Make a rope using box2d in javascript

Rope

In this experiment we shall make a rope like thing in box2d. There is no rope like structure that box2d supports directly. But if multiple small units are connected together at their edges using a revolutejoint, then it was act somewhat like a rope.

Lets take a look

You can move any part of the rope using your mouse. Now in this rope, higher the number of parts in the rope more realistic would the rope like behaviour. But more objects mean more processing for box2d as well.

Source

/**
	Make a rope in 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
	, b2World = Box2D.Dynamics.b2World
	, b2PolygonShape = Box2D.Collision.Shapes.b2PolygonShape
	, b2DebugDraw = Box2D.Dynamics.b2DebugDraw
	, b2MouseJointDef =  Box2D.Dynamics.Joints.b2MouseJointDef
	, b2RevoluteJointDef =  Box2D.Dynamics.Joints.b2RevoluteJointDef
	;

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

//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();
	
	//write some text
	ctx.textAlign = 'right';
	ctx.fillStyle = '#fff';
	ctx.font = 'bold 15px arial';
	ctx.fillText('Rope using box2d', canvas_width - 10, canvas_height - 10);
}

//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);
	
	world.SetDebugDraw(debugDraw);
	
	//ceiling
	ground = ceiling = createBox(world, canvas_width_m / 2, canvas_height_m - 0.5, 16 , 1, {type : b2Body.b2_staticBody});
	
	var last_link = ceiling;
	var last_anchor_point = new b2Vec2(0, -0.5);
	var revolute_joint = new b2RevoluteJointDef();
	
	//height of rope elements, in metres ofcourse
	var r_height = 1.1;
	
	//rope
	for (var i = 1; i <= 10; i++) 
	{
		var body = createBox(world, canvas_width_m / 2 , canvas_height_m - 1 - i * 1.5, 0.25 , r_height);
		
		//revolute joint
		revolute_joint.bodyA = last_link;
		revolute_joint.bodyB = body;
		revolute_joint.localAnchorA = last_anchor_point;
		revolute_joint.localAnchorB = new b2Vec2(0 , r_height/2);
		
		last_anchor_point = new b2Vec2(0, -1 * r_height/2);
		
		//create the joint in world
		world.CreateJoint(revolute_joint);
		
		// saving the reference of the last placed link
		last_link = body;
	}
	
	var body = createBox(world, canvas_width_m / 2 , canvas_height_m - 1 - i * 1.5, r_height , r_height, {density : 5.0});
		
	//revolute joint
	revolute_joint.bodyA = last_link;
	revolute_joint.bodyB = body;
	revolute_joint.localAnchorA = last_anchor_point;
	revolute_joint.localAnchorB = new b2Vec2(0 , r_height/2);
	
	last_anchor_point = new b2Vec2(0, -1 * r_height/2);
	
	//create the joint in world
	world.CreateJoint(revolute_joint);
		
	return world;
}		

//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() 
{
	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;
	canvas_width_m = canvas_width / scale;
	
	//first create the world
	world = createWorld();
	
	
	//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();
});






Last Updated On : 8th January 2013

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3 Comments + Add Comment

  • Thanks a lot for your example! It was really helpful!

    Not sure if you did it on purpose, but maybe you could change the variables e.pageX and e.pageY inside the mousemove handler into e.offsetX and e.offsetY to get the local canvas-position instead of the full webpage-position.

    var p = get_real(new b2Vec2(e.offsetX/scale, e.offsetY/scale));

    On this website it makes no difference, ’cause the animation plays inside an iframe, but if you’re having the code and canvas on the page itself and the canvas is not positioned at the exact top-left corner of the page, mouse interactions are not working correct.

    But again, thanks a lot for sharing your knowledge! It helped me a lot and your demo looks great! Even without the actual rope-joints!

  • I want to color the rope! Can you help me regarding this?

    • this demo shows box2d’s debugview. I don’t know if it’s possible to change colors in debugview, but the endresult normaly isn’t debugview, but a view with rendered graphics, for example with EaselJS

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