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Game Obstacles


Push the buttons to move the red square:







Add Some Obstacles

Now we want to add som obstacles to our game.

Add a new component to the gaming area. Make it green, 10px wide, 200px high, and place it 300px to the right and 120px down.

Also update the obstacle component in every frame:

Example

var myGamePiece;
var myObstacle;

function startGame() {
    myGamePiece = new component(30, 30, "red", 10, 120);
    myObstacle = new component(10, 200, "green", 300, 120);
    myGameArea.start();
}

function updateGameArea() {
    myGameArea.clear();
    myObstacle.update();
   
myGamePiece.newPos();
    myGamePiece.update();
}
Try it yourself »

Hit The Obstacle = Game Over

In the example above, nothing happens when you hit the obstacle. In a game, that is not very satisfying.

How do we know if our red square hits the obstacle?

Create a new method in the component constructor, that chekcs if the component crashes with another component. This method should be called every time the frames updates, 50 times per second.

Also add a stop() method to the myGameArea object, which clears the 20 milliseconds interval.

Example

var myGameArea = {
    canvas : document.createElement("canvas"),
    start : function() {
        this.canvas.width = 480;
        this.canvas.height = 270;
        this.context = this.canvas.getContext("2d");
        document.body.insertBefore(this.canvas, document.body.childNodes[0]);
        this.interval = setInterval(updateGameArea, 20);
    },
    clear : function() {
        this.context.clearRect(0, 0, this.canvas.width, this.canvas.height);
    },
    stop : function() {
        clearInterval(this.interval);
    }
}

function component(width, height, color, x, y) {
    this.width = width;
    this.height = height;
    this.speedX = 0;
    this.speedY = 0;
    this.x = x;
    this.y = y;
    this.update = function() {
        ctx = myGameArea.context;
        ctx.fillStyle = color;
        ctx.fillRect(this.x, this.y, this.width, this.height);
    }
    this.newPos = function() {
        this.x += this.speedX;
        this.y += this.speedY;
    }
    this.crashWith = function(otherobj) {
        var myleft = this.x;
        var myright = this.x + (this.width);
        var mytop = this.y;
        var mybottom = this.y + (this.height);
        var otherleft = otherobj.x;
        var otherright = otherobj.x + (otherobj.width);
        var othertop = otherobj.y;
        var otherbottom = otherobj.y + (otherobj.height);
        var crash = true;
        if ((mybottom < othertop) ||
               (mytop > otherbottom) ||
               (myright < otherleft) ||
               (myleft > otherright)) {
           crash = false;
        }
        return crash;
    }
}

function updateGameArea() {
    if (myGamePiece.crashWith(myObstacle)) {
        myGameArea.stop();
    } else {
        myGameArea.clear();
        myObstacle.update();
        myGamePiece.newPos();
        myGamePiece.update();
    }
}
Try it yourself »

Moving Obstacle

The obstacle is of no danger when it is static, so we want it to move.

Change the property value of myObstacle.x at every update:

Example

function updateGameArea() {
    if (myGamePiece.crashWith(myObstacle)) {
        myGameArea.stop();
    } else {
        myGameArea.clear();
        myObstacle.x += -1;
        myObstacle.update();
        myGamePiece.newPos();
        myGamePiece.update();
    }
}
Try it yourself »

Multiple Obstacles

How about adding multiple obstacles?

For that we need a property for counting frames, and a method for execute something at a given frame rate.

Example

var myGameArea = {
    canvas : document.createElement("canvas"),
    start : function() {
        this.canvas.width = 480;
        this.canvas.height = 270;
        this.context = this.canvas.getContext("2d");
        document.body.insertBefore(this.canvas, document.body.childNodes[0]);
        this.frameNo = 0;       
        this.interval = setInterval(updateGameArea, 20);
    },
    clear : function() {
        this.context.clearRect(0, 0, this.canvas.width, this.canvas.height);
    },
    stop : function() {
        clearInterval(this.interval);
    }
}

function everyinterval(n) {
    if ((myGameArea.frameNo / n) % 1 == 0) {return true;}
    return false;
}

The everyinterval function returns true if the current framenumber corresponds with the given interval.

To define multiple obstacles, first declare the obstacle variable as an array.

Second, we need to make some changes in the updateGameArea function.

Example

var myGamePiece;
var myObstacles = [];

function updateGameArea() {
    var x, y;
    for (i = 0; i < myObstacles.length; i += 1) {
        if (myGamePiece.crashWith(myObstacles[i])) {
            myGameArea.stop();
            return;
        }
    }
    myGameArea.clear();
    myGameArea.frameNo += 1;
    if (myGameArea.frameNo == 1 || everyinterval(150)) {
        x = myGameArea.canvas.width;
        y = myGameArea.canvas.height - 200
        myObstacles.push(new component(10, 200, "green", x, y));
    }
    for (i = 0; i < myObstacles.length; i += 1) {
        myObstacles[i].x += -1;
        myObstacles[i].update();
    }
    myGamePiece.newPos();
    myGamePiece.update();
}
Try it yourself »

In the updateGameArea function we must loop through every obstacle to see if there is a crash. If there is a crash, the updateGameArea function will stop, and no more drawing is done.

The updateGameArea function counts frames and adds an obstacle for every 150th frame.


Obstacles of Random Size

To make the game a bit more difficult, and fun, we will send in obstacles of random sizes, so that the red square must move up and down to not crash.

Example

function updateGameArea() {
    var x, height, gap, minHeight, maxHeight, minGap, maxGap;
    for (i = 0; i < myObstacles.length; i += 1) {
        if (myGamePiece.crashWith(myObstacles[i])) {
            myGameArea.stop();
            return;
        }
    }
    myGameArea.clear();
    myGameArea.frameNo += 1;
    if (myGameArea.frameNo == 1 || everyinterval(150)) {
        x = myGameArea.canvas.width;
        minHeight = 20;
        maxHeight = 200;
        height = Math.floor(Math.random()*(maxHeight-minHeight+1)+minHeight);
        minGap = 50;
        maxGap = 200;
        gap = Math.floor(Math.random()*(maxGap-minGap+1)+minGap);
        myObstacles.push(new component(10, height, "green", x, 0));
        myObstacles.push(new component(10, x - height - gap, "green", x, height + gap));
    }
    for (i = 0; i < myObstacles.length; i += 1) {
        myObstacles[i].x += -1;
        myObstacles[i].update();
    }
    myGamePiece.newPos();
    myGamePiece.update();
}
Try it yourself »