Wwwebber's Blog

Box2d 2.1 How to use a b2Controller

Posted in actionscript 3, Box2d by wwwebber on July 9, 2010

Box2d 2.1 brought some awesome improvements
1) improved class structure.
2) buyouncy support
3) a set of Controller Classes that are well described by the docs as ” a convience for encapsulating common per-step functionality.” (read about them here)

These improvements were actually worth the fact that none of your prior Box2d code (and I mean none) would compile with the new class structure.

If you used earlier versions of Bo2D, you already used something like these controllers–the Gravity Vector that you passed to the world object at construction.
This was a b2Vec2 (the gravity) that the world applied to all non-static objects. Ie the walls wouldn’t be affected by the gravity vector but all the moving objects would be affected by a constant force downwards (unless your gravity had an unusual orientation)

There are several types of controllers and they all extends b2Controller . The TestBuoyancy class that comes with the TestBed shows an example of how to use the b2BuoyancyController .

All of the controllers have the following methods

AddBody(body:b2Body):void
RemoveBody(body:b2Body):void

I was interested in using a b2ConstantAccelController. There aren’t really any examples out there for this (at least that I could find) so here’s how easy it is to implement a b2ConstantAccelController–which will just apply an additional vector each time step to any bodies added to that controller.

Now, unlike the GravityVector you can arbitrarily add and remove objects from the controller. How could this be useful? Imagine that you have gusts of wind blowing sideways…you can add your objects to a Contoller and have them pushed sideways…then when the wind passes you could remove them from that controller.

How to create a b2ConstantAccelController :

//construct object and its acceleration vector
var myAccelController:b2ConstantAccelController = new b2ConstantAccelController();
const WIND_VEC :b2Vec2 = new b2Vec2(10,0);
//associate controller with vector
myAccelController.A = WIND_VEC

Henceforth, all b2bodies that are stored in myAccelController will have the WIND_VEC applied to them during the b2World.step() function .. you don’t have to do anything else except add and remove bodies to the controller.

//now let’s add something to the Controller
myAccelController.AddBody(someB2Body);

…when you want to remove the body
myAccelController.RemoveBody(someB2Body);

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One Response

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  1. oh said, on March 2, 2011 at 1:27 am

    you left out a key line of code:

    world.AddController(myAccelController);

    still, very helpful postl!! :0D


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