Scaling objects with Box2D

I would like to start talking about a Shrink it prototype but I noticed it’s most based upon objects resizing.

So before trying to replicate the game, let’s start with some info about object scaling in Box2D

The first interesting thing about Box2D scaling is you can’t scale a Box2D body.

And the tutorial is over :)

Now, what about deleting an existing object and replacing it with a smaller/bigger one?

That’s the right way you can do it.

In this example, I am going to resize the simplest object ever… a circle. You know to scale a circle you simply have to change its radius.

So the trick is delete the previous circle, and create a new one.

But it’s not that easy.

In Box2D, there are bodies and shapes. A body is a collection of shapes. A circle, in this specific case, is a body with one circle shape. So to scale the circle, we must remove the initial circle shape from the body, and add a new one. We don’t have to touch the body, only the shape.

Then, since different circles have different masses, we must assign a new mass to the body.

In this script I assume you know how to select a Box2D object with the mouse. If you aren’t familiar with it, read Understanding pixels and meters with Box2D and how to select an object with mouse – part 1 and part 2.

Once you know how to select an object, it’s all a matter of geometry: to scale a circle you just have to change its radius, and every regular polygon has its formula. To tell the truth, I should improve my geometry skills so if you have the formula to resize hexagons or similar shapes, you’re welcome!

So this code has nothing new except lines 87-106 that are fully commented:

And this is the result:

Click on the circle to scale it down by 10%

Download the source code.

Next time, I’ll show you how to scale squares and triangles.

  • How about making it smoothly scale? I think that’d be a pretty cool effect.

  • Nice post.
    @Cataclysm: you can use TweenLite/Max and animate the sprite/movieclip each click, and then every time you “resize” a Box2D item, the sprite will smoothly scale to the new object’s dimmensions.

    Emanuele, feel free to suggest another creative solution ;)

  • Andy

    The exact thing I was trying to work out last week! Have now binned that part of project and am doing my scaling with tweens… ah well. Cheers anyway! ;)

  • Kala


    it is possible create conveyor?
    I want to make simple simulation and I generate randomly some object that fall down on one side and then I want to move it automatically by conveyor.

    Thank you. Kala

  • seren

    Is there a way to do this with custom shapes without * the coords by a scale each time you recreate the shape?

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  • shefyg

    There is more elegant way to do it – just change the shape radius – see here:

  • Well, this at least works for the polygons, should work circles as well:

    1. Edit the vertices in the shapelist.
    2. Add this function to b2PolygonShape (copied from the constructor with some mods):

    public function RecalculateAfterChangesInVertices():void {

    // Compute the polygon centroid.
    m_centroid = ComputeCentroid(m_vertices, m_vertexCount);

    // Compute the oriented bounding box.
    ComputeOBB(m_obb, m_vertices, m_vertexCount);

    3. Call the world’s Refilter to this body.

  • Uh, and call that RecalculateAfterChangesInVertices() for the shape, before calling Refilter(). :)

  • Ninjakannon

    You can ‘resize’ the circle far more simply:

    circle.GetFixtureList().GetShape().Set(new b2CircleShape(newRadius));

    Note: this example assumes the circle has a single fixture only.

  • haje

    thanks man, just what i needed :)

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  • Example with Phaser please! :P

    Thanks in advance,
    Regards, Nicholls