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kittykatattack / ga

The world's tiniest, cutest and funnest game engine
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Ga

Ga

"Ga!" - A baby's exclamation of surprise.

Ga is a tiny, cute and friendly system for making HTML5 games or any other kind interactive media. You can use it to make any kind of 2D action game you can imagine, with unbelievably tiny file sizes (under 6.5k!)

Take a look at the feature list and the examples folder to get started. Keep scrolling, and you'll find a complete beginner's tutorial ahead. If you've never made a game before, the tutorials are the best place to start.

Ga is "finsihed" software. It's perfect and bug-free, which is why there have been so few recent updates. Go ahead and use it - forever!

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Table of contents:

  1. Features
  2. The Plugins
  3. Coming soon...
  4. Ga's philosophy and technical constraints
  5. Minifying, crushing and compressing
  6. Contributions and Licencing
  7. Hexi
  8. Tutorials
    1. Treasure Hunter
    2. Setting up the HTML container page
    3. Initializing the Ga engine
    4. Define your "global" variables
    5. Initialize your game with a setup function
      1. Customizing the canvas
      2. Creating the chimes sound object
      3. Creating game scenes
      4. Making sprites
      5. Positioning sprites
      6. Assigning dynamic properties
      7. Creating the enemy sprites
      8. The health bar
      9. The game over scene
      10. Keyboard interactivity
      11. Setting the game state
    6. Game logic with the play function loop
      1. Moving the player sprite
      2. Containing sprites inside the screen boundaries
      3. Collision with the enemies
      4. Collision with the treasure
      5. Ending the game
    7. Using images
      1. Individual images
      2. Loading image files
      3. Making sprites with images
      4. Fine-tuning the containment area
    8. Using a texture atlas
      1. Preparing the images
      2. loading the texture atlas
    9. Alien Armada
    10. Load and use a custom font
    11. Scale and center the game in the browser
    12. A loading progress bar
    13. Shooting bullets
    14. Sprite states
    15. Generating random aliens
      1. Timing the aliens
      2. The aliens' random start positions
    16. Moving the aliens
    17. Making the aliens explode
    18. Displaying the score
    19. Ending and resetting the game
    20. Flappy Fairy!
    21. Launch a game in fullscreen mode
    22. Make a button
    23. Making the fairy fly
    24. Make a scrolling background
    25. The fairy dust explosions
    26. Use a particle emitter
    27. Creating and moving the pillars
  9. A Guide to the examples

Features

Here's Ga's core feature list:

And the coolest part? If you were alone on a desert island with only a solar powered laptop, an unlimited supply of coconuts, and a copy of ga.js you could recreate the entire history of 2D video games, from SpaceWar! to Flappy Bird. And all of it would fit on a 3.5 inch floppy disk.

The plugins

But there's more! Ga comes with a plugins.js file that includes a huge number of useful tools for making games. You can use as many or as few of these tools as you want to. Here are some of the goodies you'll find in plugins.js:

To use the plugins, just copy/paste the code you want to use from plugins.js into your game. Or, if you're not worried about the extra size, just link the whole thing; it's really tiny anyway!

If you want to get fancy, you can alternatively create your own custom.js file that contains a small custom sub-set of the plugins you want to use for your game. Your custom.js file can load at compile time, so it's ready to use before your game code runs. (See the plugins.js file for instructions on how to do this).

Coming soon...

Ga's philosophy and technical constraints

Minifying, crushing and compressing

The Ga repository doesn't include the minified and compressed version of the source code, because you should probably optimize that yourself. I recommend first minifying the code using with Google Closure Compiler (Simple mode only) or UglifyJS2. Google Closure will give you best minification, and Ga's source code is optimized for it.

Then, zip it. I recommend gzip.

For more aggressive optimization, you could further try running the minified code through JSCrush. Although it sometimes makes things worse rather than better - you'll have to test it with your code.

Note: If you're using Google Closure Compiler from the command line, set the --language_in flag to ECMASCRIPT5, like this:

java -jar ~/compiler.jar --language_in=ECMASCRIPT5 --js ga.js --js_output_file ga.min.js

Contributions and Licencing

It's Ga's ambition to be the world's tiniest, cutest and funnest game engine. Please help! If you find something that's bad, please help to fix it. If you find something good, please help to make it better. Ga welcomes any and all contributions!

+1 Bonus Points for removing code and simplifying the architecture. +2 Bonus Points for making the code easier to understand. The aim of this project is to discover the smallest universal set of reusable components required to make the widest variety of games possible with the least amount of code. What is the fundamental alphabet, or the primary colours, of game design? That's what we're searching. Can you help?

Checkout the dev branch to make experimental changes and bug fixes, and we'll merge it with the master branch when we can confirm that everything is stable. Make sure that any code changes you make are compatible with Google Closure Compiler.

Please feel free to PR (Pull Request) any bug fixes and minor code improvements and optimizations. Any changes to the user-facing public API will need to be discussed in the Issues first. If we make any public API changes, we'll need to commit to updating all affected example and tutorial files as well. Also, any major changes to the engine will need to be discused too.

Coding style: Unconventionally, Ga uses functional composition patterns (the module pattern and mixins) for object creation instead of inheritance. Why? It's really just an experiment in coding like that. It also means the code becomes a little more compact.

Licensing? Ga is vehemently unlicenesed. That means its freer than free.

It's like a pebble. You can pick it up and throw into the sea.

Hexi

Do you like Ga, but wished that it had a powerful WebGL renderer and a gazillion other features that you will probably never use? Then checkout Ga's sister game engine: Hexi. It uses almost exactly the same API as Ga, but is built on top of the latest stable version of the powerful, full-featured Pixi renderer. What that means is that you can prototype your games for js13k, and port 99% of that code unchanged into Hexi to build your prodction version. If you don't care about small file sizes, and need a highly flexible, mobile-optimized and production-ready game engine using the world's most streamlined API, then check out Hexi!

Tutorials

How do you make a video game? These tutorials will show you how.

But first, you should have a reasonable understanding of HTML and JavaScript. You don't have to be an expert, just an ambitious beginner with an eagerness to learn. If you don't know HTML and JavaScript, the best place to start learning it is this book:

Foundation Game Design with HTML5 and JavaScript

I know for a fact that it's the best book, because I wrote it :)

There are also some good internet resources to help get you started:

Khan Academy: Computer Programming

Code Academy: JavaScript

Ok, got it? Do you know what JavaScript variables, functions, arrays and objects are and how to use them? Good, then read on!

Treasure Hunter

The first game we're going to make is a simple object collection and enemy avoidance game called Treasure Hunter. Open the file 01_treasureHunter.html in a web browser. (You'll find it in Ga's tutorials folder, and you'll need to run it in a webserver). If you don't want to bother setting up a webserver, use a text-editor like Brackets that will launch one for you automatically (see Brackets' documentation for this feature).

Treasure Hunter

(Follow the link in the image above to play the game.) Use the keyboard to move the explorer (the blue square), collect the treasure (the yellow square), avoid the monsters (the red squares) and reach the exit (the green square.) Yes, you have to use your imagination - for now.

Don't be fooled by it's apparent simplicity. Treasure Hunter contains everything a video game needs:

(What's juice? Watch this video and read this article to learn about this essential game design ingredient.)

If you can make a simple game like Treasure Hunter, you can make almost any other kind of game. Yes, really! Getting from Treasure Hunter to Skyrim or Zelda is just a matter of lots of small steps; adding more detail as you go. How much detail you want to add is up to you.

In the first stage of this tutorial you'll learn how the basic Treasure Hunter game was made, and then we'll add some fun features like images and character animation that will give you a complete overview of how the Ga game engine works.

If you're an experienced game programmer and quick self-starter, you might find the code in Ga's examples folder to be a more productive place to start learning - check it out. The fully commented code in the examples folder also details specific, and advanced uses of features, that aren't covered in these tutorials. When you're finished working through these tutorials, the examples will take you on the next stage of your journey.

Setting up the HTML container page

Before you can start programming in JavaScript, you need to set up a minimal HTML container page. The HTML page loads ga.js and plugins.js which are the two files you need to use all of Ga's features. You'll write all your game code inside the last pair of <script> tags before the closing <body> tag.

<!doctype html>
<meta charset="utf-8">
<title>Treasure hunter</title>
<body>
<!-- Import the Ga game engine files -->
<script src="https://github.com/kittykatattack/ga/raw/master/./ga.js"></script>
<script src="https://github.com/kittykatattack/ga/raw/master/./plugins.js"></script>
<script>

//All of your game code will go here

</script>
</body>

This is the minimum amount of HTML code you need for a valid HTML5 document.

Initializing the Ga engine

The next step is to write some JavaScript code that initializes and starts the Ga game engine, according to some parameters that you specify. This bit of code below initializes a game with a screen size of 512 by 512 pixels. It also pre-loads the chimes.wav sound file from the sounds folder.

var g = ga(
  512, 512, setup,
  [
    "sounds/chimes.wav"
  ]
);

//Start the Ga engine.
g.start();

You can see that the result of the ga function is being assigned to an variable called g. 

var g = ga(

Now, whenever you want to use any of Ga's custom methods or objects in your game, just prefix it with g. (You don't have to use g to represent the game engine, you can use any variable name you want. g is just nice, short, and easy to remember; g = "game".)

In this example Ga creates a canvas element with a size of 512 by 512 pixels. That's specified by the first two arguments:

512, 512, setup,

The third argument, setup, means that as soon as Ga is initialized, it should look for and run a function in your game code called setup. Whatever code is in the setup function is entirely up to you, and you'll soon see how you can used it to initialize a game. (You don't have to call this function setup, you can use any name you like.)

Ga lets you pre-load game assets with an optional 4th argument, which is an array of file names. In this first example, you only need to preload one file: chimes.wav You can see that the full file path to chimes.wav is listed as a string in the initialization array:

[
  "sounds/chimes.wav"
]

You can list as many game assets as you like here, including images, fonts, and JSON files. Ga will load all these assets for you before running any of the game code.

The last thing you need to do is call Ga's start method. 

g.start();

This is the switch that turns the Ga engine on.

Define your "global" variables

After Ga has been started, declare all the variables that your game functions will need to use.

var dungeon, player, treasure, enemies, chimes, exit,
    healthBar, message, gameScene, gameOverScene;

Because they're not enclosed inside a function, these variables are "global" in the sense that you can use them across all of your game functions. (They're not necessarily "global" in the sense that they inhabit the global JavaScript name-space. If you want to ensure that they aren't, wrap all of your JavaScript code in an enclosing immediate function to isolate it from the global space.

Initialize your game with a setup function

As soon as Ga starts, it will look for and run a function in your game code called setup (or whatever other name you want to give this function.) The setup function is only run once, and lets you perform one-time setup tasks for your game. It's a great place to create and initialize objects, create sprites, game scenes, populate data arrays or parse loaded JSON game data.

Here's an abridged, birds-eye view of the setup function in Treasure Hunter, and the tasks that it performs.

function setup() {
  //Set the canvas border and background color
  //Create the `chimes` sound object
  //Create the `gameScene` group
  //Create the `exit` door sprite
  //Create the `player` sprite
  //Create the `treasure` sprite
  //Make the enemies
  //Create the health bar
  //Add some text for the game over message
  //Create a `gameOverScene` group 
  //Assign the player's keyboard controllers

  //set the game state to `play`
  g.state = play;
}

The last line of code, g.state = play is perhaps the most important because it starts the play function. The play function runs all the game logic in a loop. But before we look at how that works, let's see what the specific code inside the setup function does.

Customizing the canvas

The first two lines in the setup function give the canvas a black dashed border and set its background color to white.

g.canvas.style.border = "1px black dashed";
g.backgroundColor = "white";

Here's the effect these two lines have on the Ga canvas:

Treasure Hunter

Ga uses an ordinary 2D canvas element to display the game graphics, and you can access it in your code at any time with g.canvas. You can modify it with any ordinary HTML/CSS properties.

Creating the chimes sound object

You'll remember from the code above that we preloaded a sound file into the game called chimes.wav. Before you can use it in your game, you have to make a reference to it using Ga's sound method, like this:

chimes = g.sound("sounds/chimes.wav");

Alternatively, you can access any assets that you've loaded via Ga's assets object, like this:

g.assets["sounds/chimes.wav"]

Any assets that you've preloaded like this are accessible in the assets object.

Many assets that you might want to use, like sounds, fonts, and JSON files, can only be loaded by the browser if your code is running inside a web server. If you're trying to load or use an asset, and the browser is giving your a strange security related error message, check to make sure that the web server is initialized.

Creating game scenes

Ga has a useful method called group that lets you group game objects together so that you can work with them as one unit. Groups are used for grouping together special objects called sprites (which you'll learn all about in the next section.) But they're also used for making game scenes.

Treasure Hunter uses two game scenes: gameScene which is the main game, and gameOverScene which is displayed when the game is finished. Here's how the gameScene is made using the group method:

gameScene = g.group();

After you've made the group, you can add sprites (game objects) to the gameScene, using the addChild method.

gameScene.addChild(anySprite);

Or, you can add multiple sprites at one time with the add method, like this:

gameScene.add(spriteOne, spriteTwo, spriteThree);

Or, if you prefer, you can create the game scene after you've made all the sprites, and group all the sprites together with one line of code, like this:

gameScene = g.group(spriteOne, spriteTwp, spriteThree);

You'll see a few different examples of how to add sprites to groups in the examples ahead.

But what are sprites, and how do you make them?

Making sprites

Sprites are the most important elements in any game. Sprites are just graphics (shapes or images) that you can control with special properties. Everything you can see in your games, like game characters, objects and backgrounds, are sprites. Ga lets you make 5 kinds of basic sprites: rectangle, circle, line, text, and sprite (an image-based sprite). You can make almost any 2D action game with these basic sprite types. (If they aren't enough, you can also define your own custom sprite types.) This first version of Treasure Hunter only uses rectangle sprites. You can make a rectangle sprite like this:

var box = g.rectangle(
  widthInPixels, 
  heightInPixels, 
  "fillColor", 
  "strokeColor", 
  lineWidth, 
  xPosition, 
  yPosition
);

You can use Ga's circle method to make a circular shaped sprite:

var ball = g.circle(
  diameterInPixels, 
  "fillColor", 
  "strokeColor", 
  lineWidth,
  xPosition, 
  yPosition 
);

It's often useful to prototype a new game using only rectangle and circle sprites, because that can help you focus on the mechanics of your game in a pure, elemental way. That's what this first version of Treasure Hunter does. Here's the code from the setup function that creates the exit, player and treasure sprites.

//The exit door
exit = g.rectangle(48, 48, "green");
exit.x = 8;
exit.y = 8;
gameScene.addChild(exit);

//The player sprite
player = g.rectangle(32, 32, "blue");
player.x = 68;
player.y = g.canvas.height / 2 - player.halfHeight;
gameScene.addChild(player);

//Create the treasure sprite
treasure = g.rectangle(16, 16, "gold");

//Position the treasure next to the right edge of the canvas
treasure.x = g.canvas.width - treasure.width - 32;
treasure.y = g.canvas.height / 2 - treasure.halfHeight;

//Create a `pickedUp` property on the treasure to help us figure
//out whether or not the treasure has been picked up by the player
treasure.pickedUp = false;

//Add the treasure to the gameScene
gameScene.addChild(treasure);

Notice that after each sprite is created, it's added to the gameScene using addChild. Here's what the above code produces:

Treasure Hunter

Let's find out a little more about how these sprites are positioned on the canvas.

Positioning sprites

All sprites have x and y properties that you can use to precisely position sprites on the canvas. The x and y values refer to the sprites' pixel coordinates relative to the canvas's top left corner. The top left corner has x and y values of 0. That means any positive x and y values you assign to sprites will position them left (x) and down (y) relative to that corner point. For example, Here's the code that positions the exit door (the green square). 

exit.x = 8;
exit.y = 8;

You can see that this code places the door 8 pixel to the right and 8 pixels below the canvas's top left corner. Positive x values position sprites to the right of the canvas's left edge. Positive y values position them below the canvas's top edge.

Sprites also have width and height properties that tell you their width and height in pixels. If you need to find out what half the width or half the height of a sprite is, use halfWidth and halfHeight.

Ga also has a some convenience methods that help you quickly position sprites relative to other sprites: putTop, putRight, putBottom, putLeft and putCenter. For example, here are the lines from the code above that position the treasure sprite (the gold box). The code places the treasure 26 pixels to the left of the canvas's right edge, and centers it vertically.

treasure.x = g.canvas.width - treasure.width - 32;
treasure.y = g.canvas.height / 2 - treasure.halfHeight;

That's a lot of complicated positioning code to write. Instead, you could use Ga's built-in putCenter method to achieve the same effect like this:

g.stage.putCenter(treasure, 220, 0);

What is the stage? It's the root container for all the sprites, and has exactly the same dimensions as the canvas. You can think of the stage as a big, invisible sprite, the same size as the canvas, that contains all the sprites in your game, as well as any containers those sprites might be grouped in (Like the gameScene). putCenter works by centering the treasure inside the stage, and then offsetting its x position by 220 pixels. Here's the format for using putCenter:

anySprite.putCenter(anyOtherSprite, xOffset, yOffset);

You can use the other put methods in the same way. For example, if you wanted to position a sprite directly to the left of another sprite, without any offset, you could use putLeft, like this:

spriteOne.putLeft(spriteTwo);

This would place spriteTwo directly to the left of spriteOne, and align it vertically .You'll see many examples of how to use these put methods throughout these tutorials.

Assigning dynamic properties

Before we continue, there's one small detail you need to notice. The code that creates the sprites also adds a pickedUp property to the treasure sprite:

treasure.pickedUp = false;

You'll see how we're going to use treasure.pickedUp later in the game logic to help us determine the progress of the game. You can dynamically assign any custom properties or methods to sprites like this, if you need to.

Creating the enemy sprites

There are 6 enemies sprites (red squares) in Treasure Hunter. They're spaced evenly horizontally but but have random initial vertical positions. All the enemies sprites are created in a for loop using this code in the setup function:

//Make the enemies
var numberOfEnemies = 6,
    spacing = 48,
    xOffset = 150,
    speed = 2,
    direction = 1;

enemies = [];

//Make as many enemies as there are `numberOfEnemies`
for (var i = 0; i < numberOfEnemies; i++) {

  //Each enemy is a red rectangle
  var enemy = g.rectangle(32, 32, "red");

  //Space each enemey horizontally according to the `spacing` value.
  //`xOffset` determines the point from the left of the screen
  //at which the first enemy should be added.
  var x = spacing * i + xOffset;

  //Give the enemy a random y position
  var y = g.randomInt(0, g.canvas.height - enemy.height);

  //Set the enemy's direction
  enemy.x = x;
  enemy.y = y;

  //Set the enemy's vertical velocity. `direction` will be either `1` or
  //`-1`. `1` means the enemy will move down and `-1` means the enemy will
  //move up. Multiplying `direction` by `speed` determines the enemy's
  //vertical direction
  enemy.vy = speed * direction;

  //Reverse the direction for the next enemy
  direction *= -1;

  //Push the enemy into the `enemies` array
  enemies.push(enemy);

  //Add the enemy to the `gameScene`
  gameScene.addChild(enemy);
}

Here's what this code produces:

Treasure Hunter

The code gives each of the enemies a random y position with the help of Ga's randomInt method:

var y = g.randomInt(0, g.canvas.height - enemy.height);

randomInt will give you a random number between any two integers that you provide in the arguments. (If you need a random decimal number, use randomFloat instead).

All sprites have properties called vx and vy. They determine the speed and direction that the sprite will move in the horizontal direction (vx) and vertical direction (vy). The enemies in Treasure Hunter only move up and down, so they just need a vy value. Their vy is speed (2) multiplied by direction (which will be either 1 or -1).

enemy.vy = speed * direction;

If direction is 1, the enemy's vy will be 2. That means the enemy will move down the screen at a rate of 2 pixels per frame. If direction is -1, the enemy's speed will be -2. That means the enemy will move up the screen at 2 pixels per frame.

After the enemy's vy is set, direction is reversed so that the next enemy will move in the opposite direction.

direction *= -1;

You can see that each enemy that's created is pushed into an array called enemies.

enemies.push(enemy);

Later in the code you'll see how we'll access all the enemies in this array to figure out if they're touching the player.

The health bar

You'll notice that when the player touches one of the enemies, the width of the health bar at the top right corner of the screen decreases.

Treasure Hunter

How was this health bar made? It's just two rectangle sprites at the same position: a black rectangle behind, and a green rectangle in front. They're grouped together to make a single compound sprite called healthBar. The healthBar is then added to the gameScene.

//Create the health bar
var outerBar = g.rectangle(128, 16, "black"),
    innerBar = g.rectangle(128, 16, "yellowGreen");

//Group the inner and outer bars
healthBar = g.group(outerBar, innerBar);

//Set the `innerBar` as a property of the `healthBar`
healthBar.inner = innerBar;

//Position the health bar
healthBar.x = g.canvas.width - 148;
healthBar.y = 16;

//Add the health bar to the `gameScene`
gameScene.addChild(healthBar);

You can see that a property called inner has been added to the healthBar. It just references the innerBar (the green rectangle) so that it will be convenient to access later.

healthBar.inner = innerBar;

You don't have to do this; but, hey why not! It means that if you want to control the width of the innerBar, you can write some smooth code that looks like this:

healthBar.inner.width = 30;

That's pretty neat and readable, so we'll keep it!

The game over scene

If the player's health drops to zero, or the player manages to carry the treasure to the exit, the game ends and the game over screen is displayed. The game over scene is just some text that displays "You won!" or "You lost!" depending on the outcome.

Treasure Hunter

How was this made? The text is made with a text sprite. 

var anyText = g.text(
  "Hello!", "CSS font properties", "fillColor", xPosition, yPosition
);

The first argument, "Hello!" in the above example, is the text content you want to display. Use the content property to change the text sprite's content later.

anyText.content = "Some new content";

Here's how the game over message text is created in the setup function. 

//Add some text for the game over message
message = g.text("Game Over!", "64px Futura", "black", 20, 20);
message.x = 120;
message.y = g.canvas.height / 2 - 64;

Next, a new group is created called gameOverScene. The message text is added to it. The gameOverScene's visible property is set to false so that it's not visible when the game first starts.

//Create a `gameOverScene` group and add the message sprite to it
gameOverScene = g.group(message);

//Make the `gameOverScene` invisible for now
gameOverScene.visible = false;

At the end of the game we'll set the gameOverScene's visible property to true to display the text message. We'll also set the gameScene's visible property to false so that all the game sprites are hidden.

Keyboard interactivity

You control the player (the blue square) with the keyboard arrow keys. Ga has a built-in key object with keyboard bindings to the arrow keys and space bar. Access them like this: key.leftArray, key.rightArrow, key.upArrow, key.downArrow, key.space. All these keys have press and release methods that you can define. Here's code in the setup function that customizes the press and release methods of
Ga's pre-defined arrow keys to control the player character: 

//Left arrow key `press` method
g.key.leftArrow.press = function() {
  //Change the player's velocity when the key is pressed
  player.vx = -5;
  player.vy = 0;
};

//Left arrow key `release` method
g.key.leftArrow.release = function() {
  //If the left arrow has been released, and the right arrow isn't down,
  //and the player isn't moving vertically:
  //Stop the player
  if (!g.key.rightArrow.isDown && player.vy === 0) {
    player.vx = 0;
  }
};

g.key.upArrow.press = function() {
  player.vy = -5;
  player.vx = 0;
};

g.key.upArrow.release = function() {
  if (!g.key.downArrow.isDown && player.vx === 0) {
    player.vy = 0;
  }
};

g.key.rightArrow.press = function() {
  player.vx = 5;
  player.vy = 0;
};

g.key.rightArrow.release = function() {
  if (!g.key.leftArrow.isDown && player.vy === 0) {
    player.vx = 0;
  }
};

g.key.downArrow.press = function() {
  player.vy = 5;
  player.vx = 0;
};

g.key.downArrow.release = function() {
  if (!g.key.upArrow.isDown && player.vx === 0) {
    player.vy = 0;
  }
};

You can see that the value of the player's vx and vy properties is changed depending on which keys are being pressed or released. A positive vx value will make the player move right, a negative value will make it move left. A positive vy value will make the player move down, a negative value will make it move up.

Is that too much typing? Because controlling a player character with 4 keyboard keys is such a common requirement, Ga has a built-in function called fourKeyController that accomplishes all this in one line of code.

g.fourKeyController(player, 5, 38, 39, 40, 37);

The first argument is the sprite you want to control: player. The second argument is the number of pixels that the sprite should move each frame: 5. The last four arguments are the ascii key code numbers for the top, right, bottom and left keys. (You can remember this because their order is listed clockwise, starting from the top.)

Reference to the arrow keys and space key are built-in to Ga, but you if want to use other keys, you can easily create and assign your own with Ga's keyboard method:

var customKey = g.keyboard(asciiCode);

Your new customKey has press and release methods that you can program in the same way as the examples above.

Setting the game state

The game state is the function that Ga is currently running. When Ga first starts, it runs the setup function (or whatever other function you specify in Ga's constructor function arguments.) If you want to change the game state, assign a new function to Ga's state property. Here's how:

g.state = anyFunction;

In Treasure Hunter, when the setup function is finished, the game state is set to play:

g.state = play;

This makes Ga look for and run a function called play. By default, any function assigned to the game state will run in a continuous loop, at 60 frames per second. (You can change the frame rate at any time by setting Ga's fps property). Game logic usually runs in a continuous loop, which is known as the game loop. Ga handles the loop management for you, so you don't need to worry about how it works. (In case you're curious, Ga uses a requestAnimationFrame loop with a fixed logic time step and variable rendering time. It also does sprite position interpolation to smoothe out any inconsistent spikes in the frame rate.)

If you ever need to pause the loop, just use Ga's pausemethod, like this:

g.pause();

You can start the game loop again with the resume method, like this:

g.resume();

Now let's find out how Treasure Hunter's play function works.

Game logic with the play function loop

As you've just learned, everything in the play function runs in a continuous loop.

function play() {
  //This code loops from top to bottom 60 times per second  
}

This is where all the game logic happens. It's the fun part, so let's find out what the code inside the play function does.

Moving the player sprite

Treasure Hunter uses Ga's move method inside the play function to move the sprites in the game.

g.move(player);

This is the equivalent of writing code like this:

player.x += player.vx;
player.y += player.vy;

It just updates the player's x and y position by adding its vx and vy velocity values. (Remember, those values were set by the key press and release methods.) Using move just saves you from having to type-in and look-at this very standard boilerplate code.

You can also move a whole array of sprites with one line of code by supplying the array as the argument.

g.move(arrayOfSprites);

So now you can easily move the player, but what happens when the player reaches the edges of the screen?

Containing sprites inside the screen boundaries

Use Ga's contain method to keep sprites inside the boundaries of the screen.

g.contain(player, g.stage.localBounds);

The first argument is the sprite you want to contain, and the second argument is any JavaScript object with an x, y, width, and height property. As a convenience, all Ga sprites have a property called localBounds that return an object with this information.

As you learnt earlier, stage is the root container object for all Ga's sprites, and it has the same width and height as the canvas. That means you can use its localBounds property to keep the sprite contained inside the canvas.

But you can alternatively supply a custom object to do the same thing. Here's how:

g.contain(
  player, 
  {
    x: 0,
    y: 0,
    width: 512,
    height: 512
  }
);

This will contain the player sprite to an area defined by the dimensions of the object. This is really convenient if you want to precisely fine-tune the area in which the object should be contained.

contain has an extra useful feature. If the sprite reaches one of the containment edges, contain will return a string that tells you which edge it reached: "top", "right", "bottom", or "left". Here's how you could use this feature to find out which edge of the canvas the sprite is touching:

var playerHitsEdges = g.contain(player, g.stage.localBounds);

//Display the edge of canvas that the player hit
if (playerHitsEdges) {
  message.content 
    = "The player hit the " + playerHitsEdges + " of the canvas";
}

Collision with the enemies

When the player hits any of the enemies, the width of the health bar decreases and the player becomes semi-transparent.

Treasure Hunter

How does this work?

Ga has a full suite of useful 2D geometric and tile-based collision detection methods. You can read all about them in Ga's examples folder. Treasure Hunter only uses one of these collision methods: hitTestRectangle. It takes two rectangular sprites and tells you whether they're overlapping. It will return true if they are, and false if they aren't.

g.hitTestRectangle(spriteOne, spriteTwo);

Here's how the code in the play function uses hitTestRectangle to check for a collision between any of the enemies and the player.

//Set `playerHit` to `false` before checking for a collision
var playerHit = false;

//Loop through all the sprites in the `enemies` array
enemies.forEach(function(enemy) {

  //Move the enemy
  g.move(enemy);

  //Check the enemy's screen boundaries
  var enemyHitsEdges = g.contain(enemy, g.stage.localBounds);

  //If the enemy hits the top or bottom of the stage, reverse
  //its direction
  if (enemyHitsEdges === "top" || enemyHitsEdges === "bottom") {
    enemy.vy *= -1;
  }

  //Test for a collision. If any of the enemies are touching
  //the player, set `playerHit` to `true`
  if(g.hitTestRectangle(player, enemy)) {
    playerHit = true;
  }
});

//If the player is hit...
if(playerHit) {

  //Make the player semi-transparent
  player.alpha = 0.5;

  //Reduce the width of the health bar's inner rectangle by 1 pixel
  healthBar.inner.width -= 1;

} else {

  //Make the player fully opaque (non-transparent) if it hasn't been hit
  player.alpha = 1;
}

This bit of code creates a variable called playerHit, which is initialized to false just before the forEach loop checks all the enemies for a collision.

var playerHit = false;

(Because the play function runs 60 times per second, playerHit will be reinitialized to false on every new frame.)

If hitTestRectangle returns true, the forEach loop sets playerHit to true.

if(g.hitTestRectangle(player, enemy)) {
  playerHit = true;
}

If the player has been hit, the code makes the player semi-transparent by setting its alpha value to 0.5. It also reduces the width of the healthBar's inner sprite by 1 pixel.

if(playerHit) {

  //Make the player semi-transparent
  player.alpha = 0.5;

  //Reduce the width of the health bar's inner rectangle by 1 pixel
  healthBar.inner.width -= 1;

} else {

  //Make the player fully opaque (non-transparent) if it hasn't been hit
  player.alpha = 1;
}

You can set the alpha property of sprites to any value between 0 (fully transparent) to 1 (fully opaque). A value of 0.5 makes it semi-transparent.b (Alpha is a well-worn graphic design term that just means transparency.)

This bit of code also uses the move method to move the enemies, and contain to keep them contained inside the canvas. The code also uses the return value of contain to find out if the enemy is hitting the top or bottom of the canvas. If it hits the top or bottom, the enemy's direction is reversed with the help of this code:

//Check the enemy's screen boundaries
var enemyHitsEdges = g.contain(enemy, g.stage.localBounds);

//If the enemy hits the top or bottom of the stage, reverse
//its direction
if (enemyHitsEdges === "top" || enemyHitsEdges === "bottom") {
  enemy.vy *= -1;
}

Multiplying the enemy's vy (vertical velocity) value by negative 1 makes it go in the opposite direction. It's a really simple bounce effect.

Collision with the treasure

If the player touches the treasure (the yellow square), the chimes sound plays. The player can then carry the treasure to the exit. The treasure is centered over the player and moves along with it.

Treasure Hunter

Here's the code from the play function that achieves these effects.

//Check for a collision between the player and the treasure
if (g.hitTestRectangle(player, treasure)) {

  //If the treasure is touching the player, center it over the player
  treasure.x = player.x + 8;
  treasure.y = player.y + 8;

  if(!treasure.pickedUp) {

    //If the treasure hasn't already been picked up,
    //play the `chimes` sound
    chimes.play();
    treasure.pickedUp = true;
  };
}

You can see that the code uses hitTestRectangle inside an if statement to test for a collision between the player and the treasure.

if (g.hitTestRectangle(player, treasure)) {

If it's true, the treasure is centered over the player.

treasure.x = player.x + 8;
treasure.y = player.y + 8;

If treasure.pickedUp is false, then you know that the treasure hasn't already been picked up, and you can play the chimes sound:

chimes.play();

In addition to play Ga's sound objects also have a few more methods that you can use to control them: pause, restart and playFrom. (Use playFrom to start playing the sound from a specific second in the sound file, like this: soundObject.playFrom(5). This will make the sound start playing from the 5 second mark.)

You can also set the sound object's volume by assigning a value between 0 and 1. Here's how to set the volume to mid-level (50%).

soundObject.volume = 0.5;

You can set the sound object's pan by assigning a value between -1 (left speaker) to 1 (right speaker). A pan value of 0 makes the sound equal volume in both speakers. Here's how you could set the pan to be slightly more prominent in the left speaker.

soundObject.pan = -0.2;

If you want to make a sound repeat continuously, set its loop property to true.

soundObject.loop = true;

Ga uses a lightweight wrapper for the WebAudio API to achieve all these effects.

Because you don't want to play the chimes sound more than once after the treasure has been picked up, the code sets treasure.pickedUp to true just after the sound plays.

treasure.pickedUp = true;

Now that the player has picked up the treasure, how can you check for the end of the game?

Ending the game

There are two ways the game can end. The player's health can run out, in which case the game is lost. Or, the player can successfully carry the treasure to the exit, in which case the game is won. If either of these two conditions are met, the game's state is set to end and the message text's content displays the outcome. Here's the last bit of code in the play function that does this:

//Does the player have enough health? If the width of the `innerBar`
//is less than zero, end the game and display "You lost!"
if (healthBar.inner.width < 0) {
  g.state = end;
  message.content = "You lost!";
}

//If the player has brought the treasure to the exit,
//end the game and display "You won!"
if (g.hitTestRectangle(treasure, exit)) {
  g.state = end;
  message.content = "You won!";
}

The end function is really simple. It just hides the gameScene and displays the gameOverScene.

function end() {
  gameScene.visible = false;
  gameOverScene.visible = true;
}

And that's it for Treasure Hunter! Before you continue, try making your own game from scratch using some of these same techniques. When you're ready, read on!

Using images

There are three main ways you can use images in your Ga games.

All three ways of making image sprites use Ga's sprite method. Here's the simplest way of using it to make an image sprite.

var imageSprite = g.sprite("images/theSpriteImage.png");

In this next section we'll update Treasure Hunter with image sprites, and you'll learn all three ways of adding images to your games.

(All the images in this section were created by Lanea Zimmerman. You can find more of her artwork here. Thanks, Lanea!)

Individual images

Open and play the next version of Treasure Hunter: 02_treasureHunterImages.html (you'll find it in the tutorials folder.) It plays exactly the same as the first version, but all the colored squares have been replaced by images.

Treasure Hunter

(Click the image and follow the link to play the game.) Take a look at the source code, and you'll notice that the game logic and structure is exactly the same as the first version of the game. The only thing that's changed is the appearance of the sprites. How was this done?

Loading image files

Each sprite in the game uses an individual PNG image file. You'll find all the images in the tutorials' images sub-folder.

Treasure Hunter

Before you can use them to make sprites, you need to pre-load them into Ga's assets. The easiest way to do this is to list the image names, with their full file paths, in Ga's assets array when you first initialize the engine.

var g = ga(
  512, 512, setup,
  [
    "images/explorer.png",
    "images/dungeon.png",
    "images/blob.png",
    "images/treasure.png",
    "images/door.png",
    "sounds/chimes.wav"
  ]
);
g.start();

(If you open up the JavaScript console in the web browser, you can monitor the loading progress of these assets.)

Now you can access any of these images in your game code like this:

g.image("images/blob.png")

This is just a short-cut for accessing the image directly in the assets object like this:

g.assets["images/blob.png"]

You can use whichever style you prefer. In any case, the image file is just an ordinary JavaScript Image object, so you can use it the same way you would any other Image object.

Although pre-loading the images and other assets is the simplest way to get them into your game, you can also load assets at any other time using the assets object's load method. Just supply an array of strings that list the asset names and their file paths.

g.assets.load([
  "images/imageOne.png", 
  "images/imageTwo.png",
  "sounds/chimes.wav"
]);

Next, assign a callback function called whenLoaded that will run when the assets have loaded. 

g.assets.whenLoaded = function() {
  //Do something when the assets have loaded
};

Now that you've loaded the images into the game, let's find out how to use them to make sprites.

Making sprites with images

Create an image sprite using the sprite method using the same format you learnt earlier. Here's how to create a sprite using the dungeon.png image. (dungeon.png is a 512 by 512 pixel background image.)

dungeon = g.sprite("images/dungeon.png");

That's all! Now instead of displaying as a simple colored rectangle, the sprite will be displayed as a 512 by 512 image. There's no need to specify the width or height, because Ga figures that our for you automatically based on the size of the image. You can use all the other sprite properties, like x, y, width, and height, just as you would with ordinary rectangle sprites.

Here's the code from the setup function that creates the dungeon background, exit door, player and treasure, and adds them all to the gameScene group. 

//The dungeon background
dungeon = g.sprite("images/dungeon.png");

//The exit door
exit = g.sprite("images/door.png");
exit.x = 32;

//The player sprite
player = g.sprite("images/explorer.png");
player.x = 68;
player.y = g.canvas.height / 2 - player.halfWidth;

//Create the treasure
treasure = g.sprite("images/treasure.png");

//Position it next to the left edge of the canvas
treasure.x = g.canvas.width - treasure.width - 32;
treasure.y = g.canvas.height / 2 - treasure.halfHeight;

//Create a `pickedUp` property on the treasure to help us Figure
//out whether or not the treasure has been picked up by the player
treasure.pickedUp = false;

//Create the `gameScene` group and add all the sprites
gameScene = g.group(dungeon, exit, player, treasure);

(As a slightly more efficient improvement to the original version of this code, group creates the gameScene and groups the sprites in a single step.)

Look familiar? That's right, the only code that has changed are the lines that create the sprites. This modularity is a feature of Ga that lets you create quick game prototypes using simple shapes that you can easily swap out for detailed images as your game ideas develops. The rest of the code in the game can remain as-is.

Fine-tuning the containment area

One small improvement that was made to this new version Treasure Hunter is the new way that the sprites are contained inside the walls of the dungeon. They're contained in such a way that naturally matches the 2.5D perspective of the artwork, as shown by the green square in this screen shot:

Treasure Hunter

This is a very easy modification to make. All you need to do is supply the contain method with a custom object that defines the size and position of the containing rectangle. Here's how:

g.contain(
  player,
  {
    x: 32, y: 16,
    width: g.canvas.width - 32,
    height: g.canvas.height - 32
  }
);

Just tweak the x, y, width and height values so that the containing area looks natural for the game you're making.

Using a texture atlas

If you’re working on a big, complex game, you’ll want a fast and efficient way to work with images. A texture atlas can help you do this. A texture atlas is actually two separate files that are closely related:

Using a texture atlas is a big time saver. You can arrange the tileset’s sub-images in any order and the JSON file will keep track of their sizes and positions for you. This is really convenient because it means the sizes and positions of the sub-images aren’t hard-coded into your game program. If you make changes to the tileset, like adding images, resizing them, or removing them, just re-publish the JSON file and your game will use that updated data to display the images correctly. If you’re going to be making anything bigger than a very small game, you’ll definitely want to use a texture atlas.

The de-facto standard for tileset JSON data is the format that is output by a popular software tool called Texture Packer (Texture Packer's "Essential" license is free ). Even if you don’t use Texture Packer, similar tools like Shoebox output JSON files in the same format. Let’s find out how to use it to make a texture atlas with Texture Packer, and how to load it into a game.

Preparing the images

You first need individual PNG images for each image in your game. You've already got them for Treasure Hunter, so you're all set. Open Texture Packer and choose the {JS} configuration option. Drag your game images into its workspace. You can also point Texture Packer to any folder that contains your images. Texture Packer will automatically arrange the images on a single tileset image, and give them names that match their original image file names. It will give them a 2 pixel padding by default.

Texture Packer

Each of the sub-images in the atlas is called a frame. Although it's just one big image, the texture atlas has 5 frames. The name of each frame is the same its original PNG file name: "dungeon.png", "blob.png", "explorer.png", "treasure.png" and "door.png". These frames names are used to help the atlas reference each sub-image.

When you’re done, make sure the Data Format is set to JSON (Hash) and click the Publish button. Choose the file name and location, and save the published files. You’ll end up with a PNG file and a JSON file. In this example my file names are treasureHunter.json and treasureHunter.png. To make your life easier, just keep both files in your project’s images folder. (Think of the JSON file as extra metadata for the image file.)

loading the texture atlas

To load the texture atlas into your game, just include the JSON file in Ga's assets array when you initialize the game.

var g = ga(
  512, 512, setup,
  [
    "images/treasureHunter.json",
    "sounds/chimes.wav"
  ]
);
g.start();

That's all! You don't have to load the PNG file - Ga does that automatically in the background. The JSON file is all you need to tell Ga which tileset frame (sub-image) to display.

Now if you want to use a frame from the texture atlas to make a sprite, you can do it like this:

anySprite = g.sprite("frameName.png");

Ga will create the sprite and display the correct image from the texture atlas's tileset.

Here's how to you could create the sprites in Treasure Hunter using texture atlas frames:

//The dungeon background image
dungeon = g.sprite("dungeon.png");

//The exit door
exit = g.sprite("door.png");
exit.x = 32;

//The player sprite
player = g.sprite("explorer.png");
player.x = 68;
player.y = g.canvas.height / 2 - player.halfWidth;

//The treasure
treasure = g.sprite("treasure.png");

That's all! Ga knows that those are texture atlas frame names, not individual images, and it displays them directly from the tileset.

If you ever need to access the texture atlas's JSON file in your game, you can get it like this:

jsonFile = g.json("jsonFileName.json");

Take a look at treasureHunterAtlas.html file in the tutorials folder to see a working example of how to load a texture atlas and use it to make sprites.

Alien Armada

The next example game in this series of tutorials is Alien Armada. Can you destroy 60 aliens before one of them lands and destroys the Earth? Click the image link below to play the game:

Alien Armada

Use the arrow keys to move and press the space bar to shoot. The aliens descend from the top of the screen with increasing frequency as the game progresses. Here's how the game is played:

Alien Armada gameplay

Alien Armada illustrates some new techniques that you'll definitely want to use in your games:

You'll find the fully commented Alien Armada source code in the tutorials folder. Make sure to take a look at it so that you can see all of this code in its proper context. Its general structure is identical to Treasure Hunter, with the addition of these new techniques. Let's find out how they were implemented.

Load and use a custom font

Alien Armada uses a custom font called emulogic.ttf to display the score at the top right corner of the screen. The font file is preloaded with the rest of the asset files (sounds and images) in the assets array that initializes the game. 

var g = ga(
  480, 320, setup,
  [
    "images/alienArmada.json",
    "sounds/explosion.mp3",
    "sounds/music.mp3",
    "sounds/shoot.mp3",
    "fonts/emulogic.ttf"  //<- The custom font.
  ],
  load
);

To use the font, create a text sprite in the game's setup function. The text method's second argument is a string that describes the font's point size and name: "20px emulogic". 

scoreDisplay = g.text("0", "20px emulogic", "#00FF00", 400, 10);

You can and load and use any fonts in TTF, OTF, TTC or WOFF format.

Scale and center the game in the browser

You'll notice that when you play Alien Armada, the game is centered inside the browser window, and automatically fills to the window's maximum width and height.

Alien Armada gameplay

The browser background that borders the game is set to a dark gray color. This is thanks to one of Ga's built-in features: the scaleToWindow method. To use it, call scaleToWindow just after you call Ga's start method, like this:

g.start();
g.scaleToWindow();

scaleToWindow will center your game for the best fit. Long, wide game screens are centered vertically. Tall or square screens are centered horizontally. If you want to specify your own browser background color that borders the game, supply it in scaleToWindow's arguments, like this:

g.scaleToWindow("seaGreen");

For best results, make sure you set the default margins and paddings on all your HTML elements to 0. The following bit of CSS does the trick: 

<style> * {margin: 0; padding: 0;} </style>

Here's how this <style> tag is inserted into Alien Armada's HTML container page:

<!doctype html>
<meta charset="utf-8">
<title>Alien Armada</title>
<style> * {margin: 0; padding: 0;} </style>

Optionally, if you want to make sure that your game dynamically re-sizes and re-centers itself if the user changes the browser window size, just drop in this bit of code: 

window.addEventListener("resize", function(event){ 
  g.scaleToWindow();
});

Add it just after you've called scaleToWindow the first time. Here's what all this code looks like in context:

//...Initialize Ga...

g.start();
g.scaleToWindow();
window.addEventListener("resize", function(event){ 
  g.scaleToWindow();
});

//...The rest of your game code...

If you want to find out how it works, or you want to customize it further, you'll find the scaleToWindow method in Ga's plugins.js file.

A loading progress bar

Alien Armada loads three MP3 sound files: a shooting sound, an explosion sound, and music. The music sound is about 2 MB in size so on a slow network connection this sound could take a few seconds to load. While this is happening players would just see the blank canvas while Alien Armada loads. Some players might think the game has frozen, so the game helpfully implements a loading bar to inform players that the assets are loading. It's a blue rectangle that expands from left to right, and displays a number that tells you the percentage of game assets loaded so far.

Loading progress bar

This is a feature that's built into the Ga engine. Ga has a optional loading state that runs while game assets are being loaded. You can decide what you want to have happen during the loading state. All you need to do is write a function with code that should run while the assets are loading, and tell Ga what the name of that function is. Ga's engine will automatically run that function in a loop until the assets have finished loading.

Let's find out how this works in Alien Armada. The game code tells Ga to use a function called load during the loading state. It does this by listing load as the final argument in Ga's initialization constructor. (Look for load in the code below):

var g = ga(
  480, 320, setup,
  [
    "images/alienArmada.json",
    "sounds/explosion.mp3",
    "sounds/music.mp3",
    "sounds/shoot.mp3",
    "fonts/emulogic.ttf"
  ],
  load  //<- This is the function that will run while loading.
);

This tells Ga to run the load function in a loop while the assets are loading.

Here's the load function from Alien Armada. It creates a progressBar object, and then calls the progress bar's update method each frame. 

function load(){

    //Use Ga's built in `progressBar` to display a loading progress
    //percentage bar while the assets are loading.
    g.progressBar.create(g.canvas, g.assets);

    //Call the `progressBar`'s `update` method each frame. 
    g.progressBar.update();
}

After the assets have loaded, the setup state runs automatically. The first thing it does is call the progressBar's remove method to make the bar disappear:

function setup() {

  g.progressBar.remove();

  //... the rest of the setup function...
}

You'll find the progressBar code in the plugins.js file. It's meant to be a very simple example that you can use as the basis for writing your own custom loading animation, if you want to. You can run any code you like in the load function, so it's entirely up to you to decide what should happen or what is displayed while your game is loading.

Shooting bullets

How can you make the cannon fire bullets?

When you press the space bar, the cannon fires bullets at the enemies. The bullets start from the end of the cannon's turret, and travel up the canvas at 7 pixels per frame. If they hit an alien, the alien explodes. If a bullet misses and flies past the top of the stage, the game code removes it.

Firing bullets

To implement a bullet-firing system in your game, the first thing you need is an array to store the all the bullet sprites.

bullets = [];

This bullets array is initialized in the game's setup function.

You can then use Ga's custom shoot method to make any sprite fire bullets in any direction. Here's the general format you can use to implement the shoot method.

g.shoot(
  cannon,      //The shooting sprite
  4.71,        //The angle, in radians, at which to shoot (4.71 is up)
  16,          //The bullet's offset from the center of the sprite
  7,           //The bullet's speed (pixels per frame)
  bullets,     //The array used to store the bullets

  //A function that returns the sprite that should
  //be used to make each bullet
  function() {
    return g.sprite("bullet.png");
  }
);

The second argument determines the angle, in radians, at which the bullet should travel. 4.71 radians, used in this example, is up. 0 is to the right, 1.57 is down, and 3.14 is to the left.

The last argument is a function that returns a sprite that should be used as the bullet. In this example the bullet is created using using the "bullet.png" frame from the game's loaded texture atlas.

function() {
  return g.sprite("bullet.png");
}

Replace this function with your own to create any kind of custom bullet you might need.

When will your bullets be fired? You can call the shoot method whenever you want to make bullets, at any point in your code. In Alien Armada, bullets are fired when the player presses the space key. So the game implements this by calling shoot inside the space key's press method. Here's how:

g.key.space.press = function() {

  g.shoot(
    cannon,      //The shooting sprite
    4.71,        //The angle at which to shoot (4.71 is up)
    16,          //The bullet's offset from the center
    7,           //The bullet's speed (pixels per frame)
    bullets,     //The array used to store the bullets

    //A function that returns the sprite that should
    //be used to make each bullet
    function() {
      return g.sprite("bullet.png");
    }
  );

  //Play the shoot sound.
  shootSound.play();
};

You can see that the press method also makes the shootSound play. (The code above is initialized in the game's setup function.)

There's one more thing you need to do: you have to make the bullets move. You can do this with some code inside the game's looping play function. Use Ga's move method and supply the bullets array as an argument:

g.move(bullets);

The move method automatically loops through all the sprites in the array and updates their x and y positions with the value of their vx and vy velocity values.

So now you know how the bullets are created and animated. But what happens when they hit one of the aliens?

Sprite states

When a bullet hits an alien, a yellow explosion image appears. This simple effect is created by giving each alien sprite two states: a normal state and a destroyed state. Aliens are created with their states set to normal. If they're hit, their states change to destroyed.

The sprite's states

How does this system work?

First, let's take a look at the Alien Armada tileset, shown here:

The Alien Armada tileset

You can see two image frames that define these two states: alien.png and explosion.png. Before you create the sprite, first create an array that lists these two frames: 

var alienFrames = [
  "alien.png", 
  "explosion.png"
];

Next use the alienFrames array to initialize the alien sprite.

alien = g.sprite(alienFrames);

If you prefer, you could combine these two steps into one, like this:

alien = g.sprite([
  "alien.png", 
  "explosion.png"
]);

This loads the sprite up with two frames. Frame 0 is the alien.png frame, and frame 1 is the explosion.png frame. Frame 0 is displayed by default by when the sprite is first created.

You can use the sprite's show method to display any other frame number on the sprite, like this:

alien.show(1);

The code above will set the alien to frame number one, which is the explosion.png frame.

To make your code a little more readable, its a good idea to define your sprite's states in a special states object. Give each state a name, with a value that corresponds to that state's frame number. Here's how you could define two states on the alien: normal and destroyed:

alien.states = {
  normal: 0,
  destroyed: 1
};

alien.states.normal now has the value 0, and alien.states.destroyed now has the value 1. That means you could display the alien's normal state like this:

alien.show(alien.states.normal);

And display the alien's destroyed state like this:

alien.show(alien.states.destroyed);

This makes your code a little more readable because you can tell at a glance which sprite state is being displayed.

(Note: Ga also has a lower-level gotoAndStop method that does exactly the same thing as show. Although you're free use gotoAndStop in your game code, by convention it's only used internally by Ga's rendering engine.)

Generating random aliens

Alien Armada generates aliens at any 1 of 14 randomly chosen positions just above the top boundary of the stage. The aliens first appear infrequently, but gradually start to appear at an ever-increasing rate. This makes the game gradually more difficult as it progresses. Let's find out how these two features are implemented.

Timing the aliens

When the game starts, the first new alien is generated after 100 frames have elapsed. A variable called alienFrequency, initialized in the game's setup function is used to help track this. it's initialized to 100.

alienFrequency = 100;

Another variable called alienTimer is used to count the number of of frames that have elapsed between the previously generated alien, and the next one. 

alienTimer = 0;

alienTimer is updated by 1 each frame in the play function (the game loop). When alienTimer reaches the value of alienFrequency, a new alien sprite is generated. Here's the code from the play function that does this. (This code omits the actual code that generates the alien sprite - we'll look at that ahead)

//Add one to the alienTimer.
alienTimer++;

//Make a new alien if `alienTimer` equals the `alienFrequency`.
if(alienTimer === alienFrequency) {

  //... Create the alien: see ahead for the missing code that does this...

  //Set the `alienTimer` back to zero.
  alienTimer = 0;

  //Reduce `alienFrequency` by one to gradually increase
  //the frequency that aliens are created
  if(alienFrequency > 2){  
    alienFrequency--;
  }
}

You can see in the code above that alienFrequency is reduced by 1 after the sprite has been created. That will make the next alien appear 1 frame earlier than the previous alien, and which is why the rate of falling aliens slowly increases. You can also see that the alienTimer is set back to 0 after the sprite has been created so that it can restart counting towards making the next new alien.

The aliens' random start positions

Before we generate any aliens, we need an array to store all the alien sprites. An empty array called aliens is initialized in the setup function for this purpose.

aliens = [];

Each alien is then created in the play function, inside the same if statement we looked at above. This code has a lot of work to do:

Here's the full code that does all this:

//Add one to the alienTimer.
alienTimer++;

//Make a new alien if `alienTimer` equals the `alienFrequency`.
if(alienTimer === alienFrequency) {

  //Create the alien.
  //Assign two frames from the texture atlas as the 
  //alien's two states.
  var alienFrames = [
    "alien.png", 
    "explosion.png"
  ];

  //Initialize the alien sprite with the frames
  var alien = g.sprite(alienFrames);

  //Define some states on the alien that correspond
  //to its two frames.
  alien.states = {
    normal: 0,
    destroyed: 1
  };

  //Set its y position above the screen boundary.
  alien.y = 0 - alien.height;

  //Assign the alien a random x position.
  alien.x = g.randomInt(0, 14) * alien.width;

  //Set its speed.
  alien.vy = 1;

  //Push the alien into the `aliens` array.
  aliens.push(alien);

  //Set the `alienTimer` back to zero.
  alienTimer = 0;

  //Reduce `alienFrequency` by one to gradually increase
  //the frequency that aliens are created
  if(alienFrequency > 2){  
    alienFrequency--;
  }
}

You can see in the code above that th alien's y position places it out of sight just above the stage's top boundary.

alien.y = 0 - alien.height;

It's x position, however, is random. 

alien.x = g.randomInt(0, 14) * alien.width;

This code places it in one of 15 possible random positions (0 to 14) above the top of the stage. Here's an illustration of these positions:

The Alien Armada tileset

Finally, and very importantly, the code pushes the alien sprite into the aliens array.

aliens.push(alien);

All this code starts pumping out aliens at a steadily increasing rate.

Moving the aliens

How do we make the aliens move? In exactly the same way made the bullets move. You'll notice in the code above that each alien is initialized with a vy (vertical velocity) value of 1.

alien.vy = 1;

When this value is applied to the alien's y position, it will make the alien move down, towards the bottom of the stage, at the rate of 1 pixel per frame. All the alien sprites in the game are in the aliens array. So to make all of them move you need to loop through each sprite in the aliens array each frame and add their vy values to their y positions. Some code like this in the play function would work:

aliens.forEach(function(alien){
  alien.y += alien.vy;
});

However, its easier just to use Ga's convenient built-in move function. Just supply move with the array of sprites that you want to move, like this:

g.move(aliens);

This updates the aliens positions with their velocities automatically.

Making the aliens explode

Now that you know how to change the alien's state, how can you use this skill to create the explosion effect? Here's the simplified code from Alien Armada that shows you how to do this. Use hitTestRectangle to check for a collision between an alien and bullet. If a collision is detected, remove the bullet, show the alien's destroyed state, and then remove the alien after a delay of one second.

if (g.hitTestRectangle(alien, bullet)) {

  //Remove the bullet sprite.
  g.remove(bullet);

  //Show the alien's `destroyed` state.
  alien.show(alien.states.destroyed);

  //Wait for 1 second (1000 milliseconds) then 
  //remove the alien sprite.
  g.wait(1000, function(){
    g.remove(alien);
  });
}

You can use Ga's universal remove function to remove any sprite from a a game, like this:

g.remove(anySprite);

You can optionally use it to remove more than one sprite at a time by listing the sprites to remove in the arguments, like this:

g.remove(spriteOne, spriteTwo, spriteThree);

You can even use it to remove all the sprites in an array of sprites. Just supply the sprite array as remove's only argument:

g.remove(arrayOfSprites);

This will both make the sprites disappear from the screen, and also empty them out of the array that they were in.

Ga also has a convenient method called wait that will run a function after any delay (in milliseconds) that you specify. The Alien Armada game code uses wait to remove the alien after a one second delay, like this:

g.wait(1000, function(){
  g.remove(alien);
});

This allows the alien to display its explosion image state for one second before it disappears from the game.

These are the basic techniques involved in making the aliens explode and removing the aliens and bullets from the game when they collide. But the actual code used in Alien Armada is a little more complex. That's because the code uses nested filter loops to loop through all the bullets and aliens so that they can be checked against each other for collisions. The code also plays an explosion sound when a collision occurs, and updates the score by 1. Here's all the code from the game's play function that does this. (If you're new to JavaScript's filter loops, you can read about how to use them here.)

//Check for a collision between the aliens and the bullets.
//Filter through each alien in the `aliens` array.
aliens = aliens.filter(function(alien) {

  //A variable to help check if the alien is
  //alive or dead.
  var alienIsAlive = true;

  //Filter though all the bullets.
  bullets = bullets.filter(function(bullet) {

    //Check for a collision between an alien and bullet.
    if (g.hitTestRectangle(alien, bullet)) {

      //Remove the bullet sprite.
      g.remove(bullet);

      //Show the alien's `destroyed` state.
      alien.show(alien.states.destroyed);

      //You could alternatively use the frame number,
      //like this:
      //alien.show(1);

      //Play the explosion sound.
      explosionSound.play();

      //Stop the alien from moving.
      alien.vy = 0;

      //Set `alienAlive` to false so that it can be
      //removed from the array.
      alienIsAlive = false;

      //Wait for 1 second (1000 milliseconds) then 
      //remove the alien sprite.
      g.wait(1000, function(){
        g.remove(alien);
      });

      //Update the score.
      score += 1;

      //Remove the bullet from the `bullets array.
      return false;

    } else {

      //If there's no collision, keep the bullet in the
      //bullets array.
      return true;
    }
  });

  //Return the value of `alienIsAlive` back to the 
  //filter loop. If it's `true`, the alien will be
  //kept in the `aliens` array. 
  //If it's `false` it will be removed from the `aliens` array.
  return alienIsAlive;
});

As long as the filter loops return true, the current sprite being checked will remain in the array. If there's a collision, however, the loops return false and the current alien and bullet will be removed from their arrays.

And that's how the game's collision works!

Displaying the score

Another new feature introduced by Alien Armada is a dynamic score display. Each time an alien is hit, the score at the top right corner of the game screen increases by one. How does this work?

Alien Armada initializes a text sprite called scoreDisplay in the game's setup function.

scoreDisplay = g.text("0", "20px emulogic", "#00FF00", 400, 10);

You saw in the previous section that 1 is added to the game's score variable each time an alien is hit:

score += 1;

To visibly update the score, all you need to do is set the score value as the scoreDisplay's content, like this:

scoreDisplay.content = score;

And that's all there is to it!

Ending and resetting the game

There are two ways the game can end. Either the player shoots down 60 aliens, in which case the player wins. Or, one of the aliens has to travel beyond the bottom edge of the stage, in which case the aliens win.

A simple if statement in the play function checks for this. If either condition becomes true, the winner is set to either "player" or "aliens" and the game's state is changed to end.

//The player wins if the score matches the value
//of `scoreNeededToWin`, which is 60
if (score === scoreNeededToWin) {

  //Set the player as the winner.
  winner = "player";

  //Change the game's state to `end`.
  g.state = end;
}

//The aliens win if one of them reaches the bottom of
//the stage.
aliens.forEach(function(alien){

  //Check to see if the `alien`'s `y` position is greater
  //than the `stage`'s `height`
  if (alien.y > g.stage.height) { 

    //Set the aliens as the winner.
    winner = "aliens";

    //Change the game's state to `end`.
    g.state = end;
  }
});

The end function pauses the game, so that the animation freezes. It then displays the gameOverMessage, which will either be "Earth Saved!" or "Earth Destroyed!", depending on the outcome. As an extra touch, the music volume is also set to 50%. Then after a delay of 3 seconds, a function named reset is called. Here's the complete end function that does all this:

function end() {

  //Pause the game loop.
  g.pause();

  //Create the game over message text.
  gameOverMessage = g.text("", "20px emulogic", "#00FF00", 90, 120);

  //Reduce the music volume by half.
  //1 is full volume, 0 is no volume, and 0.5 is half volume.
  music.volume = 0.5;

  //Display "Earth Saved!" if the player wins.
  if (winner === "player") {
    gameOverMessage.content = "Earth Saved!";
    gameOverMessage.x = 120;
  }

  //Display "Earth Destroyed!" if the aliens win.
  if (winner === "aliens") {
    gameOverMessage.content = "Earth Destroyed!";  
  }

  //Wait for 3 seconds then run the `reset` function.
  g.wait(3000, function(){
    reset(); 
  });
}

The reset function resets all of the game variables back to their starting values. It also turns the music volume back up to 1. It uses the remove function to remove any remaining sprites from the aliens and bullets arrays, so that those arrays can be re-populated when the game starts again. remove is also used to remove the gameOverMessage, and the cannon sprite is re-centered at the bottom of the stage. Finally, the game state is set back to play, and the game loop is un-paused by calling Ga's resume method.

function reset() {

  //Reset the game variables.
  score = 0;
  alienFrequency = 100;
  alienTimer = 0;
  winner = "";

  //Set the music back to full volume.
  music.volume = 1;

  //Remove any remaining alien and bullet sprites.
  //The universal `remove` method will loop through
  //all the sprites in an array of sprites, removed them
  //from their parent container, and splice them out of the array.
  g.remove(aliens);
  g.remove(bullets);

  //You can also use the universal `remove` function to remove.
  //a single sprite.
  g.remove(gameOverMessage);

  //Re-center the cannon.
  g.stage.putBottom(cannon, 0, -40);

  //Change the game state back to `play`.
  g.state = play;
  g.resume();
}

And this is all the code needed to start the game again. You can play Alien Armada as many times as you like and it will reset and restart itself like this endlessly.

Flappy Fairy!

Flappy Fairy is a homage to one of the infamous games ever made: Flappy Bird. Click the image link below to play the game:

FlappyFairy

Click the "Go" button, and game will launch in fullscreen mode. Tap anywhere on the screen to make the fairy fly, and help her navigate through the gaps in 15 pillars to reach the finish. A trail of multicolored fairy dust follows the fairy as she flies through the maze. If she hits one of the green blocks she explodes in a shower of dust. But if she manages to navigate through the increasingly narrowing gaps between all 15 pillars, she reaches a big floating “Finish” sign.

Flappy Fairy gameplay

If you can make a game like Flappy Fairy, you can make almost any other kind of 2D action game. In addition to using the all techniques you've already learnt, Flappy Fairy introduces some exciting new ones:

You'll find the fully commented Flappy Fairy source code in the tutorials folder. Make sure to take a look at it so that you can see all of this code in its proper context. Its general structure is identical to the other games in this tutorial, with the addition of these new techniques. Let's find out how they were implemented.

Launch a game in fullscreen mode

When you start Flappy Fairy by clicking the "Go" button, the game expands to fill your entire screen. This is done with the help of a built-in method called enableFullscreen.

g.enableFullsreen(listOfAsciiExitKeyCodes);

It's one optional argument is a list of Ascii key codes. They refer to keyboard keys that could be used to exit fullscreen mode. For example, if you want fullscreen mode to exit if a user presses upper-case "X" or lower-case "x", list their Ascii code values in the arguments like this:

g.enableFullscreen(88, 120);

(88 is "X" and 120 is "x".) You can list as many key codes as you like. If you leave these arguments out, the default esc key will do the trick.

enableFullscreen's behaviour is very simple: it just launches fullscreen mode whenever the user releases the pointer (mouse or touch) over the canvas. Add it just below your game's start method, like this:

var g = ga(
  910, 512, setupTitleScreen,
  [
    "images/flappyFairy/flappyFairy.json"
  ]
);

g.start();
g.enableFullscreen(88, 120); //<- Add it here

It's a quick and easy way to make any games run fullscreen.

(Note: Fullscreen mode is different than scaleToWindow because it completely takes over the user's screen. And I mean completely: the browser disappears and the only thing on the screen is your game. That's cool, but many users will find it disorienting and become stressed or panicked if they can't figure out how to exit your game. So you do decide to run your game in fullscreen mode, be confident that users will know how to exit it. Or, play it safe and just use scaleToWindow, which still looks great but doesn't take over the entire browser UI.)

Make a button

The game starts when you press the "Go" button. The "Go" button is a special sprite called a button. button sprites have 3 image frame states: up, over and down. You can create a button with three states like this:

goButton = g.button([
  "up.png",
  "over.png",
  "down.png"
]);

up.png is an image that shows what the button should look like when the it's not interacting with the pointer. over.png shows what the button looks like when the pointer is over it, and down.png is the image that is displayed when the pointer presses down on the button.

Button states

(The down.png image is offset slightly down and to the right, so it looks like its being pressed down.) You can assign any images you like to these states, and the button will display them automatically based on how the pointer is interacting with it.

(Note: If your game is touch-only, you might have only two button states: up and down. In that case, just assign two image frames, and Ga will assume they refer to the up and down states.)

Buttons have special methods that you can define: press, release,over, out and tap. You can assign any code you like to these methods. For example, here's how you could change the game's state when the user releases the playButton:

goButton.release = function(){
  g.state = setupGame;
};

Buttons also have a Boolean (true/false) property called enabled that you can set to false if you want to disable the button. (Set enabled to true to re-enable it.) You can also use the button's state property to find out if the button state is currently "up", "over" or "down". (These state values are strings.)

Important! You can give any sprite the qualities of button just by setting its interactive property to true, like this:

anySprite.interactive = true;

This will give the sprite press, release, over, out and tap methods, and the same state property as ordinary buttons. This means that you can make any sprite click-able, which is really useful for a wide variety of interactive games.

You can also make the stage object interactive, which turns the whole game screen into an interactive button:

g.stage.interactive = true;

For more detail on how to use buttons, see the buttons.html file in the examples folder.

Animating sprites

A neat feature of Flappy Fairy is that the fairy character flaps her wings when she's flying up. This animation was created by rapidly displaying 3 simple images in a continuous loop. Each image displays a slightly different frame of the animation, as shown below:

Animation frames

These three images are just three ordinary frames in the game's texture atlas, called 0.png, 1.png and 2.png. But how can you turn a sequence of frames like this into a sprite animation?

First, create an array that defines the frames of the animation, like this:

var fairyFrames = [
  "0.png", 
  "1.png", 
  "2.png"
];

Then create a sprite using those frames, like this:

var fairy = g.sprite(fairyFrames);

Or, if you prefer, you can combine this into one step:

var fairy = g.sprite([
  "0.png", 
  "1.png", 
  "2.png"
]);

Any sprite with more than one image frame automatically becomes an animated sprite. If you want the animation frames to start playing, just call the sprite's play method:

fairy.play();

The frames will automatically play in a continuous loop. If you don't want them to loop, set loop to false.

fairy.loop = false;

Use the stop method to stop an animation:

fairy.stop();

If you want to know whether or not a sprite's animation is currently playing, use the Boolean (true/false) playing property to find out.

How quickly or slowly do you want the animation to play? You can set the animation's frames-per-second (fps) like this:

fairy.fps = 24;

A sprite animation's frame rate is independent of the game's frame rate. That gives you a lot of flexibility to fine-tune sprite animations.

What if you don't want to use all the sprite's image frames in the animation, only some of them? You can use the playSequence method. For example, imagine that you have a sprite with 30 frames, but you only want to play frames 10 to 15 as part of the animation. Use the playSequence method and supply it with an array containing two numbers: the first and last frames of the sequence you want to play.

animatedSprite.playSequence([10, 15]);

Now only the frames between 10 to 15 will play as part of the animation. To make this more readable, you can define the sequence as an array that describes what those animated frames actually do. For example, perhaps they define a character's walk cycle. You could create an array called walkCycle that defines those frames:

var walkCycle = [10, 15];

Then use that array with playSequence, like this:

animatedSprite.playSequence(walkCycle);

That's a bit more code to write, but much more readable!

For more details on Ga's sprite animation system and what you can do with it, see the keyframeAnimation.html, textureAtlasAnimation.html and animationStates.html file in the examples folder.

Making the fairy fly

Now that you know how to animate a sprite, how is Flappy Fairy's flying animation triggered when you tap on the game screen?

A value of 0.05, which represents gravity, is subtracted from the fairy's y position each frame in the play function. This is the gravity effect that pulls the fairy to the bottom of the screen. 

fairy.vy += -0.05;
fairy.y -= fairy.vy;

But when you tap the screen, the fairy flies up. This is thanks to Ga's built-in pointer object. It has a tap method which you can define to perform any action you like. In Flappy Fairy, the tap method increases the fairy's vertical velocity, vy, by 1.5 pixels each time you tap.

g.pointer.tap = function() {
  fairy.vy += 1.5;
};

Ga's built-in pointer object also has press and release methods that you can define in the same way. It also has Boolean (true/false) isUp, isDown and tapped properties that you can use to find the pointer's state, if you need to.

But you'll notice that the fairy only flaps her wings when she's starting to fly up, and stops flapping when she looses momentum and starts going down. To make this work, you need to know whether the fairy is currently on the way up, or on the way down, based on a change in the fairy's vertical velocity (vy) value. The game implements a well-worn old trick to help figure this out. The play function captures the fairy's velocity for this current frame in a new value called oldVy. But it does this only after the fairy's position has changed. 

function play(){

  //...
  //... all of the code that moves the fairy comes first...
  //...

  //Then, after the fairy's position has been changed, capture
  //her velocity for this current frame
  fairy.oldVy = fairy.vy;
}

This means that when the next game frame swings around, oldVy will still be storing the fairy's velocity value from the previous frame. And that means you can use that value to figure out the change in the fairy's velocity from the previous frame to the current frame. If she's starting to go up (if vy is greater than oldVy), play the fairy's animation: 

if (fairy.vy > fairy.oldVy) {
  if(!fairy.playing) {
    fairy.play();
  }
}

If she's starting to go down, stop the animation and just show the fairy's first frame.

if (fairy.vy < 0 && fairy.oldVy > 0) {
  if (fairy.playing) fairy.stop();
  fairy.show(0);
}

And that's how the fairy flies!

Make a scrolling background

A fun new feature of Flappy Fairy is that it has an infinitely scrolling background of clouds moving from right to left.

Scrolling background

The background moves at a slower rate than the green pillars, and that creates the illusion that the clouds are further away. (This is a shallow, pseudo 3D effect called paralax scrolling.)

The background is just a single image.

Scrolling background

The image has been designed so that the clouds tile seamlessly: the clouds on the top and left match up with the clouds on the right and bottom. That means you can connect multiple instances of the same image and they will appear to create a single, unbroken continuous image. (Image from OpenGameArt.)

Because this is really useful for games, Ga has a sprite type called a tilingSprite that's designed just for such infinite scrolling effects. Here's how to create a tilingSprite: 

sky = g.tilingSprite(
  g.canvas.width,        //The width
  g.canvas.height,       //The height
  "sky.png"              //The image to use
);

The first two arguments are the sprite's width and height, and the last is the image your want to use.

Tiling sprites have the same properties as normal sprites, with the addition of two new properties: tileX and tileY. Those two properties let you set the image offset from the sprite's top left corner. If you want to make a tiling sprite scroll continuously, just increase its tileX value by some small amount each frame in the game loop, like this:

sky.tileX -= 1;

And that's all you need to do to make an infinitely scrolling background.

Particle effects

How do you create effects like fire, smoke, magic, and explosions? You make lots of tiny sprites; dozens, hundreds or thousands of them. Then apply some physical or gravitational constraints to those sprites so that they behave like the element you’re trying to simulate. You also need to give them some rules about how they should appear and disappear, and what kinds of patterns they should form. These tiny sprites are called particles. You can use them to make a wide range of special effects for games.

Ga has a versatile built-in method called particleEffect that can create most kinds of particle effects you'll need for games. Here's the format for using it:

particleEffect(
  pointer.x,                                     //The particle’s starting x position
  pointer.y,                                     //The particle’s starting y position
  function(){return sprite("images/star.png")},  //Particle function
  20,                                            //Number of particles
  0.1,                                           //Gravity
  true,                                          //Random spacing
  0, 6.28,                                       //Min/max angle
  12, 24,                                        //Min/max size
  1, 2,                                          //Min/max speed
  0.005, 0.01,                                   //Min/max scale speed
  0.005, 0.01,                                   //Min/max alpha speed
  0.05, 0.1                                      //Min/max rotation speed
);

You can see that most of the arguments describe a range between the minimum and maximum values that should be used to change the sprites’ speed, rotation, scale, or alpha. You can also assign the number of particles that should be created, and add optional gravity. You can make particles using any sprites by customizing the third argument. Just supply a function that returns the kind of sprite you want to use for each particle:

function(){return sprite("images/star.png")},

If you supply a sprite that has multiple frames, the particleEffect function will automatically choose a random frame for each particle. The minimum and maximum angle values are important for defining the circular spread of particles as they radiate out from the origin point. For a completely circular explosion effect, use a minimum angle of 0 and a maximum angle of 6.28.

0, 6.28,

(These values are radians; the equivalent in degrees is 0 and 360.) 0 starts at the 3 o’clock position, pointing directly to the right. 3.14 is the 9 o’clock position, and 6.28 takes you around back to 0 again. If you want to constrain the particle range to a narrower angle, just supply the minimum and maximum values that describe that range. Here are values you could use to constrain the angle to a pizza-slice with the crust pointing left.

2.4, 3.6,

You could use a constrained angle range like this to create a particle stream, like those used to create a fountain or rocket engine flames. (You’ll see exactly how to do this ahead.) The random spacing value (the sixth argument) determines whether the particles should be spaced evenly (false) or randomly (true) within this range. By carefully choosing the sprite for the particle and finely adjusting each parameter, you can use this all-purpose particleEffect method to simulate everything from liquid to fire. In Flappy Fairy, it's used to create fairy dust.

The fairy dust explosions

When Flappy Fairy hits a block, she disappears in a puff of dust.

Fairy dust explosion

How does that effect work?

Before we can create the explosion effect, we have to define an array that lists the images we want to use for each particle. As you learned above, the particleEffect method will randomly display a frame on a sprite, if that sprite contains multiple frames. To make this work, first define an array of texture atlas frames that you want to use for the fairy's dust explosion:

dustFrames = [
  "pink.png",
  "yellow.png",
  "green.png",
  "violet.png"
];

The explosion happens when the fairy hits one of the green blocks. The game loop does this with the help of the hitTestRectangle method. The code loops through the blocks.children array and tests for a collision between each green block and the fairy. If hitTestRectangle returns true, the loop quits and a collision object called fairyVsBlock becomes true.

var fairyVsBlock = blocks.children.some(function(block){
  return g.hitTestRectangle(fairy, block, true);  
});

hitTestRectangle’s third argument needs to be true so that the collision detection is done using the sprite’s global coordinates (gx and gy). That’s because the fairy is a child of the stage, but each block is a child of the blocks group. That means they don’t share the same local coordinate space. Using the blocks sprites' global coordinates forces hitTestRectangle to use their positions relative to the canvas.

If fairyVsBlock is true, and the fairy is currently visible, the collision code runs. It makes the fairy invisible, creates the particle explosion, and calls the game’s reset function after a delay of 3 seconds.

if (fairyVsBlock && fairy.visible) {

  //Make the fairy invisible
  fairy.visible = false;

  //Create a fairy dust explosion
  g.particleEffect(
    fairy.centerX, fairy.centerY, //x and y position
    function() {                  //Particle sprite
      return g.sprite(dustFrames);
    },     
    20,                           //Number of particles
    0,                            //Gravity
    false,                        //Random spacing
    0, 6.28,                      //Min/max angle
    16, 32,                       //Min/max size
    1, 3                          //Min/max speed
  );

  //Stop the dust emitter that's trailing the fairy.
  //(More about this ahead!)
  dust.stop();

  //Wait 3 seconds and then reset the game
  g.wait(3000, reset);
}

Use a particle emitter

A particle emitter is just a simple timer that creates particles at fixed intervals. That means instead of just calling the particleEffect function once, the emitter calls it periodically. Ga has a built-in emitter method that let's you do this easily. Here’s how to use it:

var particleStream = g.emitter(
  100,                                     //The interval
  function() {return g.particleEffect(     //The `particleEffect` function
    //Assign particle parameters...
  )
);

The emitter method just wraps around the particleEffect method. Its first argument is a number, in milliseconds, that determines how frequently the particles should be created. The second argument is the particleEffect method, which you can customize however you like. The emitter method returns an object with play and stop methods that you can use to control the particle stream. You can use them just like the play and stop methods you use to control a sprite’s animation.

particleStream.play();
particleStream.stop();

The emitter object also has a playing property that will be either true or false depending on the emitter’s current state. (See the particleEmitter.html file in the examples folder for more details on how to create and use a particle emitter.)

A particle emitter is used in Flappy Fairy to make the fairy emit a stream of multicolored particles while she’s flapping her wings. The particles are constrained to an angle between 2.4 and 3.6 radians, so they’re emitted in a cone-shaped wedge to the left of the fairy.

Emitting fairy dust

The particle stream randomly emits pink, yellow, green, or violet particles, each of which is a separate frame on the texture atlas.

Here's the code that creates this effect: 

dust = g.emitter(
  300,                                   //The interval
  function() {
    return g.particleEffect(             //The function
      fairy.x + 8,                       //x position
      fairy.y + fairy.halfHeight + 8,    //y position
      function() {                       //Particle sprite
        return g.sprite(dustFrames)
      },          
      3,                                 //Number of particles
      0,                                 //Gravity
      true,                              //Random spacing
      2.4, 3.6,                          //Min/max angle
      12, 18,                            //Min/max size
      1, 2,                              //Min/max speed
      0.005, 0.01,                       //Min/max scale speed
      0.005, 0.01,                       //Min/max alpha speed
      0.05, 0.1                          //Min/max rotation speed
    );
  }
);

You can now control the dust emitter with play and stop methods.

Creating and moving the pillars

You now know how Flappy Fairy implements some of Ga's special features for some fun and useful effects. But, if you're new to game programming, you might also be wondering how the world that Flappy Fairy flies through was created. Let's take a quick look at the code that creates and moves the green pillars that the fairy has to navigate to reach the Finish sign.

There are fifteen green pillars in the game. Every five pillars, the gap between the top and bottom sections becomes narrower. The first five pillars have a gap of four blocks, the next five have a gap of three blocks and the last five have a gap of two blocks. This makes the game increasingly difficult as Flappy Fairy flies further. The exact position of the gap is random for each pillar, and different every time game is played. Each pillar is spaced by 384 pixels, and here's how they would look like if they were right next to each other.

The green pillars

You can see how the gap gradually narrows from four spaces on the left down to two on the right.

All the blocks that make up the pillars are in a group called blocks.

blocks = g.group();

A nested for loop creates each block and adds it to the blocks container. The outer loop runs 15 times; once to create each pillar. The inner loop runs eight times; once for each block in the pillar. The blocks are only added if they’re not occupying the range that’s been randomly chosen for the gap. Every fifth time the outer loop runs, the size of the gap narrows by one.

//What should the initial size of the gap be between the pillars?
var gapSize = 4;

//How many pillars?
var numberOfPillars = 15;

//Loop 15 times to make 15 pillars
for (var i = 0; i < numberOfPillars; i++) {

  //Randomly place the gap somewhere inside the pillar
  var startGapNumber = g.randomInt(0, 8 - gapSize); 

  //Reduce the `gapSize` by one after every fifth pillar. This is
  //what makes gaps gradually become narrower
  if (i > 0 && i % 5 === 0) gapSize -= 1; 

  //Create a block if it's not within the range of numbers
  //occupied by the gap
  for (var j = 0; j < 8; j++) {
    if (j < startGapNumber || j > startGapNumber + gapSize - 1) {
      var block = g.sprite("greenBlock.png");
      blocks.addChild(block);

      //Space each pillar 384 pixels apart. The first pillar will be
      //placed at an x position of 512
      block.x = (i * 384) + 512;
      block.y = j * 64;
    }
  }

  //After the pillars have been created, add the finish image
  //right at the end
  if (i === numberOfPillars - 1) {
    finish = g.sprite("finish.png");
    blocks.addChild(finish);
    finish.x = (i * 384) + 896;
    finish.y = 192;
  }
}

The last part of the code adds the big finish sprite to the world, which Flappy Fairy will see if she manages to make it through to the end.

The game loop moves the group of blocks by 2 pixels to the right each frame, but only while the finish sprite is off-screen:

if (finish.gx > 256) {
  blocks.x -= 2;
}

When the finish sprite scrolls into the center of the canvas, the blocks container will stop moving. Notice that the code uses the finish sprite’s global x position (gx) to test whether it’s inside the area of the canvas. Because global coordinates are relative to the canvas, not the parent container, they’re really useful for just these kinds of situations where you want to want to find a nested sprite’s position on the canvas.

Make sure you check out the complete Flappy Fairy source code in the examples folder so that you can see all this code in its proper context.

Coming soon: A guide to the examples