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HTML5 Game Development – Lesson 10

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HTML5 Game Development – Lesson 10

Finally we can continue the series of articles on game development in HTML5 using Canvas. Today I have prepared a new game – SkyWalker. Basically – this is a flight simulator with a plane and multiple enemies. Our goal – to get to the finish line. These are the key features: using sprites for the plane and explosions, possibility to press multiple keys (for example – you can move and attack at the same time), a certain level length, enhanced collision detections (now enemies can damage our plane), life and scores parameters.

Our previous article you can read here: Developing Your First HTML5 Game – Lesson 9.

Now you can check our demo and download the sources:

Live Demo

download in package

If you are ready – let’s start coding !

Step 1. HTML

As usual (for all canvas-based demos) we have a basic html markup:

<!DOCTYPE html>
<html lang="en" >
    <head>
        <meta charset="utf-8" />
        <meta name="author" content="Script Tutorials" />
        <title>HTML5 Game Development - Lesson 10 (SkyWalker) | Script Tutorials</title>

        <!-- add styles -->
        <link href="css/main.css" rel="stylesheet" type="text/css" />

        <!-- add scripts -->
        <script src="js/jquery.js"></script>
        <script src="js/script.js"></script>
    </head>
    <body>
        <div class="container">
            <canvas id="scene" width="700" height="700" tabindex="1"></canvas>
        </div>
    </body>
</html>

Step 2. JS

Now, please create an empty file ‘script.js’ (in ‘js’ folder) and put this code inside (this is full JS code of our SkyWalker game). I will explain the main functionality below this code.

js/script.js

// inner variables
var canvas, ctx;

// images
var backgroundImage;
var oRocketImage;
var oExplosionImage;
var introImage;
var oEnemyImage;

var iBgShiftY = 9300; //10000 (level length) - 700 (canvas height)
var bPause = true; // game pause
var plane = null; // plane object
var rockets = []; // array of rockets
var enemies = []; // array of enemies
var explosions = []; // array of explosions
var planeW = 200; // plane width
var planeH = 110; // plane height
var iSprPos = 2; // initial sprite frame for plane
var iMoveDir = 0; // move direction
var iEnemyW = 128; // enemy width
var iEnemyH = 128; // enemy height
var iRocketSpeed = 10; // initial rocket speed
var iEnemySpeed = 5; // initial enemy speed
var pressedKeys = []; // array of pressed keys
var iScore = 0; // total score
var iLife = 100; // total life of plane
var iDamage = 10; // damage per enemy plane
var enTimer = null; // random timer for a new enemy
// -------------------------------------------------------------

// objects :
function Plane(x, y, w, h, image) {
    this.x = x;
    this.y = y;
    this.w = w;
    this.h = h;
    this.image = image;
    this.bDrag = false;
}
function Rocket(x, y, w, h, speed, image) {
    this.x = x;
    this.y = y;
    this.w = w;
    this.h = h;
    this.speed = speed;
    this.image = image;
}
function Enemy(x, y, w, h, speed, image) {
    this.x = x;
    this.y = y;
    this.w = w;
    this.h = h;
    this.speed = speed;
    this.image = image;
}
function Explosion(x, y, w, h, sprite, image) {
    this.x = x;
    this.y = y;
    this.w = w;
    this.h = h;
    this.sprite = sprite;
    this.image = image;
}
// -------------------------------------------------------------
// get random number between X and Y
function getRand(x, y) {
    return Math.floor(Math.random()*y)+x;
}

// Display Intro function
function displayIntro() {
    ctx.clearRect(0, 0, ctx.canvas.width, ctx.canvas.height);
    ctx.drawImage(introImage, 0, 0,700, 700);
}

// Draw Main scene function
function drawScene() {

    if (! bPause) {
        iBgShiftY -= 2; // move main ground
        if (iBgShiftY < 5) { // Finish position
            bPause = true;

            // draw score
            ctx.font = '40px Verdana';
            ctx.fillStyle = '#fff';
            ctx.fillText('Finish, your score: ' + iScore * 10 + ' points', 50, 200);
            return;
        }

        // process pressed keys (movement of plane)
        processPressedKeys();

        // clear canvas
        ctx.clearRect(0, 0, ctx.canvas.width, ctx.canvas.height);

        // draw background
        ctx.drawImage(backgroundImage, 0, 0 + iBgShiftY, 700, 700, 0, 0, 700, 700);

        // draw plane
        ctx.drawImage(plane.image, iSprPos*plane.w, 0, plane.w, plane.h, plane.x - plane.w/2, plane.y - plane.h/2, plane.w, plane.h);

        // draw rockets
        if (rockets.length > 0) {
            for (var key in rockets) {
                if (rockets[key] != undefined) {
                    ctx.drawImage(rockets[key].image, rockets[key].x, rockets[key].y);
                    rockets[key].y -= rockets[key].speed;

                    // if a rocket is out of screen - remove it
                    if (rockets[key].y < 0) {
                        delete rockets[key];
                    }
                }
            }
        }

        // draw explosions
        if (explosions.length > 0) {
            for (var key in explosions) {
                if (explosions[key] != undefined) {
                    // display explosion sprites
                    ctx.drawImage(explosions[key].image, explosions[key].sprite*explosions[key].w, 0, explosions[key].w, explosions[key].h, explosions[key].x - explosions[key].w/2, explosions[key].y - explosions[key].h/2, explosions[key].w, explosions[key].h);
                    explosions[key].sprite++;

                    // remove an explosion object when it expires
                    if (explosions[key].sprite > 10) {
                        delete explosions[key];
                    }
                }
            }
        }

        // draw enemies
        if (enemies.length > 0) {
            for (var ekey in enemies) {
                if (enemies[ekey] != undefined) {
                    ctx.drawImage(enemies[ekey].image, enemies[ekey].x, enemies[ekey].y);
                    enemies[ekey].y -= enemies[ekey].speed;

                    // remove an enemy object if it is out of screen
                    if (enemies[ekey].y > canvas.height) {
                        delete enemies[ekey];
                    }
                }
            }
        }

        if (enemies.length > 0) {
            for (var ekey in enemies) {
                if (enemies[ekey] != undefined) {

                    // collisions with rockets
                    if (rockets.length > 0) {
                        for (var key in rockets) {
                            if (rockets[key] != undefined) {
                                if (rockets[key].y < enemies[ekey].y + enemies[ekey].h/2 && rockets[key].x > enemies[ekey].x && rockets[key].x + rockets[key].w < enemies[ekey].x + enemies[ekey].w) {
                                    explosions.push(new Explosion(enemies[ekey].x + enemies[ekey].w / 2, enemies[ekey].y + enemies[ekey].h / 2, 120, 120, 0, oExplosionImage));

                                    // delete enemy, rocket, and add +1 to score
                                    delete enemies[ekey];
                                    delete rockets[key];
                                    iScore++;
                                }
                            }
                        }
                    }

                    // collisions with plane
                    if (enemies[ekey] != undefined) {
                        if (plane.y - plane.h/2 < enemies[ekey].y + enemies[ekey].h/2 && plane.x - plane.w/2 < enemies[ekey].x + enemies[ekey].w && plane.x + plane.w/2 > enemies[ekey].x) {
                            explosions.push(new Explosion(enemies[ekey].x + enemies[ekey].w / 2, enemies[ekey].y + enemies[ekey].h / 2, 120, 120, 0, oExplosionImage));

                            // delete enemy and make damage
                            delete enemies[ekey];
                            iLife -= iDamage;

                            if (iLife <= 0) { // Game over
                                bPause = true;

                                // draw score
                                ctx.font = '38px Verdana';
                                ctx.fillStyle = '#fff';
                                ctx.fillText('Game over, your score: ' + iScore * 10 + ' points', 25, 200);
                                return;
                            }
                        }
                    }
                }
            }
        }

        // display life and score
        ctx.font = '14px Verdana';
        ctx.fillStyle = '#fff';
        ctx.fillText('Life: ' + iLife + ' / 100', 50, 660);
        ctx.fillText('Score: ' + iScore * 10, 50, 680);
    }
}

// Process Pressed Keys function
function processPressedKeys() {
    if (pressedKeys[37] != undefined) { // 'Left' key
        if (iSprPos > 0) {
            iSprPos--;
            iMoveDir = -7;
        }
        if (plane.x - plane.w / 2 > 10) {
            plane.x += iMoveDir;
        }
    }
    else if (pressedKeys[39] != undefined) { // 'Right' key
        if (iSprPos < 4) {
            iSprPos++;
            iMoveDir = 7;
        }
        if (plane.x + plane.w / 2 < canvas.width - 10) {
            plane.x += iMoveDir;
        }
    }
}

// Add Enemy function (adds a new enemy randomly)
function addEnemy() {
    clearInterval(enTimer);

    var randX = getRand(0, canvas.height - iEnemyH);
    enemies.push(new Enemy(randX, 0, iEnemyW, iEnemyH, - iEnemySpeed, oEnemyImage));

    var interval = getRand(1000, 4000);
    enTimer = setInterval(addEnemy, interval); // loop
}

// Main Initialization
$(function(){
    canvas = document.getElementById('scene');
    ctx = canvas.getContext('2d');

    // load background image
    backgroundImage = new Image();
    backgroundImage.src = 'images/levelmap.jpg';
    backgroundImage.onload = function() {
    }
    backgroundImage.onerror = function() {
        console.log('Error loading the background image.');
    }

    introImage = new Image();
    introImage.src = 'images/intro.jpg';

    // initialization of empty rocket
    oRocketImage = new Image();
    oRocketImage.src = 'images/rocket.png';
    oRocketImage.onload = function() { }

    // initialization of explosion image
    oExplosionImage = new Image();
    oExplosionImage.src = 'images/explosion.png';
    oExplosionImage.onload = function() { }

    // initialization of empty enemy
    oEnemyImage = new Image();
    oEnemyImage.src = 'images/enemy.png';
    oEnemyImage.onload = function() { }

    // initialization of plane
    var oPlaneImage = new Image();
    oPlaneImage.src = 'images/plane.png';
    oPlaneImage.onload = function() {
        plane = new Plane(canvas.width / 2, canvas.height - 100, planeW, planeH, oPlaneImage);
    }

    $(window).keydown(function (evt){ // onkeydown event handle
        var pk = pressedKeys[evt.keyCode];
        if (! pk) {
            pressedKeys[evt.keyCode] = 1; // add all pressed keys into array
        }

        if (bPause && evt.keyCode == 13) { // in case of Enter button
            bPause = false;

            // start main animation
            setInterval(drawScene, 30); // loop drawScene

            // and add first enemy
            addEnemy();
        }
    });

    $(window).keyup(function (evt) { // onkeyup event handle
        var pk = pressedKeys[evt.keyCode];
        if (pk) {
            delete pressedKeys[evt.keyCode]; // remove pressed key from array
        }
        if (evt.keyCode == 65) { // 'A' button - add a rocket
            rockets.push(new Rocket(plane.x - 16, plane.y - plane.h, 32, 32, iRocketSpeed, oRocketImage));
        }
        if (evt.keyCode == 37 || evt.keyCode == 39) {
            // revert plane sprite to default position
            if (iSprPos > 2) {
                for (var i = iSprPos; i >= 2; i--) {
                    iSprPos = i;
                    iMoveDir = 0;
                }
            } else {
                for (var i = iSprPos; i <= 2; i++) {
                    iSprPos = i;
                    iMoveDir = 0;
                }
            }
        }
    });

    // when intro is ready - display it
    introImage.onload = function() {
        displayIntro(); // Display intro once
    }
});

In the main initialization the script loads all necessary images (level map, intro screen, rocket, explosion sprite, enemy and plane sprite). Then, in order to handle with multiple pressed keys we have to use Array (pressedKeys) to keep all pressed keys (then, during rendering of the main scene, we will use this array to manipulate with our plane), and finally, once the Intro page is loaded – we display intro screen. One of important moments – handling of multiple pressed keys, look at this code:

var pressedKeys = []; // array of pressed keys

$(window).keydown(function (evt){ // onkeydown event handle
    var pk = pressedKeys[evt.keyCode];
    if (! pk) {
        pressedKeys[evt.keyCode] = 1; // add all pressed keys into array
    }
});

$(window).keyup(function (evt) { // onkeyup event handle
    var pk = pressedKeys[evt.keyCode];
    if (pk) {
        delete pressedKeys[evt.keyCode]; // remove pressed key from array
    }
});

This technique allows us to handle multiple keys. Well, during rendering of the main scene we draw next objects: level background, the main plane, rockets (of our plane), enemies and explosions. Once we hit an enemy – we draw explosion sprite at the last place of the enemy. And finally, our opponents are not harmless, as soon as they touch our plane – they explode and cause damage to our aircraft. And, if our life value is under zero – game over. To implement collisions and explosions, I used the following code:

if (plane.y - plane.h/2 < enemies[ekey].y + enemies[ekey].h/2 && plane.x - plane.w/2 < enemies[ekey].x + enemies[ekey].w && plane.x + plane.w/2 > enemies[ekey].x) {
    explosions.push(new Explosion(enemies[ekey].x + enemies[ekey].w / 2, enemies[ekey].y + enemies[ekey].h / 2, 120, 120, 0, oExplosionImage));

    // delete enemy and make damage
    delete enemies[ekey];
    iLife -= iDamage;

    if (iLife <= 0) { // Game over
        bPause = true;

        // draw score
        ctx.font = '38px Verdana';
        ctx.fillStyle = '#fff';
        ctx.fillText('Game over, your score: ' + iScore * 10 + ' points', 25, 200);
        return;
    }
}

Step 3. Custom graphics

enemy.png, explosion.png, intro.jpg, levelmap.jpg, plane.png, rocket.png

All used images are available in our package.

Live Demo

download in package

Conclusion

Do you like the new SkyWalker game? :-) I will be glad to see your comments. Good luck!

 

 

 

Create data driven applications in Qlik’s free and easy to use coding environment, brought to you in partnership with Qlik.

Topics:

Published at DZone with permission of Andrey Prikaznov, DZone MVB. See the original article here.

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