Implementing physics-based interactions in XR

Physics-based interactions are crucial for creating immersive VR/AR/MR experiences. This guide covers implementation strategies, optimization techniques, and best practices for realistic object interactions in XR environments.

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1. Core Physics Interaction Types

A. Basic Object Interactions

Interaction	Implementation	Example Use Case
Grab & Throw	Rigidbody + XR Grab Interactable (Unity)	Picking up tools in VR
Push/Pull	Force-based movement + haptic feedback	Moving furniture in AR
Stacking	Colliders + friction/gravity tuning	Warehouse simulation

B. Advanced Interactions

• Breakable Objects (pre-fractured meshes + destruction physics)
• Liquids/Particles (GPU-accelerated fluid sims like Zibra Liquids)
• Cloth/Softbody (Obi Softbody for Unity, Chaos Cloth in Unreal)

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2. Implementation Workflow

A. Unity (XR Interaction Toolkit)

// Basic grab implementation
using UnityEngine.XR.Interaction.Toolkit;

public class GrabableObject : XRGrabInteractable
{
    protected override void OnSelectEntered(XRBaseInteractor interactor)
    {
        base.OnSelectEntered(interactor);
        // Add custom grab logic
    }
}

Key Components:

1. XR Grab Interactable component
2. Rigidbody with proper mass/drag settings
3. Colliders (prefer convex mesh colliders)

B. Unreal Engine (Physics Handle)

1. Enable Chaos Physics in Project Settings
2. Use Physics Handle component for grab mechanics
3. Implement force feedback via Haptic Feedback Effect

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3. Optimization Techniques

A. Performance-Critical Settings

Parameter	Mobile XR Value	PC XR Value
Physics Solver Iters	4-6	8-12
Max Depenetration Vel	1-2	4-10
Fixed Timestep	0.0167s	0.0111s

B. Advanced Optimization

✅ Spatial Partitioning

• Use Unity’s Physics.OverlapSphereNonAlloc instead of standard collision checks

✅ LOD Physics

• Swap to simplified colliders at distance

✅ Object Pooling

• Recycle physics objects instead of instantiation/destruction

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4. Haptic Feedback Integration

A. Standard Implementation

// Unity haptic pulse example
inputDevice.SendHapticImpulse(0, 0.5f, 0.1f);

Best Practices:

• Match haptic intensity to collision force
• Use audio-driven haptics for enhanced realism

B. Platform-Specific Features

• Meta Quest Controller vibrations
• Apple Vision Pro Taptic Engine integration
• Index Controllers Finger-specific feedback

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5. Troubleshooting Common Issues

Problem	Solution
Objects jitter when held	Enable kinematic on grab, disable on release
Hands pass through walls	Use continuous collision detection
Poor stacking behavior	Increase friction + stabilize rotation

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6. Emerging Technologies

• AI Physics Prediction (NVIDIA PhysX 5)
• Quantum Physics SDKs for molecular-level interactions
• Neural Physics (machine-learned object behavior)

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Key Takeaways

✔ Start simple – Basic rigidbody interactions first
✔ Profile early – Use Unity Profiler/Unreal Insights
✔ Tune for comfort – Avoid unrealistic physics that cause motion sickness