Understanding Physics Desynchronization in Unity

Unity's physics engine runs in a separate update cycle from rendering, using a fixed timestep to process physics calculations. When this synchronization breaks, objects may move inconsistently, pass through colliders, or experience unintended physics behavior.

Common Causes of Physics Desynchronization

• Inconsistent FixedUpdate and Update timings: Physics calculations do not align with the game loop.
• Frame rate fluctuations: Variable frame rates cause physics steps to skip or accumulate.
• Directly modifying Rigidbody positions: Setting transform values instead of using physics-based movement disrupts calculations.
• Incorrect interpolation/extrapolation settings: Objects jitter due to improper physics smoothing.

Diagnosing Physics Synchronization Issues

Checking Fixed Timestep Settings

Ensure Time.fixedDeltaTime is correctly configured:

Debug.Log(Time.fixedDeltaTime);

It should match a stable physics frame rate, typically:

Time.fixedDeltaTime = 1.0f / 50.0f;

Monitoring Rigidbody Updates

Log physics updates to check for inconsistencies:

void FixedUpdate()
{
    Debug.Log("FixedUpdate called at: " + Time.time);
}

Detecting Jitter Due to Manual Transform Modifications

Verify that objects are not being moved outside physics calculations:

transform.position += Vector3.forward * speed * Time.deltaTime; // Incorrect

Instead, use:

rigidbody.MovePosition(rigidbody.position + Vector3.forward * speed * Time.fixedDeltaTime);

Fixing Physics Desynchronization

Aligning FixedUpdate and Frame Rate

Ensure physics updates run at a stable rate:

Time.fixedDeltaTime = 1.0f / 60.0f;

Using Rigidbody for Movement

Avoid direct transform.position modifications:

rigidbody.velocity = new Vector3(0, 0, speed);

Configuring Interpolation for Smoother Movement

Enable Rigidbody interpolation to prevent jitter:

rigidbody.interpolation = RigidbodyInterpolation.Interpolate;

Optimizing Physics Performance

Reduce unnecessary calculations by limiting physics updates:

Physics.autoSimulation = false;

Then manually update physics:

Physics.Simulate(Time.fixedDeltaTime);

Preventing Future Physics Issues

• Keep Time.fixedDeltaTime consistent with the target frame rate.
• Use interpolation to smooth physics updates.
• Apply forces or velocities instead of modifying transform positions.

Conclusion

Physics desynchronization in Unity can lead to jittery movement and erratic collision behavior. By properly configuring physics timesteps, using Rigidbody-based movement, and enabling interpolation, developers can maintain smooth and predictable physics interactions.

FAQs

1. Why are my Unity physics interactions inconsistent?

Physics updates may be desynchronized from the frame rate due to an incorrect fixed timestep.

2. How do I prevent Rigidbody jitter?

Enable interpolation and ensure movement happens in FixedUpdate rather than Update.

3. Should I modify transform.position for physics-based objects?

No, always use rigidbody.MovePosition or apply forces for movement.

4. How does interpolation help with physics stability?

It smooths visual updates between physics calculations, reducing jitter.

5. What is the best fixedDeltaTime value for smooth physics?

A common value is 1.0f / 60.0f (for 60 physics updates per second), but this depends on the game.