Game code for week 1 of Writing Your First Game Using HTML5 and Canvas The game code uses underscore.js, and some utility methods defined in week1-util.js

Currently, we have 3 objects: Game, Drawable, and Background (which uses Drawable as its prototype.)

Drawable is the prototype for all objects that can exist in our world. It currently provides in interface and some convenience methods to deal with drawing, and will later also deal with collision detection and input handling.

var Drawable = _.inherits(function() { }, {

Store the position and velocity of the current object.

    'state': { x: 0, y: 0, vX: 0, vY: 0 },

The constructor takes an initial state of the object, and merges it in with the defaults

    'constructor': function(initialState) {
        this.state = _.extend({}, this.state, initialState);

Draw is called in the game loop, receiving the elapsed time dT and the game object.

    'draw': function(dT, game) {

Update the x and y coordinates using the current velocities. (This is sometimes called "integrating".)

        var centroid = {
            x: this.state.vX * dT + this.state.x,
            y: this.state.vY * dT + this.state.y,

Hand off to the actual drawing method. Descendant objects override this to implement custom drawing logic.

        this.doDraw(centroid, dT, game);

Finally update the state of the object.

        this.state.x = centroid.x;
        this.state.y = centroid.y;

By default Drawable draws a circle at the current position

    'doDraw': function(centroid, dT, game) {
        game.backbufferContext.beginPath(centroid.x, centroid.y);
        game.backbufferContext.arc(centroid.x - 5, centroid.y - 5, 10, 0,  2 * Math.PI);

Background is a Drawable that shows a scrolling background.

var Background = _.inherits(Drawable, {
    'image': null,
    'ready': false,

The constructor calls the constructor of Drawable, and fetches the background image.

    'constructor': function(initialState) {
        this.constructor.__super__.constructor.call(this, initialState);

        this.image = new Image();
        this.image.onload = _.bind(function() {
            this.ready = true;
        }, this);
        this.image.src = '/posts/game/game-bg.jpg';

doDraw gets called by the logic in Drawable. In this case we override it to draw the tiling background.

    'doDraw': function(centroid, dT, game) {

Don't begin drawing until the image has downloaded.


Make the background wrap around the canvas.

        if(centroid.y <= 0) 
            centroid.y = game.height;

The tiling background is drawn in two slices.

drawImage takes a rectangle from the source image stretching from the coordinates (sx, xy) to (sx + sw, sy + sh), and draws it to the canvas in a rectangle defined similarly.

We use this to draw a top slice and a bottom slice that move up in unison.

            /* sx */ 0, 
            /* sy */ this.image.naturalHeight - centroid.y, 
            /* sw */ this.image.naturalWidth, 
            /* sh */ centroid.y, 
            /* dx */ 0, 
            /* dy */ 0, 
            /* dw */ game.width, 
            /* dh */ centroid.y);

            /* sx */ 0, /* sy */ 0, 
            /* sw */ this.image.naturalWidth,
            /* sh */ this.image.naturalHeight - centroid.y, 
            /* dx */ 0,
            /* dy */ centroid.y,
            /* dw */ game.width,
            /* dh */ this.image.naturalHeight - centroid.y

Define our game object. It has methods for setting up the rendering environment, initializing objects, and running the game.

var Game = _.inherits(function() { }, {

The speed that objects in the game move at.

    'gameSpeed': -.15,
    'constructor': function() {

Initialize the lastPaint time, which we use to calculate the elapsed time (dt) between frames.

        this.lastPaint = Date.now();

Use two canvas elements to do double-buffering, which prevents the user from seeing flickering as we draw the scene.

canvas is the element currently used to display the scene, backbufferCanvas is a hidden element that we draw the scene to.

When the scene is finished drawing, we swap the two.

        this.canvii = document.querySelectorAll('#sample1 canvas');

        this.canvas = this.canvii[0];

        this.backbufferCanvas = this.canvii[1];

        this.context = this.canvas.getContext('2d');
        this.backbufferContext = this.backbufferCanvas.getContext('2d');

canvIdx holds the index of the current display buffer. We flip this back and forth between 0 and 1 to swap the two canvases.

        this.canvIdx = 0;

Convenience methods for accessing the canvas size

        this.width = this.canvas.width;
        this.height = this.canvas.height;

We will put all the drawable objects in this array, later we loop over the array to draw the objects.

        this.objects = [];


Draw a circle in the center as a placeholder for the player

        var centerObj = new Drawable({
            x: this.width/2,
            y: this.height/2

_.bind ensures that this always refers to the Game object when draw is called from other contexts. We need this since draw is called from window.requestAnimationFrame.

        this.draw = _.bind(this._draw, this);

These are used to track the framerate as a moving average.

        this.frames = 0;
        this.times = [];
        this.start = Date.now();
    'initBackground': function() {

Initialize the background and add it to beginning of the objects array. It extends Drawable, so we can make it move just by passing in an initial speed.

        this.objects.unshift(new Background({
            vY: this.gameSpeed,
            y: this.height

draw starts the game loop. It will continue to call itself using requestAnimationFrame until the game is told to stop.

    '_draw': function() {
        if(!this.stop) {

Calculate the amount of time since the loop last ran (dT), also save the time that we started drawing, for performance tracking.

        var dT = Date.now() - this.lastPaint;
        var drawStart = Date.now();

Clear the backbuffer, then ask the objects to draw themselves.

        this.backbufferContext.clearRect(0, 0, this.width, this.height);
        this.objects.forEach(function(obj) {
            obj.draw(dT, this);
        }, this);

Increment the frame counter. Every 12 frames (5 times per second, since we run at 60 fps), calculate a new moving average framerate.

        if(this.frames % 12 == 0) {
            this.fr = this.frames*1000/(Date.now() - this.start);
            this.frames = 0;
            this.start = Date.now();

Store the time drawing ended.

        this.lastPaint = Date.now();

Draw the framerate.

        this.backbufferContext.strokeText(this.fr, 900, 20);

Finally, swap the backbuffer and the display buffer, which shows the new scene to the user.


Swaps the two canvas elements, and their contexts. Neat trick courtesy of Fedor van Eldijk via StackOverflow

    'swapsies': function() {
        this.canvas = this.canvii[this.canvIdx];
        this.backbufferCanvas = this.canvii[1 - this.canvIdx];

        this.canvIdx = 1 - this.canvIdx;

        this.context = this.canvas.getContext('2d');
        this.backbufferContext = this.backbufferCanvas.getContext('2d');
        this.canvas.style.visibility = 'visible';
        this.backbufferCanvas.style.visibility = 'hidden';


A quick and dirty shim for requestAnimationFrame.

window.requestAnimationFrame =
    window.requestAnimationFrame ||
    window.webkitRequestAnimationFrame ||
    window.mozRequestAnimationFrame ||

Now simply start the game by initializing it and calling the draw method.

It can be stopped by setting game.stop to true.

var game = new Game();

document.getElementByid('stop').onclick = function() {
	game.stop = true;

That's all the code so far!

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