Collision Detection In Unity

Collision detection is the system that tells you when two GameObjects touch each other. When a collision happens, Unity automatically calls special methods in your scripts, allowing you to respond like reducing health, playing a sound, or destroying an object.

Collision Event Methods

Unity provides three collision events that you can use in your scripts:

• OnCollisionEnter: Best for most cases (damage, item pickup, death).
• OnCollisionStay: Use for continuous effects (melting, burning).
• OnCollisionExit: Use for leaving detection (stopping sound effects).

Example

public class Player : MonoBehaviour
{
    void OnCollisionEnter(Collision collision)
    {
        // Runs ONCE when two objects first touch
        Debug.Log("Collision started with: " + collision.gameObject.name);
    }
    void OnCollisionStay(Collision collision)
    {
        // Runs EVERY FRAME while objects remain touching
        Debug.Log("Still touching...");
    }
    void OnCollisionExit(Collision collision)
    {
        // Runs ONCE when objects separate
        Debug.Log("Collision ended");
    }
}

Collision Parameter

It gives you useful information about the impact:

• collision.gameObject: The object we collided with
• collision.relativeVelocity: Speed of impact (high = hard hit)
• collision.contacts[0].point: World position of collision

Requirements for Collision Events to Fire

For OnCollisionEnter/Stay/Exit to work:

• Both objects must have Collider components.
• At least one object must have a non-kinematic Rigidbody.
• Neither Collider should be marked as "Is Trigger".

Not working? Check these:

• Is Rigidbody missing? Add one.
• Is "Is Trigger" checked? Uncheck it (use triggers for overlap detection).
• Is Rigidbody kinematic? Set to non-kinematic.

Practical Examples

Example 1: Player takes damage when hitting enemy

void OnCollisionEnter(Collision collision)
{
    if (collision.gameObject.CompareTag("Enemy"))
    {
        health -= 10;
        Debug.Log("Hit! Health: " + health);
    }
}

Example 2: Bullet destroys on impact

void OnCollisionEnter(Collision collision)
{
    // Ignore collision with shooter
    if (collision.gameObject == shooter) return;
    Destroy(gameObject);  // Destroy bullet
}

Example 3: Play sound based on impact speed

void OnCollisionEnter(Collision collision)
{
    float volume = collision.relativeVelocity.magnitude / 10;
    AudioSource.PlayClipAtPoint(impactSound, transform.position, volume);
}

Collision vs Trigger

Performance Considerations

• Keep collision code lightweight as it is called frequently.
• Avoid Find() or heavy operations inside collision methods.
• Use tags or layers instead of GetComponent when possible (faster).
• Cache components in Start() instead of getting them in collision.

Common Mistakes

• Using OnTriggerEnter when objects need to physically block - Use OnCollisionEnter instead
• Forgetting Rigidbody: Collision events won't fire
• Moving objects via Transform instead of physics: Causes inconsistent collisions
• Checking collision.gameObject.tag == "Enemy" (expensive) – Use CompareTag() instead