinematicPositionBased does not collide, I'm using a RigidBody::Dynamic instead. However, that enables gravity on the object obviously.

I tried:

• Setting density to 0.0 (causing expected crash on collision as it needs to be > 0)
• Setting density to 0.0000000001 (changes nothing, worth a try)
• Finding a way to set mass to 0
• Finding a way to make KinematicPositionBased collide.

I haven't tried:

• Using a different physics layer with 0 gravity for this object only.

Any advice?

[jcornaz]

HI,

• Setting density to 0.0 (causing expected crash on collision as it needs to be > 0)
• Setting density to 0.0000000001 (changes nothing, worth a try)
• Finding a way to set mass to 0

Like in the real world, gravitational acceleration is not impacted by the mass of the object. Therefore, changing the mass (or density) is not supposed to be of any help.

Finding a way to make KinematicPositionBased collide.

By design, kinematic bodies are not supposed to be affected by forces. The "impulse" generated by contact-resolution is one such force that is not supposed to affect kinematic bodies, by design.

---

As you said, Dynamic objects are supposed to be affected by forces, and gravity is one of them. Though you may counteract the acceleration by addding an Acceleration to your object that is the inverse of your world gravity.

[TarekAS]

Thanks for the reply, and the quick physics lesson! (forgot that gravity bends light 😄)

I managed to fake removing gravity by re-initializing the rigidbody component on each frame:

fn fake_remove_gravity_system(mut q: Query<(&FlyCam, &mut RigidBody)>) {
    let (_, mut rb) = q.single_mut();
    *rb = RigidBody::Dynamic
}

This allows you to keep collision and pseudo-remove gravity.

Obviously this is a hack, but I only need it for debugging purposes anyway.
The proper way is a you suggested, applying acceleration inverse of the world gravity.

[TarekAS]

The inverse acceleration method:

<your_entity>
    .insert(RigidBody::Dynamic)
    .insert(Acceleration::from_linear(Vec3::Y * 9.81))