Physics Optimization Techniques

# CHAPTER 15

Physics Optimization Techniques #

1. Introduction #

2. Learning Objectives #

• Optimize the Collision Matrix to prevent unnecessary overlap checks.

• Balance the Fixed Timestep for performance vs. accuracy.

• Understand and configure Rigidbody Sleep mechanics.

• Replace heavy physical projectiles with cheap Raycast math.

• Avoid common CPU spikes caused by modifying colliders at runtime.

3. The Collision Matrix (Pruning the Tree) #

• This is wasted CPU power. Bullets don't need to collide with other bullets!

• The Fix: Open the Engine's Physics/Collision Matrix. Uncheck the box where the "Bullet" layer intersects with the "Bullet" layer. The CPU will instantly ignore thousands of math checks every frame.

4. Tuning the Fixed Timestep #

• By default, engines run this 50 times a second (0.02 timestep).

• If your game is a slow-paced puzzle game, 50 calculations a second is massive overkill. You can change the Fixed Timestep to 0.04 (25 times a second). You instantly cut the CPU physics load in half!

• *Warning:* If you have fast-moving cars, lowering the timestep will cause the cars to tunnel through walls.

5. Rigidbody Sleeping #

• The Optimization: Ensure your rigidbodies actually go to sleep! If you have a script that applies a tiny random jitter force to the crate every frame, it will NEVER sleep, and 100 crates will lag the game permanently.

6. Primitive Colliders vs. Mesh Colliders #

• A Sphere Collider takes exactly 1 math calculation (Distance < Radius).

• A Mesh Collider (a 10,000 polygon rock) requires the CPU to check intersection against thousands of tiny triangles.

• The Rule: NEVER use moving Mesh Colliders. Always wrap complex models in an invisible compound of 2 or 3 simple Box/Sphere colliders.

7. Avoiding Runtime Collider Changes #

• The Fix: If an object is destroyed, just move it far below the floor and disable the visual renderer (Object Pooling), rather than constantly deleting and rebuilding colliders in memory.

8. Visual Learning: The Collision Matrix #

          [Player]   [Enemy]   [Bullet]   [Environment]
[Player]     [ ]       [X]       [ ]          [X]
[Enemy]      [X]       [ ]       [X]          [X]
[Bullet]     [ ]       [X]       [ ]          [X]
[Environ]    [X]       [X]       [X]          [ ]

*Blank spaces are completely ignored by the CPU! Massive savings.*

9. Best Practices #

• Use Non-Allocating Overlaps: When creating an explosion, Physics.OverlapSphere creates a new Array in RAM every time it fires, generating Garbage. Advanced programmers use Physics.OverlapSphereNonAlloc. You provide a pre-existing empty Array, and the engine fills it. Zero garbage created!

10. Common Mistakes #

• Raycasting Everything: Raycasts are cheap, but they aren't free. If an enemy AI shoots 50 raycasts every single frame to check for obstacles, the game will lag. Use timed coroutines to only shoot Raycasts 3 or 4 times a second per enemy.

11. Mini Project: Optimized Proximity Check #

class OptimizedBomb
{
    public Transform player;
    public float explosionRadius = 5.0f;
    
    // We pre-calculate the squared radius to avoid Square Roots!
    private float sqrRadius;

    void Start()
    {
        sqrRadius = explosionRadius * explosionRadius; // 25.0f
    }

    void Update()
    {
        // Vector3.sqrMagnitude is insanely fast for the CPU
        float distToPlayer = (transform.position - player.position).sqrMagnitude;

        if (distToPlayer <= sqrRadius)
        {
            Console.WriteLine("Player is in the blast zone!");
        }
    }
}

12. Practice Exercises #

1. 1. How does utilizing the Collision Matrix optimize CPU performance?

1. 2. What happens mathematically when you change the engine's Fixed Timestep from 0.02 to 0.04?

13. MCQs with Answers #

14. Interview Questions #

• Q: Explain the concept of Rigidbody Sleeping. What programmatic actions can accidentally prevent a Rigidbody from sleeping, destroying performance?

• Q: Contrast the performance costs of a Primitive Box Collider versus a Custom Mesh Collider. Why are Mesh Colliders strictly forbidden on moving objects in professional game studios?

• Q: A mobile game is running at 20 FPS due to heavy physics load. Walk me through three specific steps you would take to optimize the physics pipeline within the engine settings.

15. FAQs #

16. Summary #

17. Next Chapter Recommendation #