Unity Physics Engine Part 2


In this tutorial, we will cover tags and layers. How to create them and what they are used for.


The Unity version used in this tutorial is 2017.3.1f1. Most of what we are going to cover applies to the previous and the new versions as well. So it's safe to follow. We will continue from the previous tutorial Unity Physics Engine and talk about the tags and layers.

Scene Setup

Before we start, let's set up our scene. I used an awesome free model called Space Robot Kyle.

Download and Import the Space Robot Kyle Package

Let's get started:

  • Create Cube and call it Ground. Set it's scale to (10, 0.1, 10) with zeros in transform and scale

  • Create a material for it and add it to a Materials folder

  • Create another cube named Bullet with scale (0.1, 0.1, 0.5)

  • Add a Rigidbody to the Bullet and uncheck Use Gravity

  • Drag and drop the Bullet in a Prefabs folder.

  • Drag and drop Robot Kyle from Robot Kyle > Model to the Hierarchy

  • Reset transform and set Y position to 0.05

  • Change the rotation to (0,180,0)

  • Add Box Collider and Adjust it's parameters

    Adjusting Collider

  • Change the Main Camera position to (0, 1, -3)

  • Change the Main Camera rotation to (0, 0, 0)

  • Save the scene in the _Scenes folder

Scene Setup


Why Tags?

Let's say, you want to start shooting stuff with bullets. You also want to increase the score every time an enemy is shot. From our previous tutorial, we would simply call an OnCollisionEnter provided by the MonoBehavior to detect collision. But this detects ANY collision where we want to gain score only when a bullet collides. For this reason, we would create a new Tag and assign it to the enemy. Now, whenever the bullet collides with Kyle, we will first check if the Tag of the collided object is enemy, then we increase the score, else we ignore it or do whatever we want.

Create and assign tag

Creating and assigning a tag are 2 different steps. Some people think that added tags are automatically assigned but you need to do it manually.

Select the Robot Kyle from the Hierarchy. In the Inspector window, Select Tag then Add Tag.

Add Tag button

Press on the + sign and name the new tag Enemy. The tag name is CASE SENSITIVE as we will write the actual string of the tag name later in our code. So, make sure you spelled it right.

Name the new tag

If you went back to check the Robot Kyle prefab now, you will see that it is still Untagged. The previous step was just creating the tag. Open the drop down and select the new tag called Enemy that we just created.

Assign a tag

Code example

Let's start shooting bullets.

  • Create two script called BulletShooting.cs and Bullet.cs add place them inside the Scripts folder

  • Add the BulletShooting.cs script to the MainCamera

  • Add the Bullet.cs script to the Bullet prefab in the Prefabs folder

    Adding Scripts

  • In the Bullet script, copy and paste this

    //The speed parameter to controll the bullet speed.Can be changed from the bullet Prefab.
    public float bulletSpeed = 10f;

    //Update is called once per frame
    void Update()
        transform.Translate(transform.forward * bulletSpeed * Time.deltaTime);

    //Detect Collision - Unity's messages
    public void OnCollisionEnter(Collision collision)
        if (collision.gameObject.tag == "Enemy")


we specified a public variable to be able to control it in the inspector

Public Bullet Speed variable is editable in the inscpector

  • In the BulletShooting script, copy and paste this
    //The prefab of the bullet that we will shoot
    public GameObject bulletPrefab;

    // Update is called once per frame
    void Update()
        if (Input.GetMouseButtonDown(0))
            var bullet = Instantiate(bulletPrefab);
            bullet.transform.position = transform.position;
            bullet.transform.rotation = transform.rotation;
            Destroy(bullet, 10f);

We simply instantiate (Spawn) a bullet each time we press the left mouse button.

We only need to specify which prefab are we going to instantiate by dragging and dropping the Bullet from the prefabs folder to the inspector.

Assigning the bullet prefab

If we pressed Play now and clicked the left mouse button anywhere in the Game tab. A bullet will be shot and will deactivate the Robot ONLY IF the Enemy tag is assigned

Affecting only the tagged object

This happened because in the Bullet.cs script inside the OnCollisionEnter message, we checked the tag to do a certain functionality which is disable the enemy.


Why layers?

Layers on the other hand are created to group physics object that shares something similar. Let's say that our bullets special enough that it can pass through walls. To do that, we will add our bullet to a different layer so that the physics engine doesn't even try calculating anything for it.

Create and assign layers

Creating layers doesn't differ from creating tags. In fact, both are done in the same menu. Be careful again that creating layers and assigning layers are two different steps.

Select any GameObject and in the Inspector window, Select Layer then Add Layer.

Add Layer button

The first 8 (0 to 7) layers are reserved for Unity we can't edit them. We will add two new layers called SpecialBullet and Special Wall.

Create 2 new layers

We need now to tell the physics system which layers should collide with what. Goto Edit>Project Settings>Physics

Physics Settings

At the bottom you see something called Layer Collision Matrix. This matrix specifies which layer should collide with what. Don't feel lost, between 2 different layers, there is a checkbox. So, just look carefully. Right now we disabled the collision between SpecialBullet layer and SpecialWall.

Collision Matrix

Assign the SpecialBullet layer to the Bullet prefab

Assign Layer

Create a new cube as a wall for testing

  • Set position to (0, 0.8, -0.7)
  • Set rotation to (0, 0, 0)
  • Set scale to (0, 0, 0.01)

Adding special wall

We will assign the SpecialWall layer to test during runtime. But, be careful because all changes that are made while the game is running are reset back after turning off the Play mode.

Press Play button and try shooting and you will find that the Robot can't be hit. Select the Cube you just created and assign the Special Wall layer to it and you will find that the Kyle got hit.

Changing layers and shooting


We have covered how to create tags and layers and how to use them. What we have covered is not everything. There are more use cases for them. Layers can be used with raycasting to create complex functionalities like teleporting inside triggers using laser pointer. Check the reference section for more info about tags and layers.


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