Playing with the mouse joint in box2d in javascript

Mouse Joint

Out of the many joints that box2d has, one is mouse joint. It is not used in physics simulations, but helps to make the physics world interactive by making a body move towards a specific point, like the mouse coordinates for example. So objects can be picked up, dragged etc by the user.

Lets take an example

Pickup an object by clicking it and drag it. It will follow the mouse. The above is inside an iframe. Right click in it and click Reload Frame to refresh it.

Creating mouse joint

The mouse joint is created between 2 objects like this

//if joint exists then create
var def = new b2MouseJointDef();

def.bodyA = ground;
def.bodyB = body;
def.target = p;

def.collideConnected = true;
def.maxForce = 1000 * body.GetMass();
def.dampingRatio = 0;

mouse_joint = world.CreateJoint(def);

body.SetAwake(true);

The mouse joint needs 2 bodies. The first one is not used but must be there. In this example we use the ground as the first body. The second body is the body that moves towards the mouse point. The target point is the point towards which the body shall move.

The maxForce specifies the amount of force that is applied on the body to pull it towards the target point. Try playing with the values of these parameters and see the effect.

Mouse Events

The mouse joint is created on mouse events. As soon as the mouse is pressed over a body and moved, a joint is created if it does not already exists, and if the joint already exists, then the target of the joint is updated to the mouse coordinates on the box2d world.

On mouseup event the existing joint is removed.

Source

Html

The html code is minimal. It creates a canvas element. It also needs to include Jquery and Box2d library.

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
	<title>Box2d Mouse Joint Demo - Javascript</title>

	<script src="jquery-1.6.4.min.js"></script>
	<script src="Box2dWeb-2.1.a.3.js"></script>
	<script src="code.js"></script>
	<style>
	*
	{
		margin:0;
		padding:0;
	}
	</style>
</head>

<body>
	<canvas height="480" id="canvas" style="background-color:#333333;" width="640"></canvas>
</body>

</html>






The javascript code is where everything is being done.

/**
	Mouse joint demo of box2d in javascript
	Silver Moon ([email protected])
*/
var b2Vec2 = Box2D.Common.Math.b2Vec2
	, b2AABB = Box2D.Collision.b2AABB
	, b2BodyDef = Box2D.Dynamics.b2BodyDef
	, b2Body = Box2D.Dynamics.b2Body
	, b2FixtureDef = Box2D.Dynamics.b2FixtureDef
	, b2Fixture = Box2D.Dynamics.b2Fixture
	, b2World = Box2D.Dynamics.b2World
	, b2MassData = Box2D.Collision.Shapes.b2MassData
	, b2PolygonShape = Box2D.Collision.Shapes.b2PolygonShape
	, b2CircleShape = Box2D.Collision.Shapes.b2CircleShape
	, b2DebugDraw = Box2D.Dynamics.b2DebugDraw
	, b2MouseJointDef =  Box2D.Dynamics.Joints.b2MouseJointDef
	, b2Shape = Box2D.Collision.Shapes.b2Shape
	, b2Joint = Box2D.Dynamics.Joints.b2Joint
	, b2Settings = Box2D.Common.b2Settings
	;

var world;
var ctx;
var canvas_width;
var canvas_height;
var mouse_pressed = false;
var mouse_joint = false;
var mouse_x, mouse_y;

//box2d to canvas scale , therefor 1 metre of box2d = 30px of canvas :)
var scale = 30;

/*
	Draw a world
	this method is called in a loop to redraw the world
*/	 
function draw_world(world, context) 
{
	//convert the canvas coordinate directions to cartesian coordinate direction by translating and scaling
	ctx.save();
	ctx.translate(0 , canvas_height);
	ctx.scale(1 , -1);
	world.DrawDebugData();
	ctx.restore();
	
	ctx.font = 'bold 18px arial';
	ctx.textAlign = 'center';
	ctx.fillStyle = '#fff';
	ctx.fillText('Box2d MouseJoint example in Javascript', canvas_width/2, 20);
	ctx.font = 'bold 14px arial';
	ctx.fillText('Click on any object and drag it', canvas_width/2, 40);
}

//Create box2d world object
function createWorld() 
{
	//Gravity vector x, y - 10 m/s2 - thats earth!!
	var gravity = new b2Vec2(0, -10);
	
	world = new b2World(gravity , true );
	
	//setup debug draw
	var debugDraw = new b2DebugDraw();
	debugDraw.SetSprite(document.getElementById("canvas").getContext("2d"));
	debugDraw.SetDrawScale(scale);
	debugDraw.SetFillAlpha(0.5);
	debugDraw.SetLineThickness(1.0);
	debugDraw.SetFlags(b2DebugDraw.e_shapeBit | b2DebugDraw.e_jointBit);
	
	world.SetDebugDraw(debugDraw);
	
	//create some objects
	ground = createBox(world, 8.5, 2, 16 , 1, {type : b2Body.b2_staticBody, 'user_data' : {'fill_color' : 'rgba(204,237,165,1)' , 'border_color' : '#7FE57F' }});
	createBox(world, 6.50, 3.80, 1 , 1, {'user_data' : {'border_color' : '#555' }});
	createBox(world, 8.50, 3.80, 1 , 1, {'user_data' : {'fill_color' : 'rgba(204,0,165,0.3)' , 'border_color' : '#555' }});
	createBall(world, 8.50, 3.80, 1 , {'user_data' : {'fill_color' : 'rgba(204,100,0,0.3)' , 'border_color' : '#555' }});
	
	return world;
}		

//Function to create a round ball, sphere like object
function createBall(world, x, y, radius, options) 
{
	var body_def = new b2BodyDef();
	var fix_def = new b2FixtureDef();
	
	fix_def.density = options.density || 1.0;
	fix_def.friction = 0.5;
	fix_def.restitution = 0.5;
	
	var shape = new b2CircleShape(radius);
	fix_def.shape = shape;
	
	body_def.position.Set(x , y);
	
	body_def.linearDamping = 0.0;
	body_def.angularDamping = 0.0;
	
	body_def.type = b2Body.b2_dynamicBody;
	body_def.userData = options.user_data;
	
	var b = world.CreateBody( body_def );
	b.CreateFixture(fix_def);
	
	return b;
}

//Create standard boxes of given height , width at x,y
function createBox(world, x, y, width, height, options) 
{
	 //default setting
	options = $.extend(true, {
		'density' : 1.0 ,
		'friction' : 1.0 ,
		'restitution' : 0.5 ,
		
		'type' : b2Body.b2_dynamicBody
	}, options);
      
    var body_def = new b2BodyDef();
	var fix_def = new b2FixtureDef();
	
	fix_def.density = options.density;
	fix_def.friction = options.friction;
	fix_def.restitution = options.restitution;
	
	fix_def.shape = new b2PolygonShape();
		
	fix_def.shape.SetAsBox( width/2 , height/2 );
	
	body_def.position.Set(x , y);
	
	body_def.type = options.type;
	body_def.userData = options.user_data;
	
	var b = world.CreateBody( body_def );
	var f = b.CreateFixture(fix_def);
	
	return b;
}

/*
	This method will draw the world again and again
	called by settimeout , self looped
*/
function step() 
{
	var fps = 60;
	var timeStep = 1.0/(fps * 0.8);
	
	//move the box2d world ahead
	world.Step(timeStep , 8 , 3);
	world.ClearForces();
	
	//redraw the world
	draw_world(world , ctx);
	
	//call this function again after 1/60 seconds or 16.7ms
	setTimeout(step , 1000 / fps);
}

//Convert coordinates in canvas to box2d world
function get_real(p)
{
	return new b2Vec2(p.x + 0, canvas_height_m - p.y);
}

function GetBodyAtMouse(includeStatic)
{
	var mouse_p = new b2Vec2(mouse_x, mouse_y);
	
	var aabb = new b2AABB();
	aabb.lowerBound.Set(mouse_x - 0.001, mouse_y - 0.001);
	aabb.upperBound.Set(mouse_x + 0.001, mouse_y + 0.001);
	
	var body = null;
	
	// Query the world for overlapping shapes.
	function GetBodyCallback(fixture)
	{
		var shape = fixture.GetShape();
		
		if (fixture.GetBody().GetType() != b2Body.b2_staticBody || includeStatic)
		{
			var inside = shape.TestPoint(fixture.GetBody().GetTransform(), mouse_p);
			
			if (inside)
			{
				body = fixture.GetBody();
				return false;
			}
		}
		
		return true;
	}
	
	world.QueryAABB(GetBodyCallback, aabb);
	return body;
}

// main entry point
$(function() 
{
	//first create the world
	world = createWorld();
	
	var canvas = $('#canvas');
	ctx = canvas.get(0).getContext('2d');
	
	//get internal dimensions of the canvas
	canvas_width = parseInt(canvas.attr('width'));
	canvas_height = parseInt(canvas.attr('height'));
	canvas_height_m = canvas_height / scale;
	
	//If mouse is moving over the thing
	$(canvas).mousemove(function(e) 
	{
		var p = get_real(new b2Vec2(e.pageX/scale, e.pageY/scale))
		
		mouse_x = p.x;
		mouse_y = p.y;
		
		if(mouse_pressed && !mouse_joint)
		{
			var body = GetBodyAtMouse();
			
			if(body)
			{
				//if joint exists then create
				var def = new b2MouseJointDef();
				
				def.bodyA = ground;
				def.bodyB = body;
				def.target = p;
				
				def.collideConnected = true;
				def.maxForce = 1000 * body.GetMass();
				def.dampingRatio = 0;
				
				mouse_joint = world.CreateJoint(def);
				
				body.SetAwake(true);
			}
		}
		else
		{
			//nothing
		}
		
		if(mouse_joint)
		{
			mouse_joint.SetTarget(p);
		}
	});
	
	$(canvas).mousedown(function() 
	{
		//flag to indicate if mouse is pressed or not
		mouse_pressed = true;
	});
	
	/*
		When mouse button is release, mark pressed as false and delete the mouse joint if it exists
	*/
	$(canvas).mouseup(function() 
	{
		mouse_pressed = false;
		
		if(mouse_joint)
		{
			world.DestroyJoint(mouse_joint);
			mouse_joint = false;
		}
	});
	
	//start stepping
	step();
});

So play with it. Can even make a game using that joint.

Last Updated On : 6th January 2013

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3 Comments + Add Comment

  • Hi Silver.

    Thanks for the post is very utill for me.

    i’m implementing Box2Dweb in a engine own, and i have a problema, when i run the program, this throw breakpoints on this line of box2dweb:

    if (jn.joint.m_islandFlag == true) {
    continue;
    }

    Do you know what happens?

    Regards

  • 169 //call this function again after 10 seconds170 setTimeout(step , 1000 / fps);
    uhhh…. i think you mean 16 milisec

    • yeah, correct.

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