2D Physics in Unity

Game Frameworks

Lecture 03: 2D Physics in Unity

Edirlei Soares de Lima

<edirlei.lima@universidadeeuropeia.pt>

Unity – Physics

•

Unity simulates physics in 2D and 3D to ensure that objects

correctly accelerate and respond to collisions, gravity, and

various other forces.

–

3D physics: Nvidia PhysX engine integration

–

2D physics: Box2D engine integration

Main components:

–

Rigidbodies

–

Colliders

Rigidbody 2D

•

Rigidbody is the main component that

enables physical behavior for a GameObject.

–

With a Rigidbody attached, the object will

immediately respond to gravity.

–

If a Collider component is also added, the

GameObject will be moved by incoming collisions.

•

Important: since a Rigidbody component controls the

movement of the GameObject, never try to move it

from a script by changing the Transform properties

such as position and rotation.

–

Instead, use the physics engine to apply forces to push the

GameObject and let the physics engine calculate the results.

Rigidbody 2D

•

Body Type:

–

Dynamic: for objects designed to move under

simulation. It uses the full set of properties, such

as finite mass and drag, and is affected by gravity

and forces.

•

A Dynamic body will collide with every other body type.

It is the most performance-expensive body type.

Never use a script to set the position or rotation of a

Dynamic Rigidbody using its Transform component.

Rigidbody 2D

•

Body Type:

–

Kinematic: for objects designed to move under

simulation, but only under very explicit user

control. It is not affected by gravity and forces.

•

It does not collide with other Kinematic or Static

Rigidbodies (only collides with Dynamic Rigidbodies).

It has a lower demand on system resources than a

Dynamic Rigidbody.

It is designed to be repositioned explicitly via

MovePosition or MoveRotation functions.

Rigidbody 2D

•

Body Type:

–

Static: for objects designed to not move under

simulation at all. If anything collides with it, the

object behaves like an immovable object.

•

Only collides with Dynamic Rigidbodies.

The least resource-intensive body type.

All GameObjects that have a Collider component and do

not have a Rigidbody, are automatically considered Static

RigidBodies.

Collider 2D

•

A Collider component defines the shape of a

GameObject for the purposes of physical

collisions.

–

A collider is invisible and does not need to be the

exact same shape as the object.

•

Rough approximations are often more efficient and

indistinguishable in gameplay).

–

The simplest (and least processor-intensive)

colliders are primitive collider types:

•

Box, Circle, and Capsule.

More complex and customizable colliders:

•

Edge and Polygon.

Physics – Example 1

•

Example 1: Hello Physics World

–

Add Box Colliders 2D to the platforms.

Add a Circle Collider 2D to the ball.

Add a RigidBody 2D to the Ball.

Physics Material 2D

•

A Physics Material is used to adjust the

friction and bounce that occurs between

physics objects when they collide.

–

Example: Ball bouncing

Scripting – Add Force

•

Example 2: Moving the Ball with a Force

public class BallMovement : MonoBehaviour

{

public float moveForce = 20;

private Rigidbody2D rigidBody;

private void Start()

{

rigidBody = GetComponent<Rigidbody2D>();

}

private void FixedUpdate()

{

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

rigidBody.AddForce(Vector2.right * horizontalInput *

moveForce);

}

–

Important: always use FixedUpdate when manipulating a Rigidbody.

Scripting – Vector2 Class

•

The Vector2 class represents 2D vectors.

–

It also contains functions and properties for common vector

operations: magnitude, normalization, dot product, cross product,

angle, etc.

Main Constants:

–

Vector2.down: shorthand for writing Vector2(0, -1).

Vector2.left: shorthand for writing Vector2(-1, 0).

Vector2.one: shorthand for writing Vector2(1, 1).

Vector2.right: shorthand for writing Vector2(1, 0).

Vector2.up: shorthand for writing Vector2(0, 1).

Vector2.zero: shorthand for writing Vector2(0, 0).

Scripting – Vector2 Class

•

Main functions:

Vector2.Angle(Vector2 from, Vector2 to): returns the unsigned angle in degrees

between from and to.

–

Vector2.ClampMagnitude(Vector2 vector, float maxLength): returns a copy of

vector with its magnitude clamped to maxLength.

–

Vector2.Distance(Vector2 a, Vector2 b): returns the distance between a and b.

Vector2.Dot(Vector2 lhs, Vector2 rhs): dot Product of two vectors.

Vector2.Lerp(Vector2 a, Vector2 b, float t): linearly interpolates between

vectors a and b by t.

–

Vector2.MoveTowards(Vector2 current, Vector2 target, float maxDistanceDelta):

moves a point current towards target.

Vector2.SmoothDamp(Vector2 current, Vector2 target, ref Vector2

currentVelocity, float smoothTime, float maxSpeed = Mathf.Infinity, float

deltaTime = Time.deltaTime): Gradually changes a vector towards a desired goal

over time.

Scripting – Velocity Change

•

Example 3: Player Movement (Platform Game)

public class PlayerMovement : MonoBehaviour

{

public float moveForce = 6;

private Rigidbody2D rigidBody;

private void Start()

{

rigidBody = GetComponent<Rigidbody2D>();

}

private void FixedUpdate()

{

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

rigidBody.velocity = new Vector2(horizontalInput *

moveForce, rigidBody.velocity.y);

}

Effectors 2D

•

Effector 2D are components that control the

forces of physics when GameObject colliders

come into contact with each other.

Effectors:

–

PlatformEffector2D: creates platform behaviors

such as one-way collisions and no side friction;

–

SurfaceEffector2D: simulates conveyor belts;

PointEffector2D: attracts or repulses against a

given source point;

–

BuoyancyEffector2D: makes GameOjects float;

AreaEffector2D: randomly vary force and angle

magnitude;

Scripting – Impulse Force & Raycast

•

Example 4: Player Jump (Platform Game)

public class PlayerMovement : MonoBehaviour{

public float moveForce = 6;

public float jumpForce = 7.2;

private Rigidbody2D rigidBody;

private float feetOffset = 0;

private void Start(){

rigidBody = GetComponent<Rigidbody2D>();

BoxCollider2D collider = GetComponent<BoxCollider2D>();

feetOffset = ((collider.size.y * transform.localScale.y) / 2)- 0.01f;

}

private bool IsOnGround(){

Vector2 playerFeet = new Vector2(transform.position.x,

transform.position.y - feetOffset);

RaycastHit2D hit = Physics2D.Raycast(playerFeet, Vector2.down, 2.0f);

if (hit.collider != null){

float distance = Mathf.Abs(hit.point.y - playerFeet.y);

if (distance < 0.1){

return true;

}

return false;

}

Scripting – Impulse Force & Raycast

•

Example 4: Player Jump (Platform Game)

.

..

private void FixedUpdate()

{

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

float jumpInput = Input.GetAxis("Jump");

if ((jumpInput > 0) && (IsOnGround()))

{

rigidBody.AddForce(Vector2.up * jumpForce, ForceMode2D.Impulse);

}

rigidBody.velocity = new Vector2(horizontalInput * moveForce,

rigidBody.velocity.y);

}

Physics 2D – Raycast

•

The Raycast function casts a ray against colliders in the Scene.

RaycastHit2D Raycast(Vector2 origin, Vector2 direction, float distance =

Mathf.Infinity, int layerMask = DefaultRaycastLayers,

float minDepth = -Mathf.Infinity, float maxDepth =

Mathf.Infinity);

–

Returns a RaycastHit object with a reference to the collider that is hit

by the ray.

•

The collider property of the result will be NULL if nothing was hit.

–

The layerMask can be used to detect objects selectively only on

certain layers.

•

In last example, the function IsOnGround() can be optimized by defining a Layer on

objects that represent ground. The Layer can then be used in the layer mask

parameter to only check collisions with relevant objects.

RaycastHit2D hit = Physics2D.Raycast(playerFeet, Vector2.down, 2.0f,

LayerMask.GetMask("Ground"));

Physics 2D Settings

•

Edit -> Project Settings

–

Physics 2D settings can be used

to change global settings for 2D

physics simulation.

Scripting – Top-Down Movement

•

Example 5: Player Movement (Top-Down Game)

public class TopDownPlayerMovement : MonoBehaviour

{

public float moveForce = 6;

private Rigidbody2D rigidBody;

private void Start()

{

rigidBody = GetComponent<Rigidbody2D>();

}

private void FixedUpdate()

{

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

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

rigidBody.velocity = new Vector2(horizontalInput * moveForce,

verticalInput * moveForce);

}

Scripting – Top-Down Movement

•

Example 6: Pushing Objects (Top-Down Game)

–

Dynamic objects must have a RigidBody2D.

Adjust the Linear Drag to make them stop faster.

•

For the physics engine, the objects are in the air, so there

is not friction.

–

Adjust the Mass to make objects heavier.

Scripting – Top-Down Movement

•

Example 7: Player Movement based on Mouse Position

public class SpaceshipMovement : MonoBehaviour{

public float moveForce = 3;

public float minMouseDistance = 50;

private Rigidbody2D rigidBody;

private void Start(){

rigidBody = GetComponent<Rigidbody2D>();

}

private void FixedUpdate(){

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

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

Vector2 mouseDirection = Input.mousePosition -

Camera.main.WorldToScreenPoint(transform.position);

if (mouseDirection.magnitude > minMouseDistance){

float rotationAngle = Mathf.Atan2(mouseDirection.normalized.y,

mouseDirection.normalized.x) * Mathf.Rad2Deg;

rigidBody.SetRotation(rotationAngle - 90);

}

rigidBody.velocity = (((transform.up * verticalInput) +

(

transform.right * horizontalInput)) * moveForce);

}

Scripting – 2D Physics Events

•

Collision Events:

–

void OnCollisionEnter2D(Collision collision) – is called when the

collider/rigidbody has begun touching another rigidbody/collider.

void OnCollisionStay2D(Collision collision) – is called once per frame for every

collider/rigidbody that is touching the rigidbody/collider.

void OnCollisionExit2D(Collision collision) – is called when the collider/rigidbody

has stopped touching another rigidbody/collider.

Trigger Events:

–

void OnTriggerEnter2D(Collider collider) – is called when the collider/rigidbody

has begun touching another collider that is a trigger.

void OnTriggerStay2D(Collider collider) – is called once per frame for every

trigger collider that is touching the rigidbody/collider.

void OnTriggerExit2D(Collider collider) – is called when the collider/rigidbody

has stopped touching another collider that is a trigger.

Scripting – 2D Physics Events

•

Example 8: Destroy the Spaceship on Collision

public class SpaceshipMovement : MonoBehaviour{

.

..

private void OnCollisionEnter2D(Collision2D collision)

{

if (collision.collider.tag == "Asteroid")

{

Destroy(gameObject);

}

Scripting – 2D Physics Events

•

Example 9: Change the color of the Spaceship when it enters the heal area.

public class HealArea : MonoBehaviour

{

private void OnTriggerEnter2D(Collider2D collision)

{

if (collision.gameObject.tag == "Player")

{

collision.gameObject.GetComponent<SpriteRenderer>().color = Color.blue;

}

private void OnTriggerExit2D(Collider2D collision)

{

if (collision.gameObject.tag == "Player")

{

collision.gameObject.GetComponent<SpriteRenderer>().color = Color.white;

}

Scripting – 2D Physics Events

•

Example 10: Shooting and Destroying Asteroids

public class TopDownPlayerMovement : MonoBehaviour

{

public float moveForce = 3;

public float minMouseDistance = 50;

public GameObject bulletPrefab;

public float bulletForce = 12;

public float bulletStartOffset = 1.2f;

public float cooldownTime = 0.5f;

private Rigidbody2D rigidBody;

private float shootTimer = 0;

private void Start(){

rigidBody = GetComponent<Rigidbody2D>();

}

private void OnCollisionEnter2D(Collision2D collision){

if (collision.collider.tag == "Asteroid"){

Destroy(gameObject);

}

.

..

Scripting – 2D Physics Events

•

Example 10: Shooting and Destroying Asteroids

.

..

private void Update()

{

float fireInput = Input.GetAxis("Fire1");

if ((fireInput > 0) && (shootTimer <= 0))

{

Vector3 mouseDirection = Input.mousePosition –

Camera.main.WorldToScreenPoint(transform.position);

GameObject bullet = Instantiate(bulletPrefab, transform.position +

(

mouseDirection.normalized * bulletStartOffset),

Quaternion.identity);

bullet.GetComponent<Rigidbody2D>().velocity = mouseDirection.normalized

*

bulletForce;

shootTimer = cooldownTime;

}

if (shootTimer >= 0)

{

shootTimer -= Time.deltaTime;

}

.

..

Scripting – 2D Physics Events

•

Example 10: Shooting and Destroying Asteroids

private void FixedUpdate()

{

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

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

Vector2 mouseDirection = Input.mousePosition –

Camera.main.WorldToScreenPoint(transform.position);

if (mouseDirection.magnitude > minMouseDistance)

{

float rotationAngle = Mathf.Atan2(mouseDirection.normalized.y,

mouseDirection.normalized.x) * Mathf.Rad2Deg;

rigidBody.SetRotation(rotationAngle - 90);

}

rigidBody.velocity = (((transform.up * verticalInput) + (transform.right *

horizontalInput)) * moveForce);

}

Exercise 1

1

) Create an enemy for a 2D platform game that can only be

killed by jumping on his head (like some enemies of Super

Mario games).

–

Start with the code created for the player jump example.

–

If the player touches the sides of the enemy, the player dies.

Enemy

Kill the enemy

Player dies

2

D Joints

•

Joints can be used to attach GameObjects together.

There are different types of 2D joints:

–

Distance Joint 2D - attaches two GameObjects and keeps them a certain distance apart.

Fixed Joint 2D - keeps two objects in a position relative to each other, so the objects are

always offset at a given position and angle.

–

Friction Joint 2D - reduces both the linear and angular velocities between two

GameObjects to zero

Hinge Joint 2D - allows a GameObject to be attached to a point in space around which it

can rotate.

Relative Joint 2D - allows two GameObjects to maintain a position based on each other’s

location.

–

Slider Joint 2D - allows a GameObject to slide along a line in space.

Spring Joint 2D - allows two GameObjects to be attached together by a spring.

Target Joint 2D - connects to a specified target, rather than another rigid body.

Wheel Joint 2D - simulates wheels and suspension.

Distance Joint 2D

•

Distance Joint 2D – Attaches two GameObjects and keeps

them a certain distance apart.

–

Update the Line Renderer positions do show a line:

private void Update()

{

Rigidbody2D attachedBall = GetComponent<DistanceJoint2D>().connectedBody;

LineRenderer line = GetComponent<LineRenderer>();

line.SetPosition(0, transform.position);

line.SetPosition(1, attachedBall.transform.position);

}

Hinge Joint 2D

•

Hinge Joint 2D - allows a GameObject to be attached to a

point in space around which it can rotate.

–

Update the Line Renderer positions do show a line:

private void Update()

{

HingeJoint2D hinge = GetComponent<HingeJoint2D>();

LineRenderer line = GetComponent<LineRenderer>();

line.SetPosition(0, transform.position);

line.SetPosition(1, hinge.connectedBody.position);

}

Spring Joint 2D

•

Spring Joint 2D - allows two GameObjects to be attached

together by a spring.

–

Update the Line Renderer positions do show a line:

private void Update(){

SpringJoint2D hinge = GetComponent<SpringJoint2D>();

if (hinge){

LineRenderer line = GetComponent<LineRenderer>();

line.SetPosition(0, transform.position);

line.SetPosition(1, hinge.connectedBody.position);

}

else{

Destroy(GetComponent<LineRenderer>());

}