NetworkRigidbody never syncs velocity, especially problematic for Distributed Authority

[Yoraiz0r]

Description

I witnessed bugged behavior when a rigidbody switches ownership in distributed authority, using the Transferrable flag without RequirePermission.

The symptom was such that once the object switched its authority from playerA to playerB, it would change how it moves.

This is because velocities (linear and angular) are not synced at all. I've confirmed as much in NGO 2.0.0-exp2's code.
All NetworkRigidbody/NetworkRigidbodyBase does is manipulate the interpolation and kinematic state, and registers itself onto the network transform.
The network transform has no awareness of the concept of neither linear nor angular velocities. It does not add any of them to its states.

Because of that, given even small amounts of lag, two objects can have wildly different opinions of how a non-kinematic rigidbody should be moving, and because of that, ownership cannot be easily transferred over.

Reproduce Steps

Though I have not used these exact steps, I have a simple project wherein I display the linear and angular velocities of a sphere being rolled by two players, the values show that despite the sphere rolling around when controlled by another player, its actual velocity values are zero'd out. See it here.

1. Create a new project using NGO 2.0.0-exp2
2. Create a scene with a NetworkManager and a way to host & join
3. Add a sphere with a NetworkRigidbody, NetworkTransform, NetworkObject and set it to have Transferrable as true.
4. Add a NetworkBehaviour that would set the rigidbody's linear velocity to a non-zero value (such as Vector3.Forward) on authority alone, on spawn.
5. Add a button to allow claiming of ownership to another player.
6. Begin a pair of player instances and make them join each other
7. Witness the sphere moving for both players in a direction
8. Let the joined player claim ownership of the sphere
9. Witness that the sphere has stopped moving, because it has no linear velocity on its new owner.

Actual Outcome

Velocities are misaligned across clients, causing mismatch on ownership transfer.

Expected Outcome

Velocities are aligned and similar (identical) across all clients, allowing easy ownership transfer.

Screenshots

See the sample here, where

1. The objects are colored based on their owner's client Id
2. The top view shows editor gizmo overlays of linear velocity (top row), and angular velocity (bottom row), of the sphere.

The editor client represents the red player, Witness that while the sphere is blue, it has no/almost no linear velocity, despite moving.

Environment

• OS: Windows 10
• Unity Version: 6000.0.0b13
• Netcode Version: 2.0.0-exp2

Additional Context

I don't see any easy way to solve this one without editing the NetworkTransform class such that its synchronized state includes linear and angular velocities. I recognize this is far from an ideal solution, though.

There is no way to append new information to NetworkTransform state, nor is there any way to match its update rates, or interpolations.

At best, right now I'm experimenting with making a network behavior that uses network variables to store linear and angular velocities, and on ownership change, if you became the owner, apply those. I don't think this is the best nor right solution, and it lacks the whole 'unreliable deltas' etc based details that NetworkTransform supports.

Would love to know what's the proper way to go about solving this in the immediate, please!

[Yoraiz0r]

In attempt to further mitigate problems with ownership change in rigidbodies, I tried making ownership changes occur purely by distance to closest player. This clearly shows how the velocity is lost as soon as ownership is transferred.

Unity_2024-04-28_20-09-08.mp4

The jitter is a result of the remaining linear and angular velocity on the client that originally pushed the object. They don't get cleared by the transfer.

Adding a NetworkBehaviour that synchronizes and applies linear and angular velocity helps mitigate the problem but ownership transfer is still subject to the lag that comes with the switch, arguably worse looking than on just touch.

Unity_2024-04-28_20-15-26.mp4

Given the position is rolling back, I believe this is more of a NetworkTransform issue than a Rigidbody issue, though.

[krytoou]

Hello! Have you fixed it? I'm making a soccer game and can't sync the ball object in any way...

[Yury93]

Hi, have you solved this problem by any chance, can you suggest a solution or some direction to look in?

[NoelStephensUnity]

We are planning on providing an update that will include the ability to automatically synchronize velocities, however until that becomes available there is an experimental Asteroids Project you can take a look at that will provide you with a reference on how this can be achieved.

The components of interest:

• BaseObjectMotionHandler
  • Derives from NetworkTransform (recommended for creating custom motion related components).
• PhysicsObjectMotion
  • Derives from BaseObjectMotionHandler

Within PhysicsObjectMotion, you can find several NetworkVariable properties to handle synchronizing physics related values:

• Mass
• AngularVelocity
• Velocity
• Torque
• Force

The two primary things you want to synchronize to keep the object moving in the same relative direction with the same rotations are the angular and linear velocity properties.

The next area of interest would be the OnAuthorityPushTransformState that is invoked whenever the authority of the NetworkTransform has detected enough change on the synchronized axis to push a state update out to all non-authority instances. The idea behind this is if you have any form of change happening on the authority side and the object in question is non-kinematic... for your particular use case it means that most likely either the linear (i.e. position) or angular (i.e. rotation) velocities have changed which makes this a perfect time to check both values and if they exceed a threshold value then update their respective NetworkVariable.

So, at this point you have a NetworkTransform and NetworkRigidbody (configured for "Use Rigidbody for Motion") where the authority is synchronizing all non-authority (i.e. kinematic) instances of the change in the Rigidbody's position and rotation (PhysX version) as well as the "current" linear and angular velocities.

When ownership changes you just need to make sure the new owner applies the linear and angular velocities to the Rigidbody and since the new owner's instance will have changed from kinematic to non-kinematic the object should continue moving and rotating respective to the velocity values applied.

I would highly recommend updating to the most recent version of NGO (v2.3.0) as it also has some updates to the BufferedLinearInterpolator that should help you with any fine tuning needed between the change in ownership.

Let me know if this helps?