How to make a Distance Joint in Box2D in Javascript

Distance

In the previous post on mouse joints we learned how to interact with box2d objects using the mouse. Now its time to take a look at another joint in box2d, the distance joint. Distance joint connects 2 bodies maintaining a fixed distance between them, its like metal wire connecting the 2 bodies, such that the wire would neither expand nor shrink, but allow the bodies to rotate about the point of joint.

See the circle and rectangle connected with the blue wire. Now thats supposed to be invisible, its drawn here just for the sake of indicating where the joint is. In a real application it would be replaced by appropriate graphics.

Joint of that kind can be used to make something like a pendulum. Ofcourse additional force would be needed to keep a body moving non-stop.

Source

The source is mostly from the mouse joint example, except that it adds an extra joint element.

/**
	Distance 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
	, b2DistanceJointDef =  Box2D.Dynamics.Joints.b2DistanceJointDef
	, 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 Distance Joint 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});
	var a = createBox(world, 6.50, 3.80, 1 , 1);
	var b = createBox(world, 10.50, 3.80, 1 , 1);
	var c = createBall(world, 8.50, 3.80, 1);
	
	//create distance joint between b and c
	distance_joint = new b2DistanceJointDef();
	distance_joint.bodyA = b;
	distance_joint.bodyB = c;
	//connect the centers - center in local coordinate - relative to body is 0,0
	distance_joint.localAnchorA = new b2Vec2(0, 0);
	distance_joint.localAnchorB = new b2Vec2(0, 0);
	//length of joint
	distance_joint.length = 3;
	distance_joint.collideConnected = true;
	
	//add the joint to the world
	world.CreateJoint(distance_joint);
	
	return world;
}		

//Function to create a round ball, sphere like object
function createBall(world, x, y, radius, 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 || 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();
});