Emanuele Feronato

Talking about Box2D, HTML5, Javascript and Phaser.

A couple of months ago – three to tell the truth – Photon Storm guys released a Box2D plugin for Phaser.

It’s time to publish some tutorials about it, and we start with the most basic tutorial, that is a “click to create, click to remove” script.

This is what I mean:

Click anywhere, you will create a crate if you click on an empty spot, or destroy a crate if you click on it.

The code is very easy to understand, as unlike the original Box2D and its AS3 porting, where it’s up to you to add sprites and synchronize them with physics world bodies, Phaser works in the opposite way: you add the sprites and assign a body to them.

Way more intuitive, and you won’t need the debug draw tool that much.

Here is the code:

var game;

window.onload = function() {	
	game = new Phaser.Game(640, 480, Phaser.AUTO, "");
     game.state.add("PlayGame",playGame);
     game.state.start("PlayGame");
}
	
var playGame = function(game){};

playGame.prototype = {
	preload: function(){
		game.load.image("crate", "crate.png");
		game.load.image("ground", "ground.png"); 
	},
  	create: function(){
  		game.stage.backgroundColor = "#222222";
  		// in the same way we initialized other Phaser physics engines, this is how we initialize Box2D engine
		game.physics.startSystem(Phaser.Physics.BOX2D);
		// setting gravity
		game.physics.box2d.gravity.y = 500;
		// we add the sprite which will represent the ground.
		// notice are placing it as if the registration point were in the center
    		var groundSprite = game.add.sprite(320, 460, "ground");
    		// enabling the physics on the body
    		game.physics.box2d.enable(groundSprite);
    		// making the body/sprite static
    		groundSprite.body.static = true;
    		// waiting for player input
    		game.input.onDown.add(addCrate, this);
	}
}

function addCrate(e){
	// this is how we get an array of bodies at a certain cordinate, in this case player input coordinate
	var currentBody = game.physics.box2d.getBodiesAtPoint(e.x, e.y);
	// if there is at least one body (but it can't be more than one in this example)...
	if(currentBody.length>0){	
		// if the body is not static (it's not the ground)
		if(!currentBody[0].static){
			// destroy the sprite and the body
			currentBody[0].sprite.destroy();
		}
	}
	else{
		// otherwise create a crate in the same way we created the ground
		var crateSprite = game.add.sprite(e.x, e.y, "crate");
    		game.physics.box2d.enable(crateSprite);
    	}
}

This plugin is really awesome, so expect more tutorials and some games during next weeks. Mwanwhile, you can download the source code.