Ignoring collisions without using physics layers [duplicate]

Question

Trying to ignore collisions between certain prefabs, but in this case I don't want to use layers since these are specific object pairs that I want to not collide, and creating a layer for each prefab would complicate the layer matrix more than I want to.

What I tried so far:

public void OnCollisionEnter(Collision collision) {
    if (collision.gameObject.tag == "Bullet") {
        Physics.IgnoreCollision(collision.collider, collider);
    }
}

So I tried ignoring the collision in the OnCollisionEnter, but this only worked partially. The objects did go one through the other, but they both slowed down. My guess is that the first frame of the collision, the one that generated the call to OnCollisionEnter, was calculated regardless of the ignore command.

Any ideas how I can make all bullets not collide to one another, without putting them in a separate layer?

Answer 1

Trying to ignore collisions between certain prefabs, but in this case I don't want to use layers since these are specific object pairs that I want to not collide

When you have specific objects pair that you don't want to collide, using layers is definitively the best way to go.

creating a layer for each prefab would complicate the layer matrix more than I want to.

"Complicate" the layer matrix is not a problem. The matrix is limited to 32 layer, if you haven't free layers to use than you might consider other solution. Until I have a free layer I'd always go for using it.

Physics.IgnoreCollision works just fine. There are some limitations documented indeed.

Btw, the problem is that you are calling it when a collision as already occurred. You have to use it on an object pair you want ignore collision for, before the collision occurs(for example set up it inside Start method instead).

Answer 2

Ok, so I haven't seen anyone answering with a solution for ignoring collusion between certain prefabs without assigning them their own layers (which are limited to 32).

So it's clear that in current unity versions, you must call IgnoreCollision() for each pair of instantiated objects.

So the solution I found, is an old one (before the layers were introduced), but it seems like it can work. It's called a "collision manager", and it's basically a class that stores references to all the objects that are instantiated, and calls IgnoreCollision() for every pair of objects of types that should not collide. To make it work each object has to register itself with the collision manager and it's type in the Start() method. This way the collision manager can make sure that every pair of objects of types that shouldn't collide will not collide, assuming all objects register themselves.

Here's the link to the article I found, although it's very lacking in explanation:

http://wiki.unity3d.com/index.php?title=CollisionIgnoreManager