# Understanding advanced collision management with Matter physics using “Risky Road” game prototype

I hope you enjoyed my HTML5 Risky Road tutorial series.

The series had a good feedback and I am using this prototype to explain two new features: collision filtering and collision active listener.

In real world, all physics bodies collide with all other physics bodies.

This also happens by default in Matter physics.

Sometimes, for game design purposes, we might want to disable collisions between some bodies, like in multiplayer games when your bullets shouldn’t hit other players in your team.

Another frequent scenario is when we have two bodies attached using a constraint, like the wheels and the car.

In previous example, constraint length and position were calculated to keep the wheels far from the car, to avoid collisions.

But what if we wanted wheels to be overlap the car, while not colliding with it? This is when collision filtering comes into play.

When you define your body properties such as friction and restitution, you can also give your body a `collisionFilter` property which can contain a category and a mask.

Setting a mask to collision filter will make the body collide only with the bodies with the given category.

Another interesting feature to add to the game would be a bonus to give the player when the car flies in the air after taking a hill at full speed.

We normally deal with collision detection to check when a collision starts, but we rarely check when a collision is still active.

Matter’s `collisionactive` event will return all active collisions as a set of pair of bodies colliding with each other. This way we can check if the wheels are on the ground or not.

Have a look at the result:

Just tap and hold to accelerate.

You will notice how wheels are overlapping both car and crate bodies without actually colliding with them thanks to collision filtering, and when the car is in the air for more than a half second you will see “flying” text.

Here is the completely commented source code:

```var game;

var gameOptions = {

// start vertical point of the terrain, 0 = very top; 1 = very bottom
startTerrainHeight: 0.5,

// max slope amplitude, in pixels
amplitude: 100,

// slope length range, in pixels
slopeLength: [150, 350],

// a mountain is a a group of slopes.
mountainsAmount: 3,

// amount of slopes for each mountain
slopesPerMountain: 6,

// car acceleration
carAcceleration: 0.01,

// maximum car velocity
maxCarVelocity: 1
}
let gameConfig = {
type: Phaser.AUTO,
backgroundColor: 0x75d5e3,
scale: {
mode: Phaser.Scale.FIT,
autoCenter: Phaser.Scale.CENTER_BOTH,
parent: "thegame",
width: 750,
height: 1334
},
physics: {
default: "matter",
matter: {
debug: true,
debugBodyColor: 0x000000
}
},
scene: playGame
}
game = new Phaser.Game(gameConfig);
window.focus();
}
class playGame extends Phaser.Scene{
constructor(){
super("PlayGame");
}
create(){

// creation of pool arrays
this.bodyPool = [];
this.bodyPoolId = [];

// array to store mountains
this.mountainGraphics = [];

// mountain start coordinates
this.mountainStart = new Phaser.Math.Vector2(0, 0);

// loop through all mountains
for(let i = 0; i < gameOptions.mountainsAmount; i++){

// each mountain is a graphics object

// generateTerrain is the method to generate the terrain. The arguments are the graphics object and the start position
this.mountainStart = this.generateTerrain(this.mountainGraphics[i], this.mountainStart);
}

// method to add the car, arguments represent x and y position
this.addCar(250, game.config.height / 2 - 70);

// the car is not accelerating
this.isAccelerating = false;

// input management
this.input.on("pointerdown", this.accelerate, this);
this.input.on("pointerup", this.decelerate, this);

// collision check between the diamond and the car. Any other diamond collision is not allowed
this.matter.world.on("collisionstart", function(event, bodyA, bodyB){
if((bodyA.label == "diamond" && bodyB.label != "car") || (bodyB.label == "diamond" && bodyA.label != "car")){
this.scene.start("PlayGame")
}

}.bind(this));

// a text to show when we are flying
this.flyingText = this.add.text(100, 100, "FLYING!!", {
fontFamily: "Arial",
fontSize: 128,
color: "#FF8800"
});
this.flyingText.setVisible(false);

// variable to count the time flying
this.flyingTime = 0;

// this event will check all active collisions
this.matter.world.on("collisionactive", function(e){

// no wheels colliding
this.wheelsColliding = false;

// a collision made by a pair of bodies
e.pairs.forEach(function(p){

// if a colliding body's label is "wheel"...
if(p.bodyA.label == "wheel" || p.bodyB.label == "wheel"){

// at least a wheel is colliding
this.wheelsColliding = true;
}
}.bind(this))
}.bind(this))
}

// method to generate the terrain. Arguments: the graphics object and the start position
generateTerrain(graphics, mountainStart){

// array to store slope points
let slopePoints = [];

// variable to count the amount of slopes
let slopes = 0;

// slope start point
let slopeStart = new Phaser.Math.Vector2(0, mountainStart.y);

// set a random slope length
let slopeLength = Phaser.Math.Between(gameOptions.slopeLength[0], gameOptions.slopeLength[1]);

// determine slope end point, with an exception if this is the first slope of the fist mountain: we want it to be flat
let slopeEnd = (mountainStart.x == 0) ? new Phaser.Math.Vector2(slopeStart.x + gameOptions.slopeLength[1] * 1.5, 0) : new Phaser.Math.Vector2(slopeStart.x + slopeLength, Math.random());

// current horizontal point
let pointX = 0;

// while we have less slopes than regular slopes amount per mountain...
while(slopes < gameOptions.slopesPerMountain){

// slope interpolation value
let interpolationVal = this.interpolate(slopeStart.y, slopeEnd.y, (pointX - slopeStart.x) / (slopeEnd.x - slopeStart.x));

// if current point is at the end of the slope...
if(pointX == slopeEnd.x){

// increase slopes amount
slopes ++;

// next slope start position
slopeStart = new Phaser.Math.Vector2(pointX, slopeEnd.y);

// next slope end position
slopeEnd = new Phaser.Math.Vector2(slopeEnd.x + Phaser.Math.Between(gameOptions.slopeLength[0], gameOptions.slopeLength[1]), Math.random());

// no need to interpolate, we use slope start y value
interpolationVal = slopeStart.y;
}

// current vertical point
let pointY = game.config.height * gameOptions.startTerrainHeight + interpolationVal * gameOptions.amplitude;

// add new point to slopePoints array
slopePoints.push(new Phaser.Math.Vector2(pointX, pointY));

// move on to next point
pointX ++ ;
}

// simplify the slope
let simpleSlope = simplify(slopePoints, 1, true);

// place graphics object
graphics.x = mountainStart.x;

// draw the ground
graphics.clear();
graphics.moveTo(0, game.config.height * 2);
graphics.fillStyle(0x654b35);
graphics.beginPath();
simpleSlope.forEach(function(point){
graphics.lineTo(point.x, point.y);
}.bind(this))
graphics.lineTo(pointX, game.config.height * 2);
graphics.lineTo(0, game.config.height * 2);
graphics.closePath();
graphics.fillPath();

// draw the grass
graphics.lineStyle(16, 0x6b9b1e);
graphics.beginPath();
simpleSlope.forEach(function(point){
graphics.lineTo(point.x, point.y);
})
graphics.strokePath();

// loop through all simpleSlope points starting from the second
for(let i = 1; i < simpleSlope.length; i++){

// define a line between previous and current simpleSlope points
let line = new Phaser.Geom.Line(simpleSlope[i - 1].x, simpleSlope[i - 1].y, simpleSlope[i].x, simpleSlope[i].y);

// calculate line length, which is the distance between the two points
let distance = Phaser.Geom.Line.Length(line);

// calculate the center of the line
let center = Phaser.Geom.Line.GetPoint(line, 0.5);

// calculate line angle
let angle = Phaser.Geom.Line.Angle(line);

// if the pool is empty...
if(this.bodyPool.length == 0){

// create a new rectangle body
this.matter.add.rectangle(center.x + mountainStart.x, center.y, distance, 10, {
isStatic: true,
angle: angle,
friction: 1,
restitution: 0,
collisionFilter: {
category: 2
}
});
}

// if the pool is not empty...
else{

// get the body from the pool
let body = this.bodyPool.shift();
this.bodyPoolId.shift();

// reset, reshape and move the body to its new position
this.matter.body.setPosition(body, {
x: center.x + mountainStart.x,
y: center.y
});
let length = body.area / 10;
this.matter.body.setAngle(body, 0)
this.matter.body.scale(body, 1 / length, 1);
this.matter.body.scale(body, distance, 1);
this.matter.body.setAngle(body, angle);
}
}

// assign a custom "width" property to the graphics object
graphics.width = pointX - 1

// return the coordinates of last mountain point
return new Phaser.Math.Vector2(graphics.x + pointX - 1, slopeStart.y);
}

// method to build the car

// car is made by three rectangle bodies which will be merged into a compound object
let floor = Phaser.Physics.Matter.Matter.Bodies.rectangle(posX, posY, 100, 10, {
label: "car"
});
let rightBarrier = Phaser.Physics.Matter.Matter.Bodies.rectangle(posX + 45, posY - 15, 10, 20, {
label: "car"
});
let leftBarrier = Phaser.Physics.Matter.Matter.Bodies.rectangle(posX - 45, posY - 15, 10, 20, {
label: "car"
});

// this is how we create the compound object
this.body = Phaser.Physics.Matter.Matter.Body.create({

// array of single bodies
parts: [floor, leftBarrier, rightBarrier],
friction: 1,
restitution: 0
});

// add the body to the world

// the diamond. It cannot fall off the car
this.diamond = this.matter.add.rectangle(posX, posY - 40, 30, 30, {
friction: 1,
restitution: 0,
label: "diamond"
});

// add front wheel. A circle
this.frontWheel = this.matter.add.circle(posX + 35, posY + 25, 30, {
friction: 1,
restitution: 0,
collisionFilter: {
},
label: "wheel"
});

this.rearWheel = this.matter.add.circle(posX - 35, posY + 25, 30, {
friction: 1,
restitution: 0,
collisionFilter: {
},
label: "wheel"
});

// these two constraints will bind front wheel to the body
pointA: {
x: 30,
y: 0
}
});
pointA: {
x: 45,
y: 0
}
});

// same thing for rear wheel
pointA: {
x: -30,
y: 0
}
});
pointA: {
x: -45,
y: 0
}
});
}

// method to accelerate
accelerate(){
this.isAccelerating = true;
}

// method to decelerate
decelerate(){
this.isAccelerating = false;
}

update(t, dt){

// if wheels aren't colliding...
if(!this.wheelsColliding){

// add frame delta time to flying time
this.flyingTime += dt;

// we can say the car is flying when it's in the air for more than 0.5 seconds
if(this.flyingTime > 500){

// show flying text
this.flyingText.setVisible(true);
}
}

// if wheels aren colliding...
else{

// reset flying time
this.flyingTime = 0;

// hide flying text
this.flyingText.setVisible(false);
}

// zoom is calculated according to car speed.
// zoom = 1: no zoom
// zoom > 1: zoom in
// zoom < 1: zoom out
let zoom = 1 - Phaser.Math.Clamp(this.body.speed, 0, 15) / 25

// zoomTo method allows the camera to zoom at "zoom" ratio in 1000 milliseconds
// the most important argument is the 4th argument.
// If set to "false", camera won't adjust its zoom if already zooming.
this.cameras.main.zoomTo(zoom, 1000, "Linear", false);

// make the game follow the car
this.cameras.main.scrollX = this.body.position.x - game.config.width / 4 + game.config.width * (1 - this.cameras.main.zoom);
this.cameras.main.scrollY = this.body.position.y - game.config.height / 2.2;

// flyingText too should follow the car
this.flyingText.x = 100 + this.cameras.main.scrollX;

// adjust velocity according to acceleration
if(this.isAccelerating){
let velocity = this.frontWheel.angularSpeed + gameOptions.carAcceleration;
velocity = Phaser.Math.Clamp(velocity, 0, gameOptions.maxCarVelocity);

// set angular velocity to wheels
this.matter.body.setAngularVelocity(this.frontWheel, velocity);
this.matter.body.setAngularVelocity(this.rearWheel, velocity);
}

// loop through all mountains
this.mountainGraphics.forEach(function(item){

// if the mountain leaves the screen to the left...
if(this.cameras.main.scrollX > item.x + item.width + game.config.width){

// reuse the mountain
this.mountainStart = this.generateTerrain(item, this.mountainStart)
}
}.bind(this));

// get all bodies
let bodies = this.matter.world.localWorld.bodies;

// loop through all bodies
bodies.forEach(function(body){

// if the body is out of camera view to the left side and is not yet in the pool..
if(this.cameras.main.scrollX > body.position.x + game.config.width && this.bodyPoolId.indexOf(body.id) == -1){

// ...add the body to the pool
this.bodyPool.push(body);
this.bodyPoolId.push(body.id);
}
}.bind(this))
}

// method to apply a cosine interpolation between two points
interpolate(vFrom, vTo, delta){
let interpolation = (1 - Math.cos(delta * Math.PI)) * 0.5;
return vFrom * (1 - interpolation) + vTo * interpolation;
}
}
```

There’s nothing better than learning new concepts by creating actual, playable projects. Download the source code.

215 GAME PROTOTYPES EXPLAINED WITH SOURCE CODE
// 1+2=3
// 10000000
// 2 Cars
// 2048
// Avoider
// Ballz
// Block it
// Blockage
// Bloons
// Boids
// Bombuzal
// Breakout
// Bricks
// Columns
// CubesOut
// Dots
// DROP'd
// Dudeski
// Eskiv
// Filler
// Fling
// Globe
// HookPod
// Hundreds
// InkTd
// Iromeku
// Lumines
// Magick
// MagOrMin
// Maze
// Memdot
// Nano War
// Nodes
// o:anquan
// Ononmin
// Pacco
// Phyballs
// Platform
// Poker
// Pool
// Poux
// Pudi
// qomp
// Racing
// Renju
// SameGame
// Security
// Sling
// Slingy
// Sokoban
// Splitter
// Sproing
// Stack
// Stairs
// Stringy
// Sudoku
// Tetris
// Threes
// Toony
// Turn
// TwinSpin
// vvvvvv
// Wordle
// Worms
// Yanga
// Zhed
// zNumbers