Forums » Impact Engine » Box2D Conveyor belts

Is there any work taking place to port the latest Box2D code over to Impact? Specifically I am looking for the setTangentSpeed functionality for contacts (http://code.google.com/p/box2d-html5/source/browse/trunk/Box2D/Testbed/Tests/ConveyorBelt.js).

Or has one of you geniuses come up with another way to create a conveyor belt (or surface) using box2d entities?

I didn't get a response so here is how I created my conveyor belts. I added a contact listener to plugins/box2d/game.js that contains a PreSolve function. When the contact occurs, If one of the contact points belongs to a conveyor, I apply a force to the center of mass of the other object.

A couple points to note.
1) In plugins/box2d/game.js, add this.addContactListener(); to the loadLevel function. The addContactListener function is below.
2) For each entity, you will need a collision function
3) In the createBody function for each entity add these properties:
a) this.body.userData = 'yourEntityName';
b) this.body.entity = this;
4) When creating your conveyor in weltmeister, create a 'dir' key with value of either 'left' or 'right' depending on which direction you want your conveyor to roll.

Here is the addContactListener function to add to the plugins/box2d/game.js

addContactListener: function() {
       var listener = new Box2D.Dynamics.b2ContactListener();
	   
       listener.BeginContact = function(contact){
            //console.log('Add Contact', point);
			// Instead of telling you which b2Body collided, the b2Contact tells you which fixture collided.
            var fixA = contact.GetFixtureA();
			// To know which b2Body was involved in the collision you can retrieve a reference to the fixture.
            var fixB = contact.GetFixtureB(); 
            var body1 = fixA.GetBody();
            var body2 = fixB.GetBody();

			if (body1.userData !== undefined && body1.entity !== undefined) {
				// console.log('body 1 Add Contact', point);
				body1.entity.collision((body2.entity !== undefined ? body2.entity : undefined));
			}
			if (body2.userData !== undefined && body2.entity != undefined) {
				// console.log('body 2 Add Contact', point);
				body2.entity.collision((body1.entity !== undefined ? body1.entity : undefined));
			}
	  };
		
	  listener.EndContact = function (contact) {
	  };
	
	  listener.PostSolve = function (contact, impulse) {
	  };
	
	  listener.PreSolve = function (contact, oldManifold) {
		  var fixtureA = contact.GetFixtureA();
          var fixtureB = contact.GetFixtureB();
		  //console.log('PreSolve');
			 
          if (fixtureA.GetBody().userData == 'conveyor')
          {
			  //console.log('PreSolve - Apply force to fixtureB ' + fixtureB.GetBody().userData);
			  var speed = 0;
			  if ( fixtureA.GetBody().entity.dir == 'left') {
			     speed = -200.0;
			  } else {
			     speed = 200.0;
			  }
			  var vect = new Box2D.Common.Math.b2Vec2(speed, 0); 

			  // Apply force to contact body 
			  var cPos = fixtureB.GetBody().GetWorldCenter();
			  fixtureB.GetBody().ApplyForce(vect,cPos);

              //contact.SetTangentSpeed(5.0);
          }

          if (fixtureB.GetBody().userData == 'conveyor')
          {
			  //console.log('PreSolve - Apply force to fixtureA ' + fixtureA.GetBody().userData);			  
              //contact.SetTangentSpeed(-5.0);
  			  var speed = 0 ;
			  if ( fixtureB.GetBody().entity.dir == 'left' ) {
                                   speed = -200.0;
			  } else {
                                  speed = 200.0;
			  }
			  var vect = new Box2D.Common.Math.b2Vec2(speed, 0);
			   
			  var cPos = fixtureA.GetBody().GetWorldCenter();
			  fixtureA.GetBody().ApplyForce(vect,cPos);
          }
	  };
        
      ig.world.SetContactListener(listener);		
},

And here is my conveyor.js

// JavaScript Document
ig.module(
	'game.entities.conveyor'
)
.requires(
	'impact.entity',
	'plugins.box2d.entity'
)
.defines(function(){
EntityConveyor = ig.Box2DEntity.extend({
	//_wmDrawBox: true,
	//_wmBoxColor: 'rgba(0, 255, 0, 0.7)',
	size: {x: 250, y:50},
	offset: {x: 0, y: 0},
	zindex: 16,
	name: 'conveyor',
	type: ig.Entity.TYPE.A,
	checkAgainst: ig.Entity.TYPE.NONE,
	collides: ig.Entity.COLLIDES.NEVER, // Collision is already handled by Box2D!
	animSheet: new ig.AnimationSheet( 'media/conveyor.png', 250, 50 ),
	dir: 'left',	// dir is set from weltmeister info 
	bodyDef: undefined,
		
	init: function( x, y, settings ) {
		// Add the animations
		this.addAnim( 'idle', 1, [0] );
		this.addAnim( 'left', 0.1, [0,1,2,3], false );
		this.addAnim( 'right', 0.1, [3,2,1,0], false );
		this.currentAnim=this.anims.idle;
		this.parent( x, y, settings );		
	},
	
	createBody: function() {
		this.bodyDef = new Box2D.Dynamics.b2BodyDef();
		this.bodyDef.position = new Box2D.Common.Math.b2Vec2(
			(this.pos.x + this.size.x / 2) * Box2D.SCALE,
			(this.pos.y + this.size.y / 2) * Box2D.SCALE
		); 
		this.bodyDef.type = Box2D.Dynamics.b2Body.b2_staticBody;
		
		this.body = ig.world.CreateBody(this.bodyDef);
				
		this.body.userData= "conveyor";
		this.body.entity = this;
		var fixture = new Box2D.Dynamics.b2FixtureDef;
		fixture.shape = new Box2D.Collision.Shapes.b2PolygonShape();
		fixture.shape.SetAsBox(
			this.size.x / 2 * Box2D.SCALE,
			this.size.y / 2 * Box2D.SCALE
		);
		
		fixture.friction = 0.8;			
		this.body.CreateFixture(fixture);				
	},
	
	collision: function(colBody) {
		//console.log('Collision - ' + this.name + ' with ' + (colBody !== undefined ? colBody.name : 'undefined'));
	},

	update: function() {
		if (this.dir == 'left') {
		   this.currentAnim=this.anims.left;
		} else {
		   this.currentAnim=this.anims.right;
		}
		this.parent();
	}
});

});

Nice work figuring this out on your own! Thanks for sharing. :)