# Setup

The game has cars and collisions, and I'm trying to create the following system:

• When a car T-bones another, the one getting T-boned is destroyed;
• When the cars collide laterally (usually just bump into each other), a normal physics collision happens (and nothing is destroyed);
• When the cars collide head-on, the lighter one is destroyed.

The cars are all instantiated from 1 prefab. This can't be changed.

## Car Setup

The main body sans the bumper has a normal, non-trigger collider and the dynamic Rigidbody. It has the tag "Destructible".

A child object (which also houses the bumper model, but that shouldn't matter) has a normal, non-trigger collider for the bumper. It has no Rigidbody and reports to the parent's Rigidbody. It has the tag "Destroyer".

# Attempted Solutions

## 1) The transform.forward solution

Get the transform.forwards of both cars and compute their dot product.

• If the 2 vectors are opposite each other (or close to it), destroy the lighter car.
• If the vectors are facing away from each other (or are close to parallel), let a normal collision happen.
• If the vectors are perpendicular (or somewhere in that ballpark)... then I'm lost. How do I know which car T-boned which?

That brings us to the solution #2.

## 2) The 'multiple colliders' solution

Assign another collider to the bumper of the car and give it a separate tag (e.g. "Destroyer). As a consequence, give the other — main — collider a tag, too (e.g. "Destructible").

• If a Destroyer collider hits a Destructible, the latter is destroyed.
• If a Destructible hits a Destructible, a normal physics collision happens.
• If a Destroyer hits a Destroyer, destroy the lighter one.

But here, I run into an issue.

1. You can't have multiple colliders on the same game object. Ergo, create an empty child and attach it there. I won't set it to Trigger, or otherwise it won't detect collisions with another of its type (i.e. Destroyer).
2. In order to destroy colliding objects, I need a script and an OnCollisionEnter() inside it.
3. OnCollisionEnter() won't run if neither of the game objects has a non-kinematic Rigidbody. This means I need to give the bumper colliders a non-kinematic Rigidbody.
4. But if I give them dynamic Rigidbodies, they'll just... float away from the car.

In other words, I need a way to create a 2nd collider but keep it firmly attached to the car. Furthermore, if the main collider covers it, then I need to exempt it from collisions with the 2nd collider (let's call it the sub-collider), and I can't use layers to do that or otherwise the sub-collider won't detect main colliders on other cars.

If I don't cover it, then I need the sub-collider to act as a normal physics collider, too, when it hits, for example, a wall, and not have the bumper sink into it. That means it needs a non-kinematic Rigidbody.

How can I solve this issue?

• Doesn't your car already have a non-kinematic rigidbody on it? Why would the bumper collider need its own? You can have multiple child colliders all reporting their collisions up to a single shared rigidbody. Or, if you're using trigger colliders and OnTriggerEnter, even kinematic bodies will fire that message. – DMGregory Sep 17 at 13:26
• Hey, I didn't know that bit about child colliders reporting to the parent Rigidbody! However, OnCollisionEnter() does not fire at all now. – verified_tinker Sep 17 at 13:35
• Because you used trigger colliders? They'll fire OnTriggerEnter instead. – DMGregory Sep 17 at 13:41
• No, no. I'm using normal colliders. I can't use make the bumper colliders triggers, because although they'll detect normal colliders (the main, Destructible bodies), they won't detect other Destroyers. From Unity docs: "If both GameObjects have Collider.isTrigger enabled, no collision happens." Thus, head-on collisions won't work. – verified_tinker Sep 17 at 13:43
• Sounds like you should edit your question to walk us through your current setup. I'm not sure from your description what's missing. – DMGregory Sep 17 at 13:45

If the vectors are perpendicular (or somewhere in that ballpark)... then I'm lost. How do I know which car T-boned which?

Shoot a ray from the center of mass of the vehicle through the front. If the ray hits the other car, it was the other car that was t-boned. If they ray doesn't hit something, reverse the roles of both cars, shoot again.

You might need more than one ray and shoot through the front left/front right edges of the car as well.

Physics.Raycast should work well for that.

• The contact points given by the Collision data structure should give you similar info as the raycast hit, without a separate query. Think you could use those instead? – DMGregory Sep 17 at 18:34
• I've not finished yet, but I'm going for solution #3: BoxCast "stripes" right around the bumper. I'm keeping 2 colliders -- the main one for the body and a child one for the bumper -- and so, depending on which the BoxCasts hit, I know what happened. – verified_tinker Sep 17 at 20:54