Build 10 games and learn game development along the way in this fast paced game development course

Get the source code of 12 commercial games, loaded 50+ million times, and learn the secrets of game design

Learn how to make a successful Flash game from a real world example. Fully commented source code

Create realistic Flash physics games from scratch with advanced features, from Angry Birds to Crush the Castle

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.

Rate this post: 1 Star2 Stars3 Stars4 Stars5 Stars (11 votes, average: 3.27 out of 5)
Loading ... Loading ...
Learn how to make a successful commercial Flash game from a real world example: get the fully commented source code of Globez, a Flash game played millions of times which generated a four figure income. Limited copies available.

Get it now
Be my fan on Facebook and follow me on Twitter! Exclusive content for my Facebook fans and Twitter followers

This post has 14 comments

  1. Cataclysm Studios

    on December 12, 2009 at 7:56 pm

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

  2. Yarden Refaeli

    on December 12, 2009 at 8:44 pm

    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 ;)

  3. Andy

    on December 14, 2009 at 7:49 pm

    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! ;)

  4. Kala

    on December 14, 2009 at 9:10 pm

    Hello,

    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

  5. seren

    on December 15, 2009 at 4:48 pm

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

  6. Scaling objects with Box2D – part 2 : Emanuele Feronato - italian geek and PROgrammer

    on December 16, 2009 at 12:47 am

    [...] the first part I showed you how to scale a [...]

  7. Interview with Nick Pearce, the guy behind Shrink it : Emanuele Feronato - italian geek and PROgrammer

    on December 16, 2009 at 11:01 pm

    [...] Shrink it? You should, because it’s a very popular game and because it inspired me to write Scaling objects with Box2D – part 1 and [...]

  8. Shrink it Box2D prototype : Emanuele Feronato - italian geek and PROgrammer

    on January 5, 2010 at 5:22 pm

    [...] post continues Scaling objects with Box2D and Scaling objects with Box2D – part 2, with these [...]

  9. shefyg

    on September 18, 2010 at 6:38 pm

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

    http://www.box2d.org/forum/viewtopic.php?f=8&t=5601&p=26070#p26070

  10. Ville Helin

    on June 9, 2012 at 1:58 pm

    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.

  11. Ville Helin

    on June 9, 2012 at 1:59 pm

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

  12. Ninjakannon

    on August 26, 2012 at 3:39 pm

    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.

  13. haje

    on September 15, 2012 at 8:41 am

    thanks man, just what i needed :)

  14. Joe’s Anatomy | Dev Blog

    on April 11, 2014 at 12:04 pm

    […] You can read more about it here. […]

Thank you for the download!!

I hope you will find it useful and make something interesting out of it

To keep up to date with the blog, why don't you like my Facebook page and follow me on Twitter?

Want to learn more? Don't miss this:

Learn how to make a successful commercial Flash game from a real world example: get the fully commented source code of Globez, a Flash game played millions of times which generated a four figure income. Limited copies available.

Get it now

×