OnCollisionEnter vs OnTriggerEnter. What's the difference?


Júlio Rodrigues ·

They both look similar and behave similarly, what's then the difference between OnTriggerEnter and OnCollisionEnter? The key to understand this is in knowing what are triggers in Unity.

In Unity, an object might be controlled by the physics engine or by script. If you need to control an object by script but still would like to know if an object touched another, a 'collision' happened, you need to use triggers.

Collision is under quotes because, strictly under Unity's terminology, a collision only happens when object's movements are governed by the physics engine, for the other cases what we have are simply objects touching each other, also, for such event, our script can be alerted as well.

What are triggers

A trigger is a collider that's not influenced by the physics engine. It doesn't respond to forces nor gravity. But they still do have a use for the physics engine, they are used to detect whether an object passed through another. Triggers are everywhere in Unity game development, and in other engines too to be honest.

This grim repear has all its colliders as triggers

This Reaper is controlled by a simple back-and-forth walk AI, the physics engine is not used, but we still want to know when it has touched some things in the stage. For that, we can use an OnTriggerEnter

Collisions

A collision is also the result of an object touching another one, but instead of passing through, these objects push each other in a realistic way. Use OnCollisionEnter when your rigidbody colliders aren't triggers and you'd like to know when they touched each other.

On this prototype

For more information on how to create and use triggers, please see the official documentation on the subject. If you followed the documentation and something with your collision detection is not working, you may try to fix it using our comprehensive collision fixing tutorial.

TLDR;

Use triggers if you don't want/need the physics engine to control your object but still need to know if an object passed through another or reached some `zone` within the game. In that case, you'll use OnTriggerEnter().

If your object is indeed controlled by the physics engine, you'll use OnCollisionEnter() to know if an object touched another one.

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Classify in:
  • scripting
  • physics
  • collision detection
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