Introduction to Unity for 2D Game Development

Game Frameworks

Lecture 02: Introduction to Unity for 2D Game

Development

Edirlei Soares de Lima

<edirlei.lima@universidadeeuropeia.pt>

Unity – Interface

(

A) Toolbar

B) Hierarchy

C) Game View

D) Scene View

E) Inspector

F) Project

G) Status Bar

Unity – GameObjects

•

Every object in Unity is a GameObject (characters, collectible

items, lights, cameras, special effects, etc.).

–

However, a GameObject can not do anything on its own: you need to

add components so it can become a character or a special effect.

–

GameObjects act as containers for Components.

Unity – GameObjects

•

Examples:

–

A light is created by attaching a Light

component to a GameObject.

–

A solid cube object has a Mesh Filter

and Mesh Renderer component, to

draw the surface of the cube, and a

Box Collider component to represent

the object’s solid volume in terms of

physics.

Unity – Components

•

Components define the behavior of a GameObject.

•

Every GameObject has a Transform component.

–

The Transform defines the GameObject’s position, rotation, and scale in the game

world.

•

Components can be added to GameObjects through the

Components Menu or through the Component Browser (Add

Component button in the object’s inspector).

Unity – Transforms

•

The Transform component is used to store a GameObject’s

position, rotation, scale and parenting state.

–

Transforms are manipulated in 3D space in the X, Y, and Z axes or in 2D

space in just X and Y.

translate

rotate

scale

Unity – Parenting

•

When a GameObject is a Parent of another GameObject, the

Child GameObject will move, rotate, and scale exactly as its

Parent does.

•

Parent is created by dragging any GameObject

in the Hierarchy View onto another.

•

Transform values in the Inspector for any child

GameObject are displayed relative to the

Parent’s Transform values (local coordinates).

•

Local coordinates vs global coordinates.

2

D Graphics in Unity

•

Unity is famous for its 3D capabilities, but it can also be used

to create 2D games.

•

Graphic objects in 2D are known as Sprites.

•

Sprites are rendered with a Sprite Renderer

component rather than the Mesh Renderer used

with 3D objects.

•

Unity has a separate physics engine for handling

2

D physics so as to make use of optimizations only

available with 2D.

2

D Graphics – Primitive Sprites

•

Unity can import sprites created with any image editor.

However, there are also a number of primitive sprites that can

be created directly within Unity.

–

These primate sprites offer a quick way to create placeholders and

prototypes for testing purposes.

Unity – Tags

•

A Tag is a reference word which you can assign to one or more

GameObjects.

–

Tags are useful to identify GameObjects for scripting purposes: finding

an object by its tag, identify collision objects, etc.

–

Once you name a Tag, it cannot be renamed later.

A GameObject can only have one Tag assigned to it.

Unity – Layers

•

A Layer is a reference word that can be to create groups of

objects that share characteristics.

–

Layers are commonly used to render only a part of the scene or to

perform selectively collision detection.

–

A GameObject can only have one Layer assigned to it.

Unity – Sorting Layers

•

Sorting Layers are used in conjunction with Sprites to define

the overlay order of different Sprites.

–

To change their order, drag the handle at the left-hand side of each

Layer item.

Change the value of the Order in Layer to set the Renderer’s priority

among other Renderers within the same Sorting Layer.

Unity – Prefabs

•

Prefabs allows you to create, configure, and store a complete

GameObject with all its components, property values, and

child GameObjects as a reusable Asset.

–

The Prefab acts as a template from which you can create new Prefab

instances in the Scene.

–

Any edits that you make to a Prefab Asset are automatically reflected

in all instances of that Prefab.

Unity – Scripting

•

The behavior of GameObjects is controlled by the Components

that are attached to them, which can be a Unity Component or

a custom Component created using scripts.

–

Unity supports the C# programming language natively.

using UnityEngine;

using System.Collections;

public class MyScript : MonoBehaviour {

/

/ Use this for initialization

void Start () {

}

/

/ Update is called once per frame

void Update () {

}

Unity – Scripting

•

Basic MonoBehaviour Events:

–

Update: handles the frame update for the GameObject (a loop).

–

Start: is called by Unity before gameplay begins (i.e. before the Update

function is called for the first time) and is an ideal place to do any initialization.

using UnityEngine;

using System.Collections;

public class MyScript : MonoBehaviour {

/

/ Use this for initialization

void Start () {

}

/

/ Update is called once per frame

void Update () {

}

Scripting – Hello World

•

Example 1: Hello World

using UnityEngine;

using System.Collections;

public class MyScript : MonoBehaviour {

void Start () {

Debug.Log("Hello World!");

}

–

Debug.Log is a function that prints a message to Unity’s console output (usually used

for debug).

A script only defines a blueprint for a Component. Therefore, none of its code will be

executed until an instance of the script is attached to a GameObject.

•

The script can be attached by dragging the script to a GameObject in the inspector or added through the Add

Component button.

Scripting – Variables and Inspector

•

Example 2: Hello World With Name

–

Like other Components, scripts can have properties that are editable in the inspector:

using UnityEngine;

using System.Collections;

public class MyScript : MonoBehaviour {

public string myName;

void Start () {

Debug.Log("Hello World " + myName + "!");

}

–

In Unity, variables must public to be visible in the Inspector (or use the SerializeField

tag).

Scripting – Update Function

•

Example 3: Moving Transforms

public class MyScript : MonoBehaviour

{

public float distancePerFrame = 0.02f;

private void Update()

{

transform.Translate(Vector2.right * distancePerFrame);

}

–

Problem: frame time is not constant, so the object will move at an irregular speeds in

different computers. Solution: delta time (time in seconds since the last frame).

public class MyScript : MonoBehaviour

{

public float distancePerSecond = 2.0f;

private void Update()

{

transform.Translate(Vector2.right * distancePerSecond *

Time.deltaTime);

}

Scripting – Accessing Components

•

Example 4: Accessing a Rigidbody2D Component

–

Sometimes, a script needs access to other Components attached to the same

GameObject to access/change a property or call a function.

public class MyScript : MonoBehaviour

{

public float moveForce = 200.0f;

private Rigidbody2D rigidBody;

private void Start()

{

rigidBody = GetComponent<Rigidbody2D>();

rigidBody.mass = 2;

rigidBody.AddForce(Vector2.right * moveForce);

}

Scripting – Accessing Other Objects

•

Example 5: Linking GameObjects with Variables

–

The most straightforward way to find a related GameObject is to add a public

GameObject variable to the script:

public class Enemy : MonoBehaviour

{

public GameObject player;

.

..

}

–

The variable will be visible in the Inspector and can be assigned by dragging and

dropping an object from the scene.

public class Enemy : MonoBehaviour

{

public GameObject player;

void Update()

{

Debug.Log("Player is at: " + player.transform.position);

}

Scripting – Accessing Other Objects

•

Example 6: Finding GameObjects by Name

– It is always possible to locate GameObjects anywhere in the Scene hierarchy as long as

you have some information to identify them (name or tag).

public class Enemy : MonoBehaviour

{

private GameObject player;

private void Start()

{

player = GameObject.Find("Square");

}

void Update()

{

Debug.Log("Player is at: " + player.transform.position);

}

–

Important: for performance reasons, it is recommended to not use this function every

frame.

Scripting – Accessing Other Objects

•

Example 7: Finding GameObjects by Tag

– It is always possible to locate GameObjects anywhere in the Scene hierarchy as long as

you have some information to identify them (name or tag).

public class Enemy : MonoBehaviour

{

private GameObject player;

private void Start()

{

player = GameObject.FindWithTag("Player");

}

void Update()

{

Debug.Log("Player is at: " + player.transform.position);

}

–

Important: the method returns the first GameObject it finds with the specified tag. If a

scene contains multiple GameObjects with the specified tag, use the function

GameObject.FindGameObjectsWithTag (returns an array of GameObjects).

Scripting – Event Functions

•

A script in Unity is not like the traditional program where the

code runs continuously until it completes its task.

–

Instead, Unity controls scripts intermittently by calling certain functions

that are declared within the code.

–

These functions are known as event functions.

•

Update Events:

–

void Update() – Is called every frame.

–

void FixedUpdate() – has the frequency of the physics system (it is called every

fixed frame-rate frame). Always use FixedUpdate when manipulating a Rigidbody.

–

void LateUpdate() – Is called after all Update functions have been called. This is

useful to order script execution.

Scripting – Event Functions

•

Initialization Events:

–

void Start() – Is called on the frame when a script is enabled just before any of

the Update methods are called the first time.

–

void Awake() – is called when the script instance is being loaded. It is called when

the script object is initialized, regardless of whether or not the script is enabled.

Physics Events:

–

void OnCollisionEnter(Collision collision) – is called when the collider/rigidbody

has begun touching another rigidbody/collider.

void OnCollisionStay(Collision collision) – is called once per frame for every

collider/rigidbody that is touching the rigidbody/collider.

void OnCollisionExit(Collision collision) – is called when the collider/rigidbody

has stopped touching another rigidbody/collider.

Scripting – Event Functions

•

Physics Events (Triggers):

–

void OnTriggerEnter(Collider collider) – is called when the collider/rigidbody has

begun touching another collider that is a trigger.

void OnTriggerStay(Collider collider) – is called once per frame for every trigger

collider that is touching the rigidbody/collider.

void OnTriggerExit(Collider collider) – is called when the collider/rigidbody has

stopped touching another collider that is a trigger.

•

GUI Events:

– void OnGUI() – is called for rendering and handling GUI events.

List of Event Functions:

https://docs.unity3d.com/2020.2/Documentation/ScriptReference/MonoBehaviour.html

Scripting – Creating/Destroying GameObjects

•

Example 8: Creating Several Instances of a Prefab

–

Some games keep a constant number of objects in the scene, but it is very common for

characters, treasures and other object to be created and removed during gameplay.

public class GameManager : MonoBehaviour

{

public GameObject enemy;

void Start()

{

for (int i = 0; i < 5; i++) {

Instantiate(enemy);

}

–

The function Destroy is used to destroy game objects:

Destroy(gameObject, 0.5f);

•

The second parameter is optional and represents the amount of time to delay before destroying the object.

Unity – Input

•

Input allows the user to control your application using a

device, touch, or gestures.

–

Keyboards and mouse

Joysticks

Touch screens

Accelerometers or gyroscopes

VR and AR controllers

The Input Manager allows you to define input axes and their

associated actions.

–

Edit -> Project Settings (Input category).

Scripting – Input Axes

•

Example 9: Translating an Object using the Keyboard

–

We can use the input from the Horizontal and Vertical axes and the transform.Translate

method:

public class PlayerMovement : MonoBehaviour

{

public float moveSpeed = 10;

void Update()

{

float horizontalInput = Input.GetAxis("Horizontal");

float verticalInput = Input.GetAxis("Vertical");

transform.Translate(new Vector2(horizontalInput, verticalInput)

*

moveSpeed * Time.deltaTime);

}

–

Important: when using the Translate function to move an object, there is no collision

detection. Use rigidbodies when moving objects that require physical interaction.

Exercise 1

1

) Using primitive sprites (rectangles, circles, and triangles),

create in Unity a scene similar to the one illustrated below:

Important: use Unity’s

parenting system to

create a hierarchy and

combine sprites that

are part of the same

object.

Exercise 2

2

) Continue the implementation of the last exercises and create a

script to move the sun using the keyboard arrows.

–

When the sun is behind the cloud, change the color of the background

to a darker color.

Hint: the color of a sprite can be

changed in a script by accessing the

color property of the SpriteRenderer

component of the object.

Example:

background.GetComponent<SpriteRenderer>().color = Color.grey;

Building Multiplatform Projects

•

File -> Build Settings

PC

Android

Mobile Device Input

•

The Input class offers access to touchscreen, accelerometer

and geographical/location input.

–

Example 10: Touch input on mobile devices

public class TouchInput : MonoBehaviour{

public GameObject instObject;

void Update(){

foreach (Touch touch in Input.touches){

if (touch.phase == TouchPhase.Began){

Vector3 position = Camera.main.ScreenToWorldPoint(touch.position);

Instantiate(instObject, new Vector3(position.x, position.y, 0),

Quaternion.identity);

}

Mobile Device Accelerometer

•

As the mobile device moves, the accelerometer reports linear

acceleration changes along the three primary axes.

–

Example 11: Use accelerometer to change gravity on mobile devices

public class TouchInput : MonoBehaviour{

public GameObject instObject;

void Update(){

.

..

Physics2D.gravity = new Vector3(Input.acceleration.x * 9.81f,

Input.acceleration.y * 9.81f, 0);

}