Physics

Summary

A Collider is a component that defines the physical boundaries of a GameObject. This way, to detect the collision between two GameObjects, each of them must have a Collider.

In Unity, colliders can function in two different ways:

Normal Mode: the object physically interacts with other objects

Collider only

Property: No movement, static object

Useful for: walls, floor

Collider + RigidBody

Property: Moves and responds to gravity or impulses

Useful for: domino Challenge, rolling ball

Function: void OnCollisionEnter(Collision collision)

Trigger Mode: the object does not block others; no physical interaction => Ghost object

Collider + Trigger

Property: detects when a GameObject passes through

Useful for: the finish line in a car race, a trap in a cave

Function: void OnTriggerEnter(Collider other)

Colliders Shape

Setup

Navigate to Assets > Scenes

Create a new folder named « PhysX »

Create a new scene called « Colliders »

Double click to open it

Create a Sphere and assign it a color

Select the sphere

In the Inspector, notice that a Collider is already present

Uncheck the checkbox of the Mesh Renderer

Notice that the collider of the sphere is now visible on the screen

Activate the sphere Mesh Renderer

The Collider of the sphere perfectly matches its shape. Here are the colliders of other objects:

Sphere: sphere collider

Capsule: capsule collider

Cylinder: capsule collider

Cube: box collider

Two remarks:

All basic 3D objects are delivered with a built-in collider

The cylinder uses a capsule collider (Lower computational cost)

Avertissement

We recommend not using a plane because it is defined by 200 triangles. Testing its collision is equivalent in complexity to testing the collision with 200/6 = 33 cubes. As a consequence, use a cube to create a floor.

RigidBody

Basic setup

We begin our study by adding a RigidBody to the sphere.

Create a cube and and assign it a color

Scale the cube to create a ground

Select the sphere

In the Inspector, click on the button Add Component

Select RigidBody

In the RigidBody section, check: Use Gravity

If you want the camera to shoot the same view as the scene view

In the Hierarchy, select the camera

Right click and select Align With View

Enter Play mode

What happened?

The sphere has a Rigidbody, moreover it is affected by gravity so it falls

The ground don’t have a Rigidbody, it’s a static object, it cant move

Both the sphere and the ground have a Collider, allowing them to come into contact, which stops the sphere’s fall

Simulation

We add some new elements to the scene; try to reproduce the scene below.

Some hints:

Analyze which object has a RigidBody: is the object moving or not?

Detect which object is affected by gravity: it is falling or not ?

Enable all options correctly

Collider

Scene Setup

Navigate to Assets > Scenes > PhysX

Create a new Scene named Detection

Create a sphere, assign it a color and a RigidBody with gravity

Move the sphere upwards

Collider Setup

In the Hierarchy, right click and select Create Empty

Rename the object « BCollider »

In the Inspector, press the Add Component button and select Box Collider

In the Inspector, activate the Is Trigger option

In the Inspector, click on the Edit Collider icon:

Use the face gizmos to

Enlarge the box

Make it thicker than the sphere

Collider script setup

A collider trigger provides multiple functions to handle collisions:

In the PhysX folder, create a new script named Colliz

Select the GameObject BCollider

Drag and drop the script into the Inspector

Copy paste the following code into the script
using UnityEngine;

public class Colliz : MonoBehaviour
{
        public Material Mat_in,Mat_out;

        void OnTriggerEnter(Collider other)
        {
                Renderer objectRenderer = other.gameObject.GetComponent<Renderer>();
                objectRenderer.material = Mat_in;
        }

        void OnTriggerExit(Collider other)
        {
                Renderer objectRenderer = other.gameObject.GetComponent<Renderer>();
                objectRenderer.material = Mat_out;
        }
}
Drag and drop two Solid Color Materials into the Inspector

Avertissement

By reading the script, you can see that we change the sphere’s color through the Collider using the reference named other. Why not place this script directly on the sphere? We could, but that would require setting its Collider as a trigger, turning it into a ghost object and we don’t want that.

Display collider

It is possible to display the collider in Play mode:

Select the Game window (by clicking on the tab, not by activating Play mode)

On the right of the Game tab, click on Gizmos to show the collider during Play mode

In the Hierarchy, select the BCollider

Test

Enter Play mode: