OnTriggerEnter doesn't work if the colliding object is not moving. Translating it by (0,0,0) fixes it. What's the problem?

Questions & Answers
1

I have a problem getting a OnTriggerEnter event using a moving object colliding with a non moving object. Let me explain:

I have two objects in the scene. The first one is my bullet, which is a cube with a Box Collider (Is Trigger checked).

This bullet is moving towards a target with the simple following script:

void Update () {
	transform.Translate(0, 1*Time.deltaTime, 0);
}

It has an overriden OnTriggerEnter which does nothing except outputing a log message:

void OnTriggerEnter(Collider otherObject){
	Debug.Log("Collided!");	
}

The target is also a cube, with a Box Collider and a RigidBody (Is Kinematic checked). It is not moving, so there is nothing updating the transform.

Now here is the strange thing, with this setup only, the bullet does not trigger the event when it collides with the target. However, if I add a script to my target doing nothing but translating its transform with a null vector, as follows:

void Update () {
	transform.Translate(0, 0, 0);
}

it works and I do get the log message saying “Collided”.

Does anybody know what the problem is? Am I doing something wrong? Does causing the transform to refresh by translating it activate the rigidbody or something?

Edit: it also works without the null translate if I add a RigidBody (IsKinetic checked) on the Bullet.

3

I’m having a similar issue. I am spawning an object with a sphere collider trigger in front of my character (capsule collider, rigidbody). The spawned object does a bit of its own setup (when it is spawned, it will fall until it is on the ground, then set the radius of the collider). If I am not moving, and I spawn the object, I do not get any OnCollider messages at all (even though the capsule is completely inside the sphere). If I move my character at all, I get the OnTriggerEnter message and it works fine.

It seems to me that if neither object is moving*, it will not even check for trigger / collider interactions.

My fix was, when I turned on the collider and set the size for it, I waited 1 frame and then ‘nudged’ the object myTransform.position = myTransform.position + Vector3.zero;

This forces the physics system to respond, and even though it did not actually move, it processes the colliders and notices the interaction. Seems likely to me it’s an optimization based on the pretense that if 2 things are not moving they cannot hit each other.

I hope this helps others with similar issues.

5

I had a similar problem, I spawned in an object and wanted to know if it was colliding with another. Both are kinematic RigidBodies 2D with 2DColliders and the spawned in object wouldn’t call its OnTriggerEnter2D() function. The way I solved it was by changing the Rigidbodies 2D sleep options to “Never Sleep”. I believe that in the default state, “Start Asleep” an object “Wakes up” and starts checking for collisions only when it starts to move, thus the “translate (0,0,0)” thing working.

2

Try using OnTriggerStay that should work perfetly

4

I believe the problem is with the way that Unity checks for collisions. It doesnt make sense to check every single object for collisions constantly so they most likely only check the objects that have moved that frame. Thats why translating by (0,0,0) works because it marks it as “moved” for that frame.