Rendering joints with Box2D

In most Box2D projects you can see how to render bodies or attach objects to them, but I couldn’t see any tutorial about rendering joints.

If you look at the classic rendering loop it’s something like this one

for (var bb:b2Body = m_world.m_bodyList; bb; bb = bb.m_next) {
     if (bb.m_userData is Sprite) {
          bb.m_userData.x = bb.GetPosition().x;
          bb.m_userData.y = bb.GetPosition().y;
          bb.m_userData.rotation = bb.GetAngle() * (180/Math.PI);
     }
}

but as you can see, it only renders bodies.

The following script is the same you can find at Simulating a hook with Box2D but I will render the joints with lines.

package {
	import flash.display.Sprite;
	import flash.events.Event;
	import Box2D.Dynamics.*;
	import Box2D.Collision.*;
	import Box2D.Collision.Shapes.*;
	import Box2D.Common.Math.*;
	import Box2D.Dynamics.Joints.*;
	import flash.events.MouseEvent;
	public class distance_joint extends Sprite {
		var mouseJoint:b2MouseJoint;
		var mousePVec:b2Vec2 = new b2Vec2();
		var bd:b2BodyDef;
		var the_circle:b2CircleDef = new b2CircleDef();
		var circle:b2Body;
		var the_box:b2PolygonDef = new b2PolygonDef();
		var the_joint:b2DistanceJointDef = new b2DistanceJointDef();
		var joint:b2DistanceJoint;
		//
		public var m_world:b2World;
		public var m_iterations:int=10;
		public var m_timeStep:Number=1.0/30.0;
		//
		var my_canvas:Sprite = new Sprite();
		public function distance_joint() {
			addChild(my_canvas);
			// world setup
			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
			var m_sprite:Sprite;
			m_sprite = new Sprite();
			addChild(m_sprite);
			var dbgDraw:b2DebugDraw = new b2DebugDraw();
			var dbgSprite:Sprite = new Sprite();
			m_sprite.addChild(dbgSprite);
			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|b2DebugDraw.e_jointBit;
			m_world.SetDebugDraw(dbgDraw);
			// ground
			the_box.SetAsBox(9,0.5);
			the_box.density=0;
			the_box.friction=0.4;
			the_box.restitution=0.1;
			bd = new b2BodyDef();
			bd.position.Set(8.5, 13);
			var ground:b2Body=m_world.CreateBody(bd);
			ground.CreateShape(the_box);
			ground.SetMassFromShapes();
			// roof
			the_box.SetAsBox(9,0.5);
			the_box.density=0;
			the_box.friction=0.4;
			the_box.restitution=0.1;
			bd = new b2BodyDef();
			bd.position.Set(8.5, 0);
			var roof:b2Body=m_world.CreateBody(bd);
			roof.CreateShape(the_box);
			roof.SetMassFromShapes();
			// circle
			the_circle.radius=2;
			the_circle.density=1.0;
			the_circle.friction=0.4;
			the_circle.restitution=0.3;
			bd = new b2BodyDef();
			bd.position.Set(6, 2);
			circle=m_world.CreateBody(bd);
			circle.CreateShape(the_circle);
			circle.SetMassFromShapes();
			// square
			the_box.SetAsBox(1.5,1.5);
			the_box.density=1.0;
			the_box.friction=0.4;
			the_box.restitution=0.1;
			bd = new b2BodyDef();
			bd.position.Set(2, 2);
			var box:b2Body=m_world.CreateBody(bd);
			box.CreateShape(the_box);
			box.SetMassFromShapes();
			// listeners
			stage.addEventListener(MouseEvent.MOUSE_DOWN, createMouse);
			stage.addEventListener(MouseEvent.MOUSE_UP, destroyMouse);
			addEventListener(Event.ENTER_FRAME, Update, false, 0, true);
		}
		public function createMouse(evt:MouseEvent):void {
			var body:b2Body=GetBodyAtMouse(true);
			if (body) {
				the_joint.Initialize(circle, body, circle.GetWorldCenter(),new b2Vec2(mouseX/30,mouseY/30));
				the_joint.collideConnected=true;
				joint=m_world.CreateJoint(the_joint) as b2DistanceJoint;
			}
		}
		public function destroyMouse(evt:MouseEvent):void {
			if (joint) {
				m_world.DestroyJoint(joint);
				joint=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()) {
					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);
			my_canvas.graphics.clear();
			my_canvas.graphics.lineStyle(4,0x000000,1);
			for (var j:b2Joint=m_world.GetJointList(); j; j=j.GetNext()) {
				my_canvas.graphics.moveTo(j.GetAnchor1().x*30,j.GetAnchor1().y*30);
				my_canvas.graphics.lineTo(j.GetAnchor2().x*30,j.GetAnchor2().y*30);
			}
		}
	}
}

The key is in the loop at lines 134-137 and its GetJointList() function that gets the world joint list. With the returned joint, using GetNext() I get the next joint in the world list. A NULL joint indicates the end of the list.

Then, GetAnchor1 and GetAnchor2 return me the anchor point of both bodies linked by the joint in world coordinates.

And that's it... that easy...

Click on a static object (green) to create a joint on the fly and have it rendered

No need to include the source code, just replace the script at Box2D joints: Distance Joint with this one.

Get the most popular Phaser 3 book

Through 202 pages, 32 source code examples and an Android Studio project you will learn how to build cross platform HTML5 games and create a complete game along the way.

Get the book

214 GAME PROTOTYPES EXPLAINED WITH SOURCE CODE
// 1+2=3
// 100 rounds
// 10000000
// 2 Cars
// 2048
// A Blocky Christmas
// A Jumping Block
// A Life of Logic
// Angry Birds
// Angry Birds Space
// Artillery
// Astro-PANIC!
// Avoider
// Back to Square One
// Ball Game
// Ball vs Ball
// Ball: Revamped
// Balloon Invasion
// BallPusher
// Ballz
// Bar Balance
// Bejeweled
// Biggification
// Block it
// Blockage
// Bloons
// Boids
// Bombuzal
// Boom Dots
// Bouncing Ball
// Bouncing Ball 2
// Bouncy Light
// BoxHead
// Breakout
// Bricks
// Bubble Chaos
// Bubbles 2
// Card Game
// Castle Ramble
// Chronotron
// Circle Chain
// Circle Path
// Circle Race
// Circular endless runner
// Cirplosion
// CLOCKS - The Game
// Color Hit
// Color Jump
// ColorFill
// Columns
// Concentration
// Crossy Road
// Crush the Castle
// Cube Jump
// CubesOut
// Dash N Blast
// Dashy Panda
// Deflection
// Diamond Digger Saga
// Don't touch the spikes
// Dots
// Down The Mountain
// Drag and Match
// Draw Game
// Drop Wizard
// DROP'd
// Dudeski
// Dungeon Raid
// Educational Game
// Elasticity
// Endless Runner
// Erase Box
// Eskiv
// Farm Heroes Saga
// Filler
// Flappy Bird
// Fling
// Flipping Legend
// Floaty Light
// Fuse Ballz
// GearTaker
// Gem Sweeper
// Globe
// Goat Rider
// Gold Miner
// Grindstone
// GuessNext
// Helicopter
// Hero Emblems
// Hero Slide
// Hexagonal Tiles
// HookPod
// Hop Hop Hop Underwater
// Horizontal Endless Runner
// Hundreds
// Hungry Hero
// Hurry it's Christmas
// InkTd
// Iromeku
// Jet Set Willy
// Jigsaw Game
// Knife Hit
// Knightfall
// Legends of Runeterra
// Lep's World
// Line Rider
// Lumines
// Magick
// MagOrMin
// Mass Attack
// Math Game
// Maze
// Meeblings
// Memdot
// Metro Siberia Underground
// Mike Dangers
// Mikey Hooks
// Nano War
// Nodes
// o:anquan
// One Button Game
// One Tap RPG
// Ononmin
// Pacco
// Perfect Square!
// Perfectionism
// Phyballs
// Pixel Purge
// PixelField
// Planet Revenge
// Plants Vs Zombies
// Platform
// Platform game
// Plus+Plus
// Pocket Snap
// Poker
// Pool
// Pop the Lock
// Pop to Save
// Poux
// Pudi
// Pumpkin Story
// Puppet Bird
// Pyramids of Ra
// qomp
// Quick Switch
// Racing
// Radical
// Rebuild Chile
// Renju
// Rise Above
// Risky Road
// Roguelike
// Roly Poly
// Run Around
// Rush Hour
// SameGame
// SamePhysics
// Save the Totem
// Security
// Serious Scramblers
// Shrink it
// Sling
// Slingy
// Snowflakes
// Sokoban
// Space Checkers
// Space is Key
// Spellfall
// Spinny Gun
// Splitter
// Spring Ninja
// Sproing
// Stabilize!
// Stack
// Stick Hero
// String Avoider
// Stringy
// Sudoku
// Super Mario Bros
// Surfingers
// Survival Horror
// Talesworth Adventure
// Tetris
// The Impossible Line
// The Moops - Combos of Joy
// The Next Arrow
// Threes
// Tic Tac Toe
// Timberman
// Tiny Wings
// Tipsy Tower
// Toony
// Totem Destroyer
// Tower Defense
// Trick Shot
// Tunnelball
// Turn
// Turnellio
// TwinSpin
// vvvvvv
// Warp Shift
// Way of an Idea
// Whack a Creep
// Wheel of Fortune
// Where's my Water
// Wish Upon a Star
// Word Game
// Wordle
// Worms
// Yanga
// Yeah Bunny
// Zhed
// zNumbers