Use cases for rigidbody.linearVelocity

Help & Support
1

Hey,

I’ve recently had great use for the ammo.js physics engine and the entity.rigidbody.linearVelocity variable to assess an entity’s velocity, however I noticed in some cases linearVelocity is still 0 even if the entity is moving (and has a rigidbody). Namely when the entity is being moved by entity.rigidbody.teleport and entity.setPosition().

Anyone know in which cases entity.rigidbody.linearVelocity actually reflect the movement of the entity? And is there a way to accurately set the position of an entity in such a way that linearVelocity can still be used?

2

Hi @Astra!

I assume you don’t use a dynamic rigidbody?

Otherwise you can use it in combination with methods like applyForce and applyImpulse.

3

Actually I am!

But I want to move the entity along a fixed path which is pretty difficult to do accurately with applyForce and applyImpulse. So I was hoping it is possible to use something else.

(My temporary solution is to manually calculate the velocity, but that has its own set of caveats)

4

You can keep it kinematic then. The valocity property on the kinematic entity is not calculated. You can manually get the velocity of it like this:

// current velocity = (current frame position - previous frame position) / dt;
velocity.copy(currentPosition).sub(lastPosition).mulScalar(1/dt);

Just make sure to set both currentPosition and lastPosition to the same value, when teleporting entity a large distance (e.g. on respawn). Otherwise it will create a large velocity value.

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