UNITY COLLISION LAYERS & LAYER COLLISION MATRIX

1. The $O(N^2)$ Physics Broadphase Problem

By default, every layer in a new Unity project is set to collide with every other layer. If you have 500 bullets, 50 enemies, debris fragments, and terrain, PhysX evaluates pairwise bounding box overlaps across all combinations during every FixedUpdate.

Pruning the Layer Collision Matrix (in Project Settings > Physics) ensures PhysX instantly skips pairs that should never interact (e.g. PlayerProjectiles colliding with PlayerProjectiles or Debris with Debris).

2. Battle-Tested Combat Layer Setup

3. Efficient LayerMask Filtering in C#

In C#, a LayerMask is a 32-bit integer where each bit represents a layer (from 0 to 31). Never use string comparisons (hit.transform.tag == "Enemy") inside high-frequency Raycasts:

using UnityEngine;

public class HighSpeedLaser : MonoBehaviour
{
    [Header("Raycast Target Filter")]
    [Tooltip("Only evaluates hits against these specific layers.")]
    [SerializeField] private LayerMask targetLayers;

    public void FireLaser(Vector3 start, Vector3 direction, float range)
    {
        // ✅ Hardware bitmask check: PhysX completely ignores all non-target colliders
        if (Physics.Raycast(start, direction, out RaycastHit hit, range, targetLayers))
        {
            Debug.Log($"Laser impacted: {hit.collider.name} on layer {LayerMask.LayerToName(hit.collider.gameObject.layer)}");
        }
    }
}

Related Physics Guides & Tools