Unity Collider explained with OnCollisionEnter and OnTriggerEnter

Collision detection is required for all types of games. Detecting and utilizing the collision is implemented in a different manner in different game engines. In Unity, a collider can be added to any game object as a component. Also, you need to understand is how colliders and rigidbody work together to efficiently use them. Without a basic understanding of how Unity Colliders work, it will be very difficult to code the game mechanics. In this post, we will see how to use colliders in Unity and where to use OnCollisionEnter and OnTriggerEnter functions.

If you are totally new to Unity check out how to learn Unity.

Unity colliders are very simple to use. Unity has divided the colliders into 2D and 3D. So you have to be very careful which collider you are using. All your codes and collision detection will change for 2D and 3D. Further, there are types of colliders, static collider, rigidbody collider, kinematic rigidbody collider, etc. The collider used in all these are the same, the type is for how the object to which the collider is added behaves when collided with.

Unity Collider in inspector

Unity Collider Types

Static collider

Static colliders are considered to be non-moving objects by Unity. Do not confuse static gameobject with the static collider. If a rigidbody is not attached to a collider then it’s a static collider. They do not move when an object Collides to with them. Though, they can be moved using transform, moving a static collider leads to performance loss during runtime.

Rigidbody collider

A rigidbody collider works like a real-world object. It is governed by the forces of physics. If you apply force on it, it will move. In Unity, rigidbody is used on an object which will move a lot during gameplay. Check the screenshot for how a rigidbody is attached to a game object.

Kinematic rigidbody collider

A kinematic rigidbody is a rigidbody that behaves like a static object. So, the next question is then why use a kinematic rigidbody. The main reason to use a kinematic rigidbody is to tell Unity that the object does not respond to forces but will be movable by script during runtime. The performance loss is very less in moving a kinematic rigidbody compared to a Static object.

Non-trigger and Trigger collider

In Unity you can mark a collider as trigger using the check box in the inspector window. The behavior of the object changes if it is marked as a trigger.

Unity Colliders not marked as trigger

Unity will detect a collision only if one of the objects is a rigidbody. That is, if you have two gameobject marked as static colliders then collision between them will not be detected. It’s the same case with two kinematic rigidbody. If you use the OnCollisionEnter function in any of the above cases, the function will not be called during a Collision. Find the collision matrix for unity colliders in the image below.

collision matrix for unity colliders

Ref:unity docs

Unity collider marked as Trigger

When you mark a collider as a trigger, it no longer behaves like a solid object. It allows the other object to pass through it. But the time of one object entering another object’s space can trigger a function. With that, you can know if a collision has happened without actually creating a collision. You have to use the OnTriggerEnter function if the collider is Marked as a trigger. The collision matrix is a little different in the case of triggers.

Trigger collision matrix

Trigger collision doesn’t work if both the colliders are static colliders. In all the other cases the on trigger enter is called.Knowing these basic things about colliders will help you set them up easily in your game.

If you did not understand the matrix completely then here is a simple comparison to help you. Consider static colliders as object that don’t move like wall. Rigidbody collider denotes a moving object like a ball. Kinematic Rigidbody denotes that the object is not moved by physics but can be moved by script or animation.

Unity does not want to detect collision between two static objects so normal collision is detected only if one of the game object is a movable by physics. Where as trigger function works a little different. It works only if one of the collider is marked as trigger and can be moved either by physics or script.

Sample Unity collision script

unity OnCollisionEnter

void OnCollisionEnter(Collision col)
    {
        If(col.tag=="Enemy")
           {
            Destroy(col.gameobject)
           }
    }

In this script we are using a normal collision. “col” returns the collider of the game object. We further check if the game object is an enemy. If yes, we destroy it. Remember either the parent gameobject or the enemy game object needs to have a Rigidbody for this script to work.

Unity OnTriggerEnter

We can do the same thing with trigger with this code.

void OnTriggerEnter(Collider col)
    {
        If(col.tag=="Enemy")
           {
            Destroy(col.gameobject)
           }
    }

Pro tip: If the object is marked as trigger, it will pass through the obstacle and no collision physics will act.

Other features in Unity OnCollisionEnter and OntriggerEnter

Both the examples of Unity OnCollisionEnter and OnTriggerEnter above shows that you can get the gameobject of the colliding object. Apart from that you can get a lot of data from the collision class object. Here is the list of things that you can get from the collision class

  1. collider- get the collider of the colliding object
  2. relative velocity- get the relative velocity between the colliding objects.
  3. transform- get the transform of the object hit.

That summarizes the collider function in Unity. If you have any questions you can post it in the comment box below.

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.