# Basic Box2D rope

It’s time to create a rope with Box2d.

To start defining a rope, we’ll start from something similar: a chain. A chain is a series of two or more connected links. The smaller and closer the links, the more flexible the chain. So we can define a rope as a “perfect chain”, with infinite links.

In a Box2D simulation, ropes as we know them aren’t possible. But we can construct chains, and according to the number of links they can approximate a rope quite well.

Obviously, the higher the number of links, the more accurate the simulation, the slowest and CPU intensive the simulation.

I found 10 links are a good compromise between simulation and performance.

Every link is (guess what?) linked to the previous one with a revolute joint. Refer to Box2D joints: Revolute Joint if you need more information about revolute joints.

The core of the whole script is the `link` variable. I simply “save” the reference of the last chain placed on the stage and create the joint between the last chain placed and the one I am going to place.

So I’m having the first link with a joint with the ceiling, the second one with a joint on the first one, the third with a joint on the second and so on.

```package {
import flash.display.Sprite;
import flash.events.Event;
import flash.events.MouseEvent;
import Box2D.Dynamics.*;
import Box2D.Collision.*;
import Box2D.Collision.Shapes.*;
import Box2D.Common.Math.*;
import Box2D.Dynamics.Joints.*;
public class rope extends Sprite {
var body:b2Body;
public var m_world:b2World;
public var m_iterations:int=10;
public var m_timeStep:Number=1.0/30.0;
var mouseJoint:b2MouseJoint;
var mousePVec:b2Vec2 = new b2Vec2();
var bodyDef:b2BodyDef;
var boxDef:b2PolygonDef;
var circleDef:b2CircleDef;
var revolute_joint:b2RevoluteJointDef=new b2RevoluteJointDef();
public function rope() {
var worldAABB:b2AABB = new b2AABB();
worldAABB.lowerBound.Set(-100.0, -100.0);
worldAABB.upperBound.Set(100.0, 100.0);
var gravity:b2Vec2=new b2Vec2(0.0,10.0);
var doSleep:Boolean=true;
m_world=new b2World(worldAABB,gravity,doSleep);
// debug draw start
var m_sprite:Sprite;
m_sprite = new Sprite();
var dbgDraw:b2DebugDraw = new b2DebugDraw();
var dbgSprite:Sprite = new Sprite();
dbgDraw.m_sprite=m_sprite;
dbgDraw.m_drawScale=30;
dbgDraw.m_alpha=1;
dbgDraw.m_fillAlpha=0.5;
dbgDraw.m_lineThickness=1;
dbgDraw.m_drawFlags=b2DebugDraw.e_shapeBit;
m_world.SetDebugDraw(dbgDraw);
// ceiling
bodyDef = new b2BodyDef();
bodyDef.position.x=8.5;
bodyDef.position.y=0;
boxDef = new b2PolygonDef();
boxDef.SetAsBox(2, 0.5);
boxDef.density=0;
boxDef.friction=0.5;
boxDef.restitution=0.2;
body=m_world.CreateBody(bodyDef);
body.CreateShape(boxDef);
// rope
for (var i:int = 1; i <= 10; i++) {
// rope segment
bodyDef = new b2BodyDef();
bodyDef.position.x=8.5;
bodyDef.position.y=i;
boxDef = new b2PolygonDef();
boxDef.SetAsBox(0.1, 0.5);
boxDef.density=100;
boxDef.friction=0.5;
boxDef.restitution=0.2;
body=m_world.CreateBody(bodyDef);
body.CreateShape(boxDef);
// joint
m_world.CreateJoint(revolute_joint);
body.SetMassFromShapes();
// saving the reference of the last placed link
}
// final body
bodyDef.position.x=8.5;
bodyDef.position.y=11;
boxDef = new b2PolygonDef();
boxDef.SetAsBox(0.5,0.5);
boxDef.density=2;
boxDef.friction=0.5;
boxDef.restitution=0.2;
body=m_world.CreateBody(bodyDef);
body.CreateShape(boxDef);
m_world.CreateJoint(revolute_joint);
body.SetMassFromShapes();
}
public function createMouse(evt:MouseEvent):void {
var body:b2Body=GetBodyAtMouse();
if (body) {
var mouseJointDef:b2MouseJointDef=new b2MouseJointDef  ;
mouseJointDef.body1=m_world.GetGroundBody();
mouseJointDef.body2=body;
mouseJointDef.target.Set(mouseX/30, mouseY/30);
mouseJointDef.maxForce=30000;
mouseJointDef.timeStep=m_timeStep;
mouseJoint=m_world.CreateJoint(mouseJointDef) as b2MouseJoint;
}
}
public function destroyMouse(evt:MouseEvent):void {
if (mouseJoint) {
m_world.DestroyJoint(mouseJoint);
mouseJoint=null;
}
}
public function GetBodyAtMouse(includeStatic:Boolean=false):b2Body {
var mouseXWorldPhys = (mouseX)/30;
var mouseYWorldPhys = (mouseY)/30;
mousePVec.Set(mouseXWorldPhys, mouseYWorldPhys);
var aabb:b2AABB = new b2AABB();
aabb.lowerBound.Set(mouseXWorldPhys - 0.001, mouseYWorldPhys - 0.001);
aabb.upperBound.Set(mouseXWorldPhys + 0.001, mouseYWorldPhys + 0.001);
var k_maxCount:int=10;
var shapes:Array = new Array();
var count:int=m_world.Query(aabb,shapes,k_maxCount);
var body:b2Body=null;
for (var i:int = 0; i < count; ++i) {
if (shapes[i].GetBody().IsStatic()==false||includeStatic) {
var tShape:b2Shape=shapes[i] as b2Shape;
var inside:Boolean=tShape.TestPoint(tShape.GetBody().GetXForm(),mousePVec);
if (inside) {
body=tShape.GetBody();
break;
}
}
}
return body;
}
public function Update(e:Event):void {
m_world.Step(m_timeStep, m_iterations);
if (mouseJoint) {
var mouseXWorldPhys=mouseX/30;
var mouseYWorldPhys=mouseY/30;
var p2:b2Vec2=new b2Vec2(mouseXWorldPhys,mouseYWorldPhys);
mouseJoint.SetTarget(p2);
}

}
}
}```

And this is the result:

Every object is draggable, except the ceiling.

217 GAME PROTOTYPES EXPLAINED WITH SOURCE CODE
// 1+2=3
// 10000000
// 2 Cars
// 2048
// Avoider
// Ballz
// Block it
// Blockage
// Bloons
// Boids
// Bombuzal
// Breakout
// Bricks
// Columns
// CubesOut
// Dots
// DROP'd
// Dudeski
// Eskiv
// Filler
// Fling
// Globe
// HookPod
// Hundreds
// InkTd
// Iromeku
// Lumines
// Magick
// MagOrMin
// Maze
// Memdot
// Nano War
// Nodes
// o:anquan
// Ononmin
// Pacco
// Phyballs
// Platform
// Poker
// Pool
// Poux
// Pudi
// qomp
// Racing
// Renju
// SameGame
// Security
// Sling
// Slingy
// Sokoban
// Splitter
// Sproing
// Stack
// Stairs
// Stringy
// Sudoku
// Tetris
// Threes
// Toony
// Turn
// TwinSpin
// vvvvvv
// Wordle
// Worms
// Yanga
// Zhed
// zNumbers