Physics simulations in VR, AR, and MR must balance realism with performance to maintain immersion and avoid motion sickness. This guide covers best practices, engines, optimization techniques, and use cases for physics in XR.
1. Physics Engines for XR
A. Built-in Physics Systems
| Engine | Physics System | Best For |
|---|---|---|
| Unity | NVIDIA PhysX (3D) | High fidelity, rigidbody interactions |
| Unreal Engine | Chaos Physics | Destructible environments, realism |
| Godot | Bullet (3D) / Box2D (2D) | Lightweight XR projects |
B. Specialized XR Physics Tools
- Oculus Interaction SDK (Meta Quest hand physics)
- Magic Leap Hand Tracking + Physics (AR interactions)
- Apple Vision Pro’s RealityKit (Object anchoring, collisions)
2. Types of Physics Simulations in XR
A. Rigidbody Physics
- Used for objects with mass and collision (e.g., throwing a ball, stacking blocks).
- Key settings:
- Mass (avoid extreme values to prevent instability).
- Drag & Angular Drag (controls movement resistance).
- Collision Detection (use Continuous for fast-moving objects).
B. Softbody & Cloth Physics
- Simulates deformable objects (flags, clothing, squishy materials).
- Performance-heavy—best for PC VR, not mobile XR.
- Unity Cloth Component / Unreal Chaos Cloth.
C. Particle Systems (Fluids, Smoke, Debris)
- Used for fire, water splashes, magic effects.
- Optimize with:
- GPU acceleration (Unreal Niagara, Unity VFX Graph).
- Low-resolution collisions (avoid per-particle physics).
D. Vehicle & Ragdoll Physics
- Wheel Colliders (Unity) / Chaos Vehicles (Unreal) for cars.
- Ragdolls for character death animations (use simplified colliders).
3. Optimization for XR Performance
A. Reduce Physics Calculations
✔ Use Sleep Mode (disable physics on idle objects).
✔ Limit Rigidbody Count (merge small objects where possible).
✔ Disable Gravity on static objects.
B. Collision Optimization
✅ Collider Types (From Most to Least Efficient):
- Primitive Colliders (Box, Sphere, Capsule)
- Convex Mesh Colliders
- Concave Mesh Colliders (Avoid in VR—high CPU cost)
✅ Layer-Based Collisions
- Disable unnecessary interactions (e.g., UI shouldn’t collide with walls).
C. Fixed Timestep & Solver Settings
- Unity: Reduce
Fixed Timestep(default 0.02s) if physics feels sluggish. - Unreal: Adjust
Chaos Solver Iterationsfor stability.
4. XR-Specific Physics Challenges
A. Hand & Controller Interactions
- Grab Mechanics:
- Use Unity XR Interaction Toolkit (
XR Grab Interactable). - Unreal:
Motion Controller+ Physics Handle. - Collision Avoidance: Prevent hands from clipping through objects.
B. Haptic Feedback Syncing
- Oculus Haptics: Trigger pulses on collision (e.g., grabbing a virtual object).
- Apple Vision Pro: Taptic Engine feedback for AR interactions.
C. Networked Physics (Multiplayer XR)
- Prediction & Lag Compensation: Critical for synchronized object throws.
- Photon Unity Networking (PUN) / Unreal’s Replication System.
5. Use Cases & Examples
| Application | Physics Technique |
|---|---|
| VR Training (Medical, Industrial) | Rigidbody tools, gravity simulation |
| AR Furniture Placement | Collision detection with real-world surfaces |
| VR Sports (Golf, Boxing) | Trajectory prediction, force feedback |
| MR Collaborative Design | Shared physics objects in real-time |
6. Tools & Plugins for Advanced Physics
- Obi Softbody (Unity) – Optimized cloth/rope physics.
- PhysX 5 (Unreal/Unity) – Improved GPU acceleration.
- Zibra AI Liquid (Unity) – Real-time fluid simulation.
7. Troubleshooting Common Issues
🔹 Jittery Objects? → Increase solver iterations, enable interpolation.
🔹 Hands Passing Through Walls? → Use continuous collision detection.
🔹 VR Motion Sickness? → Avoid unrealistic physics (e.g., floating objects).
Key Takeaways
✔ Prioritize performance (simplify colliders, limit rigidbodies).
✔ Use engine-specific optimizations (PhysX, Chaos, Bullet).
✔ Sync physics with haptics for immersion.
✔ Test in-headset early to catch latency issues.
Future of Physics in XR
- Machine Learning Physics (NVIDIA’s AI-based simulations).
- Quantum Physics Solvers (for ultra-realistic molecular interactions).
- Holographic Physics (light-field-based collisions in MR).