Use the layer collision matrix to reduce overlaps

Reduce collisionA collision occurs when the physics engine detects that the colliders of two GameObjects make contact or overlap, when at least one has a Rigidbody component and is in motion. More info
See in Glossary calculation overhead by configuring interaction rules between GameObjectsThe fundamental object in Unity scenes, which can represent characters, props, scenery, cameras, waypoints, and more. A GameObject’s functionality is defined by the Components attached to it. More info
See in Glossary with collision layers.

Use the Layer Collision Matrix to control how objects on different layers interact with each other in the physics simulation. You can assign objects to a layer in their InspectorA Unity window that displays information about the currently selected GameObject, asset or project settings, allowing you to inspect and edit the values. More info
See in Glossary, and then use the Layer Collision Matrix to define the interaction rules between them. Defining how objects interact on a layer basis can reduce the number of overlap checks and collision calculations performed each frame.

The benefits of layer-based filtering are:

• Reduced broad phase workload: The physics system skips entire groups of objects when calculating potential collisions.
• Improved narrow phase efficiency: By ignoring irrelevant interactions, the simulation can focus computational resources on meaningful collisions.
• Better scalability: Helps maintain performance as the number of objects and complexity in your sceneA Scene contains the environments and menus of your game. Think of each unique Scene file as a unique level. In each Scene, you place your environments, obstacles, and decorations, essentially designing and building your game in pieces. More info
  See in Glossary increases.

To begin optimizing with collision layers, first define and assign layers to your GameObjects. Then, configure how these layers interact in the Layer Collision Matrix. This matrix directly tells the physics system which layers must ignore each other during collision checks, which effectively culls potential interactions early in the simulation.

The recommended best practices for layer-based collision detectionAn automatic process performed by Unity which determines whether a moving GameObject with a Rigidbody and collider component has come into contact with any other colliders. More info
See in Glossary are:

• Enable interactions only between layers that are absolutely necessary for gameplay.
• Revisit the collision matrix periodically as your project grows. Disable unused layer pairs to increase performance.
• Use clear and consistent naming conventions for your layers to simplify debugging and future maintenance.
• Use the ProfilerA window that helps you to optimize your game. It shows how much time is spent in the various areas of your game. For example, it can report the percentage of time spent rendering, animating, or in your game logic. More info
  See in Glossary to monitor the effect of your changes. In the Physics Profiler module, observe counters related to broad phase pairs, narrow phase checks, and active contacts. Reducing unnecessary layer interactions leads to a decrease in these values, especially the broad phase work.

To learn more, refer to Layer-based collision detection.

To define and assign layers to your GameObjects and configure the Layer Collision Matrix:

1. Select Edit > Project Settings to open the Project Settings window.
2. Select the Tags and Layers tab.
3. In the Layers section, define custom layers under the User Layers section. For example, you can create a “Player” “Environment”, or “Projectiles” layer.
4. In the Editor, assign a GameObject to a layer in the Inspector window. To learn more, refer to LayersLayers in Unity can be used to selectively opt groups of GameObjects in or out of certain processes or calculations. This includes camera rendering, lighting, physics collisions, or custom calculations in your own code. More info
   See in Glossary.
5. Select **Edit > Project Settings.
6. Select the Physics > Settings tab.
7. Select the Shared tab.
8. In the Layer Collision Matrix, uncheck any layer pairs that you do not want to interact. For example, you might not want projectiles to collide with other projectiles.

Additional resources

• Unity Profiler
• Managing time and frame rate
• Memory Profiler
• Physics Project Settings
• Layer-based collision detection
• Layers