Box2D: tutorial for the absolute beginners

January 27, 2009 • Actionscript 3, Box2D, Flash by Emanuele Feronato • 62 Comments

This is a tutorial about Box2D dedicated to the absolute beginners.

I received a lot of request asking for this, so here we go.

What is Box2D

Box2D is a feature rich 2d rigid body physics engine, written in C++ by Erin Catto.

There is a Flash version called Box2DFlashAS3, but among Flash developers it’s called Box2D as well, that you can download from this link.

Being a Flash porting, we will use AS3 to make our project, instead of C++ as required by the original library.

Once you downloaded and unpacked the zip file in a folder, this is what you will get:

You are now ready to begin

Your first Box2D experiment

Start Flash and create a new AS3 Flash file and call it (example) demo.fla. Save it in the same folder you used to unzip Box2D package. Also in your properties panel assign the Class name to (example) demo, this way:

Now create a new actionscript file, call id demo.as and save it in the same folder.

Your folder now should look this way:

it’s time to edit demo.as

package {
	import flash.display.Sprite;
	import flash.events.Event;
	import flash.utils.Timer;
	import flash.events.TimerEvent;
	import Box2D.Dynamics.*;
	import Box2D.Collision.*;
	import Box2D.Collision.Shapes.*;
	import Box2D.Common.Math.*;
	public class demo extends Sprite {
		public var the_world:b2World;
		var time_count:Timer=new Timer(1000);
		public function demo() {
			var environment:b2AABB = new b2AABB();
			environment.lowerBound.Set(-100.0, -100.0);
			environment.upperBound.Set(100.0, 100.0);
			var gravity:b2Vec2=new b2Vec2(0.0,10.0);
			the_world=new b2World(environment,gravity,true);
			var debug_draw:b2DebugDraw = new b2DebugDraw();
			var debug_sprite:Sprite = new Sprite();
			addChild(debug_sprite);
			debug_draw.m_sprite=debug_sprite;
			debug_draw.m_drawScale=30;
			debug_draw.m_fillAlpha=0.5;
			debug_draw.m_lineThickness=1;
			debug_draw.m_drawFlags=b2DebugDraw.e_shapeBit;
			the_world.SetDebugDraw(debug_draw);
			var final_body:b2Body;
			var the_body:b2BodyDef;
			var the_box:b2PolygonDef;
			the_body = new b2BodyDef();
			the_body.position.Set(8.5, 13);
			the_box = new b2PolygonDef();
			the_box.SetAsBox(8.5, 0.5);
			the_box.friction=0.3;
			the_box.density=0;
			final_body=the_world.CreateBody(the_body);
			final_body.CreateShape(the_box);
			final_body.SetMassFromShapes();
			addEventListener(Event.ENTER_FRAME, on_enter_frame);
			time_count.addEventListener(TimerEvent.TIMER, on_time);
			time_count.start();
 
		}
		public function on_time(e:Event) {
			var final_body:b2Body;
			var the_body:b2BodyDef;
			var the_box:b2PolygonDef;
			the_body = new b2BodyDef();
			the_body.position.Set(Math.random()*10+2, 0);
			the_box = new b2PolygonDef();
			the_box.SetAsBox(Math.random()+0.1,Math.random()+0.1);
			the_box.friction=0.3;
			the_box.density=1;
			final_body=the_world.CreateBody(the_body);
			final_body.CreateShape(the_box);
			final_body.SetMassFromShapes();
		}
		public function on_enter_frame(e:Event) {
			the_world.Step(1/30, 10);
		}
	}
}

Lines 2-5: some commons Flash libraries used to make the demo

Lines 6-9: Box2D libraries… it’s not that important at the moment to know everything about them… they’re just some libraries

Line 11: Declaring the_world variable, b2World type. b2World is the the main object to deal with the Box2d engine. It stores all the joints and bodies, handles listeners and is responsible for stepping through the simulation.

Line 14: Declaring the_environment, b2AABB type. The physics environment generated by Box2D is not infinite, and b2AABB is the container of such environment. Think about it as a bounding box. Inside this bounding box, the world is ruled by Box2D physics.

Lines 15-16 : Defining the upper and lower corners of the environment bounding box, in meters. 1 meter = 30 pixels. So our box has sides made by 6000 pixels. They’re pretty too much for a single-screen project, but if you are using scrolling, it could be useful to set up a big environment. For more information about pixels and meters refer to Understanding pixels and meters with Box2D and how to select an object with mouse – part 2.

Line 17: Declaring gravity variable, b2Vec2 type. b2Vec2 is a vector with x and y components.

Line 18: Starting up the world: the constructor has three parameters: environment, the worldAABB bounding box, gravity the world gravity vector, and a boolean set to true to improve performance by not simulating inactive bodies.

And now you created the world. Next time I’ll explain the rest of the script

Anyway, this is the result

And this is the source code to download.

I hope this will help you start playing with Box2D.

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This post has 62 comments

Superdean

on January 28, 2009 at 12:35 am

YES!
finally a box 2d tutorial i can understand!
this actually might make me migrate to AS3…
Yowan

on January 28, 2009 at 3:37 am

Awesome!!
Thanks a lot!
Shival

on January 28, 2009 at 11:03 am

You are really the rockstar of programming
Shival

on January 28, 2009 at 11:06 am

make it PROgramming
Xavi

on January 28, 2009 at 6:57 pm

thanx emanuele for making a tut for Box 2D that is for begginers
i haven’t started AS3 yet, but i think this will help…
do u have a beggining AS3 tutorial
Prankard

on January 29, 2009 at 4:04 am

Very basic for my taste. But you can’t satisfy everyone :)
Glad you got a good response from your other users.
Keep up the more advanced tutorials too.
Emanuele Feronato

on January 29, 2009 at 12:20 pm

[...] Box2D: tutorial for the absolute beginners [...]
Xodus

on January 29, 2009 at 1:58 pm

XD
FrozenHaddock

on January 29, 2009 at 2:03 pm

Excellent!

I shall be closely following both the basic Box2D tutorials tonight and start playing around with them!
Weekly Shared Items - 30. January, 2009 « toxin 2.0

on January 30, 2009 at 9:53 am

[...] Box2D: tutorial for the absolute beginners [...]
Koczo

on February 2, 2009 at 4:33 pm

It looks great, but when i’m trying to open “demo.fla” file in Adobe Flash CS3, I got the message “Unexpected file format”. Do you know why?
A smart way to manage sleeping objects with Box2D : Emanuele Feronato

on February 18, 2009 at 11:04 am

[...] you followed Box2D: tutorial for the absolute beginners, then you know you can put some object to “sleep” to improve performance by not [...]
Erase Box, a Box2D game made with my tutorials : Emanuele Feronato

on February 21, 2009 at 1:13 pm

[...] a mix of your tutorials about “Box2D for absolute begginers” and “Totem Destroyer Prototype“. I also added some other features, such as the [...]
Fabien Millerand

on February 23, 2009 at 3:43 am

Hi,

I wanted to know if there was an easy way to add a texture to those ‘boxes’..
Thx
Fabien
lol

on March 20, 2009 at 6:59 pm

uuuh, yeeeah, that’s so PROOO Mr. PROgrammer
Tobias C.

on April 16, 2009 at 4:55 pm

Thanks a lot!!!!
Fighterlegend

on April 25, 2009 at 8:47 pm

Can you put it in a Flash CS3 format?

Or do I NEED Flash CS4?

If I do, that would suck! :(
AS3 Flash Physics Engine Box2D | [mck]

on May 1, 2009 at 11:15 am

[...] Boy Wonder!!) Emanuele Feronato (an Italian PROgrammer) has written an extremely useful tutorial: box2d tutorial for the absolute beginners/. This one is a very good starting [...]
Destruction

on May 4, 2009 at 6:02 am

Nice guide, might play around with it sometimes.
Also, hi Fighterlegend!
Thempc

on May 28, 2009 at 2:09 pm

to Fighterlegend:
no you do not NEED CS4 for this,
this can also be done in CS3 with ease.

PS: awesome tutoial 5/5
Androoo

on August 6, 2009 at 2:26 pm

Why don’t you reply to peoples questions?
Mokey

on August 8, 2009 at 5:20 am

Nice tutorial. I’m still learning as3 and this will be very useful. Thanks!
Box2DFlash - Falling Text | Cyan[c] Design

on August 26, 2009 at 5:54 am

[...] want to give yet another shout out to Emanuele Feronato. If you are looking at this code, and are completely lost, he has EXCELLENT tutorials to get you [...]
cyancdesign

on August 27, 2009 at 4:24 am

This one help me start off on the right foot. And all your other tutorials are awesome! I was able to expand on this to turn the boxes into dynamic falling text.
(^_^)// Cheers.
shuping chen

on August 29, 2009 at 5:16 am

Thank you very much.^_^
Chris

on October 13, 2009 at 2:35 am

Thanks for these, really finding them useful.

One thing though, you mentioned that 1 meter = 30 pixels… is that a default or do you have to set that ratio somewhere in the code?

Thanks again.
Efe

on October 21, 2009 at 6:26 am

Thank you for the lovely tutorial, its beyond amazingly useful. As a low to mid-level Flash progrommer / animator, getting a handle on the Box2D stuff through just the examples is actually quite difficult (and not helped by the slightly more ambiguous naming conventions it uses)

But seriously, amazing stuff. Thanks :)
Sam

on November 17, 2009 at 8:37 pm

I don’t seem to have flash, to create a new AS3, I need help. Can you give me a link?
bubloobasu

on December 22, 2009 at 7:47 am

Can u pls explain the rest of the script
Sucks man...

on January 8, 2010 at 3:37 am

Errm.PART 2!!!!!!!!!!!!!!!!!!!!!?????????????????
No it doesn't!

on January 8, 2010 at 3:39 am

HAHA! I found it XD

http://www.emanueleferonato.com/2009/01/29/box2d-tutorial-for-the-absolute-beginners-step-2/

:D
clayton

on January 25, 2010 at 3:34 pm

i followed the directions exactly as listed, however when i try to run the program, it gives me a bunch of errors starting with:

1046: Type was not found or was not a compile-time constant: b2World.
1046: Type was not found or was not a compile-time constant: b2AABB.
1046: Type was not found or was not a compile-time constant: b2Vec2.

anyone have any ideas?
Rackdoll

on January 27, 2010 at 2:48 pm

@clayton

your classpaths arent correct configured..
these errors happen when flash or your code editor cannot find the class you are trying to use..
So check your linkage to the classes and try again :)

it also helps to get a basic understanding of AS3.0 and OO before you start a tutorial like this one.

;)
Angry

on January 28, 2010 at 5:40 am

ok. I tried this and it was an epic fail.

The idiots who made Box2d decided to randomly change the whole thing! The box2d looks nothing like the screenshots you had. Could you plz make a new tutorial?! thanks.
Xalvez

on January 28, 2010 at 11:12 am

I got the same error with #26.
Help pls ><
Box2D tutorial for the absolute beginners – revamped : Emanuele Feronato - italian geek and PROgrammer

on February 2, 2010 at 1:00 am

[...] About a year ago I published a Box2D tutorial for the absolute beginners. [...]
Sam

on February 6, 2010 at 4:34 pm

Hi, guys.

You need to have the following in order to successfully run Demo.fla.

1. Adobe Flash CS4
2. Box2DFlashAS3_2.0.2

Download Box2DFlashAS3_2.0.2 in here.

http://sourceforge.net/projects/box2dflash/files/

If you have any questions, ask. :-)

Thank you for the great tutorial, Emanuel! You rock!

Sam
merve

on February 13, 2010 at 9:43 pm

hello,
i liked very much the scene i see in the last animation screen.

now i want to ask whether it is possible to write an application without using flash development environment? i am a newbee in software developing but i like flash applications too. but i am a student and no have money for flash development environment.
is there a compiler for action script desperate from flash development environment?
i googled a bit but find nothing.

thanks for tutorial and any possible advises.
Gab

on February 14, 2010 at 3:55 am

I tryed this code with Adobe Flash IDE CS4 : It’s okay.

When I try with an AS3 project with Flash Develop, debugDraw doesn’t display any objects.

I use : FlashDevelop 3.0.6/Box2D 2.0.2/Flex 3.4
Tim

on February 23, 2010 at 2:25 pm

Learning AS3 and your tutorial is an awesome way to experiment. Really one of the best sites I’ve found about Flash, Actionscript, and games. Thanks for sharing your knowledge.
reb1rth

on March 11, 2010 at 4:14 am

hii those whor getting error just like #26 and #29 check out this link http://orenyomtov.com/topic/flash
i had the same error i got it sorted nw.
one1989

on May 8, 2010 at 6:05 am

Great tutorial. I’m very excited in box2d. Hope you can give more..
Physics Engine für Flash: Box2D | Multitouch and Beyond

on May 17, 2010 at 3:21 pm

[...] Einsteiger Tutorial von Emanuele Feronato: http://www.emanueleferonato.com/2009/01/27/box2d-tutorial-for-the-absolute-beginners/ [...]
Adbury

on May 20, 2010 at 12:32 pm

Hey Emanuele, Nice tutorial though the box 2d files i downloaded had nearly 60 to 70 syntax errors like the one below all was same error

1084: Syntax error: expecting identifier before lessthan.
public var m_vertices:Vector.;
the use of this bracket caused a lot of problems
claudio

on June 8, 2010 at 5:01 pm

Doesnt work i dont know why
do i have to put it in a special rute in flash installation directory?
Roshan C

on June 12, 2010 at 12:24 pm

hi,
A very simple and nice tutorial and followed all the instruction as said in tutorial but i got 10 compile time errors :- 1084: Syntax error: expecting identifier before lessthan. all the errors were same. I am using FLASH CS3. I created demo myself since i am not having CS4. Thanks for such a b’ful tutorial.
games

on June 20, 2010 at 7:48 pm

Great tutorial, with the latest version there were a lot of errors, but after downloading the previous version Box2DFlashAS3_2.0.2 all worked great without any errors.
Thanks for this tutorial!
Alexandre Colella

on June 27, 2010 at 5:48 pm

Thanks! Great easy-to-learn tutorial!
shefyg

on August 12, 2010 at 8:10 am

hi Emanuele,

10x for this tutorial

I think you should consider update it. I saw that in the latest B2Box – there is now more public b2DebugDraw.m_sprit instead we have b2DebugDraw.SetSprite()

If either of you (readers of this tutorial) encountered this problem – I hope I helped.

The way I solved this, is by going to decleration of class b2DebugDraw, and looking for m_sprite.

Best Regards, Shefy
Useful Links « Mobile training support

on September 30, 2010 at 4:57 pm

[...] http://www.emanueleferonato.com/2009/01/27/box2d-tutorial-for-the-absolute-beginners/ [...]
Neko

on November 29, 2010 at 12:03 am

Warning, don’t waste your time with this tutorial – Box2D has been updated in the 2 years since this was written and this tutorial is currently broken. :(
Danko

on December 29, 2010 at 2:32 pm

great tutorial!
stan

on March 4, 2011 at 9:17 am

You’re just taking tutorials from here and there and adding them to your site as your own tutorial.

You are not even explaining the code( which Im sure you don’t know yourself seeing as they are copied from other tutorials).

Why not make a simple original tutorial?
Draco18s

on March 4, 2011 at 5:51 pm

Because it’s a CS4 file.
Draco18s

on March 4, 2011 at 6:11 pm

Finally, something that might actually help me figure out how to START a project with Box2D. Unfortunately, they updated the engine to a new version, and this demo can’t be converted:

b2Body no longer has a CreateShape function. Or anything that appears to do even remotely the same thing.

At least the old package is still available.
satya pandey

on March 17, 2011 at 7:06 am

hi sir , Good morning
While working with above tutorial, i am facing problem with following
1. debug_draw.m_sprite=debug_sprite;
2. var the_box:b2PolygonDef;

it’s showing compile time constant, definition not found & Attempted access of inaccessible property m_sprite through a reference with static type Box2D.Dynamics:b2DebugDraw.

please sir help to resolve these problem.

Thanking you……….
Bryce Neal

on March 21, 2011 at 9:24 pm

@Satya

This tutorial is out of date. Somewhere along the line defs were renamed to shapes. Try b2PolygonShape.
AndrewThompson

on June 8, 2011 at 7:15 am

Hey Awesome tut!

I have a problem though. I am getting a “Scene 1, Layer ‘Layer 1′, Frame 1, Line 1 1037: Packages cannot be nested.” Compiler Error.

Any ideas why or how to fix it?
Rafael

on June 12, 2011 at 6:07 am

To make this tutorial Download this file:

http://sourceforge.net/projects/box2dflash/files/box2dflash/Box2DFlashAS3_2.0.2/

not the latest.

Ty Feronato
George

on August 14, 2011 at 8:29 am

Hi,
I’m new to all this. I’ve been trying to follow your tutorials but they don’t seem to be in order? I found the first one thru the web and this one by chance. I would really like to learn how to use this powerful tool and see what I can come up with. Could you please email me the link or links to all of these tutorials you are offering?
Also, since I have not found all the tutorials are you explaining how to create a game from start to finish using this box2d method?
Thanks,
George =)
Evgeney

on September 6, 2011 at 8:08 pm

Thanks a lot.
Kin Jung Il

on November 14, 2011 at 3:12 pm

I can’t do it D:

italian geek and PROgrammer

This post has 62 comments

Superdean

Xodus

Koczo

lol

Tobias C.

Fighterlegend

Destruction

Thempc

Androoo

shuping chen

Efe

Sam

bubloobasu

clayton

Xalvez

merve

Gab

Tim

reb1rth

one1989

claudio

Roshan C

Alexandre Colella

shefyg

Neko

stan

Draco18s

Draco18s

satya pandey

Bryce Neal

Rafael

George

Evgeney

Kin Jung Il

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