How to Detect Collision in Unity: Simple Guide with Examples

In Unity, you detect collisions by using the OnCollisionEnter or OnTriggerEnter methods inside a script attached to a GameObject with a Collider component. These methods run automatically when your object hits or overlaps another object with a Collider, allowing you to respond to the collision in code.

Syntax

To detect collisions, you create a script with one of these methods:

• void OnCollisionEnter(Collision collision): Called when this object's collider hits another collider and at least one has a Rigidbody component.
• void OnTriggerEnter(Collider other): Called when this object's collider marked as a trigger overlaps another collider.

Attach this script to a GameObject with a Collider component. For OnCollisionEnter, at least one object must have a Rigidbody. For OnTriggerEnter, the collider must be set as a trigger.

csharp

void OnCollisionEnter(Collision collision) {
    // Code to run when collision happens
}

void OnTriggerEnter(Collider other) {
    // Code to run when trigger overlap happens
}

Example

This example shows how to detect when your player hits an obstacle and prints a message to the console.

csharp

using UnityEngine;

public class CollisionDetector : MonoBehaviour {
    void OnCollisionEnter(Collision collision) {
        Debug.Log("Collided with " + collision.gameObject.name);
    }

    void OnTriggerEnter(Collider other) {
        Debug.Log("Triggered by " + other.gameObject.name);
    }
}

Output

When the player hits another object with a collider, the console shows: "Collided with ObjectName" or "Triggered by ObjectName" if using triggers.

Common Pitfalls

• For OnCollisionEnter to work, at least one object must have a Rigidbody component; otherwise, the method won't be called.
• For OnTriggerEnter, the collider must have "Is Trigger" checked in the Inspector.
• Mixing triggers and collisions incorrectly can cause no detection.
• Not attaching the script to the correct GameObject will prevent detection.

csharp

/* Wrong: No Rigidbody on either object, so collision won't be detected */
// GameObjects have colliders but no Rigidbody

/* Right: Add Rigidbody to one object */
// Add Rigidbody component to player or obstacle GameObject

Quick Reference

Method	When to Use	Requirements
OnCollisionEnter(Collision collision)	Detect physical collisions	Both objects have Colliders; at least one has Rigidbody; colliders not set as triggers
OnTriggerEnter(Collider other)	Detect overlaps with trigger colliders	Collider set as trigger; at least one object has Rigidbody

Key Takeaways

Use OnCollisionEnter to detect physical collisions between objects with Rigidbody and Collider components.

Use OnTriggerEnter to detect when an object enters a trigger collider area.

Ensure at least one object has a Rigidbody component for collision detection to work.

Set the collider's "Is Trigger" property correctly depending on the detection method.

Attach your collision detection script to the GameObject with the collider.