The magic of compound objects with Box2D

You can call it complex shapes, compound shapes, complex objects, compound objects or even poisoned underpants, but you all want to create complex “things” with Box2D.

Despite the “complex” word, it’s very easy to create one.

You just have to add multiple shapes to a body.

If you followed my latest Box2D tutorials, you will find the whole code I am going to show you very familiar, with some exceptions I will explain.

But first let’s take a look at what I am going to create: a perfect tile based maze, made with small squares representing the tiles and managed as a single object.

Here it is: if you drag the maze, you will see it’s a single object made (compound) with small tiles – the real objects, used as primitives.

Here it is the code:

You can find the maze at line 16 (yes, I did not create it dynamically, feel free to do it if you want), and now let’s look at the difference between a normal and a compound object.

A normal object, like the green static floor, after the CrateShape method at line 51, has a setMassFromShapes that finalizes the object itself.

If you look at the maze, inside the two cycles at lines 57-58, I only have CreateShape calls while a single setMassFromShapes is called after I completed the cycles.

This way, I am getting all objects merged in one big, compound, object.

And that’s all…

  • Very good post… as usual.
    Thanks Emanuele for this.

  • If we end up with a Box 2D platformer (with level editor) I think you should sell the source. People can make it off of your tutorials if interested, but I think there would be a big market to sell this as well. :)

  • Orion

    ;o, I can’t see anything on my own flash oO with the same code.

  • This could be a cool game idea if there is a ball at one end of the maze and you have to pick up and rotate the maze to get the ball to the end.

  • Sorry I meant start the ball at the beginning and then have it go to the end.

  • anlik

    up up up, I hope that i can see Box2D tutorial everydayO(∩_∩)O,Thanks

  • Nathan

    I was thinking the same thing as sathoro. I’ve been trying to do one of those for a long time. Now I think I know how!

  • ruslan

    How do you visualize the shapes(boxes)??

  • Pippo

    using debugsrite

  • samuvagyok

    Good article!
    Have you ever tried to use “custom” polygons?
    When I created one in box2d it seems it doesn’t use the physics – no gravity, no collision etc. and I still can’t find any helpful post in the box2d forum. Maybe I missed something that is in the setAsBox method…
    Would you write an article about custom shapes?
    Thanks, and best regards!
    S

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  • Justin Edmond

    I’m getting the same issue as some other people where nothing is showing up…

    I’m digging around the code but I can’t find a reason why it should be doing this.

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  • pbear

    For those getting no result, you need to add a line to set the flags. It was apparently dropped from the previous version. Right before these lines:

    m_world.SetDebugDraw(dbgDraw);
    // debug draw end

    Add:
    dbgDraw.m_drawFlags = b2DebugDraw.e_shapeBit | b2DebugDraw.e_jointBit | b2DebugDraw.e_coreShapeBit | b2DebugDraw.e_aabbBit | b2DebugDraw.e_obbBit | b2DebugDraw.e_pairBit | b2DebugDraw.e_centerOfMassBit;

    And you should be able to see what is going on.

  • for those who find this usefull.
    i made an editor for drawing complex shapes for box2d.
    in this version you can grab the generated xml from te process xml button.

    find it at:

    http://www.nuvorm.nl/start/index.php?option=com_content&view=article&id=83:box-2d-complex-shape-editor&catid=40:experiments&Itemid=53

  • How can I render textures movieClips in the compound object ?

  • for(var currentBody:b2Body = world.GetBodyList(); currentBody; currentBody = currentBody.GetNext()){
    if(currentBody.GetUserData()){
    if(currentBody.GetUserData().skin){
    currentBody.GetUserData().skin.x = currentBody.GetPosition().x*worldScale;
    currentBody.GetUserData().skin.y = currentBody.GetPosition().y*worldScale;
    currentBody.GetUserData().skin.rotation = currentBody.GetAngle()*(180/Math.PI);
    }
    }

    The render loop it’s the same??

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