How-To: 2D Physics with Box2D

metachris · by **metachris** » Wed Mar 04, 2009 12:47 pm

Hey guys!

Here is a brief tutorial on how to use the popular 2D physics engine Box2D in an Android Activity.

This is only part 1, including the import of the libraries, creation of the world, adding a few dynamic objects and updating the world. Not included is the drawing part of things, which is subject to a later post, or a post by someone else

Download jbox2d-2.0.1-library-only.jar and save it anywhere as jbox2d.jar
In Eclipse:
Create a folder /lib/
Right Click on /lib, select Import
Select General / File System
Choose the directory of jbox2d.jar
Import the file jbox2d.jar
Right click on the project and select "Properties"
Click on "Java Build Path" and select the "Libraries" tab
Click on "Add JARs..." and pick the file jbox2d.jar

Now we have everything set up and can write a hello world! This is the exact example from the Box2D User Manual.

Initialize a new world
Add a ground box
Create some dynamic bodies (balls)
Simulate the world

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import org.jbox2d.collision.AABB;
import org.jbox2d.collision.CircleDef;
import org.jbox2d.collision.PolygonDef;
import org.jbox2d.common.Vec2;
import org.jbox2d.dynamics.Body;
import org.jbox2d.dynamics.BodyDef;
import org.jbox2d.dynamics.World;
 
import android.util.Log;
 
public class PhysicsWorld {
    public int targetFPS = 40;
    public int timeStep = (1000 / targetFPS);
    public int iterations = 5;
 
    private Body[] bodies = new Body[100]; 
    private int count = 0;
    
    private AABB worldAABB;
    private World world;
    private BodyDef groundBodyDef;
    private PolygonDef groundShapeDef;
 
    public void create() {
        // Step 1: Create Physics World Boundaries
        worldAABB = new AABB();
        worldAABB.lowerBound.set(new Vec2((float) -100.0, (float) -100.0));
        worldAABB.upperBound.set(new Vec2((float) 100.0, (float) 100.0));
        
        // Step 2: Create Physics World with Gravity
        Vec2 gravity = new Vec2((float) 0.0, (float) -10.0);
        boolean doSleep = true;
        world = new World(worldAABB, gravity, doSleep);
        
        // Step 3: Create Ground Box
        groundBodyDef = new BodyDef();
        groundBodyDef.position.set(new Vec2((float) 0.0, (float) -10.0));
        Body groundBody = world.createBody(groundBodyDef);
        groundShapeDef = new PolygonDef();
        groundShapeDef.setAsBox((float) 50.0, (float) 10.0);
        groundBody.createShape(groundShapeDef);
    }
 
    public void addBall() {
        // Create Dynamic Body
        BodyDef bodyDef = new BodyDef();
        bodyDef.position.set((float) 6.0+count, (float) 24.0);
        bodies[count] = world.createBody(bodyDef);
        
        // Create Shape with Properties
        CircleDef circle = new CircleDef();
        circle.radius = (float) 1.8;
        circle.density = (float) 1.0;
        
        // Assign shape to Body
        bodies[count].createShape(circle);
        bodies[count].setMassFromShapes();
 
        // Increase Counter
        count += 1;        
    }
    
    public void update() {
        // Update Physics World
        world.step(timeStep, iterations);
        
        // Print info of latest body
        if (count > 0) {
            Vec2 position = bodies[count].getPosition();
            float angle = bodies[count].getAngle();
            Log.v("Physics Test", "Pos: (" + position.x + ", " + position.y + "), Angle: " + angle);
        }
    }    
}
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To use this in an Activity, you may write a code like this:

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import android.app.Activity;
import android.os.Bundle;
import android.os.Handler;
 
public class Physics extends Activity {
    PhysicsWorld mWorld;
    private Handler mHandler;
 
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        mWorld = new PhysicsWorld();
        mWorld.create();
 
        // Add 50 Balls
        for (int i=0; i<50; i++) {
            mWorld.addBall();
        }
 
        // Start Regular Update
        mHandler = new Handler();
        mHandler.post(update));
    }
 
    @Override
    protected void onPause() {
        super.onPause();
        mHandler.removeCallbacks(update);
    }
 
    private Runnable update = new Runnable() {
        public void run() {
            mWorld.update();
            mHandler.postDelayed(update, (long) (mWorld.timeStep*1000));
        }
    };
}
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This is a short version of the full post at 4feets.com. Further suggested reading:

Box2D User Manual
Box2D Class Reference
JBox2D Wiki (explains the differences from Box2D and JBox2D)
Physics Engine Comparison, 2
Speed Tests

- Chris

[edit]add: bodies=new Body[100];[/edit]

torpor · by **torpor** » Wed Mar 04, 2009 2:18 pm

Excellent! Nice one Chris - you are rapidly becoming a potent tutorial master!

Would be nice to see the next page of this tutorial implement a simple gravity-type game or something .. maybe something for a hack session this week, eh?

grzegorz_ae · by **grzegorz_ae** » Wed Mar 04, 2009 6:26 pm

The best post of the month!

MickArea · by **MickArea** » Thu Mar 05, 2009 4:29 am

pretty good, but you forget this detail :

bodies=new Body[100];

by example

metachris · by **metachris** » Thu Mar 05, 2009 4:28 pm

MickArea wrote:pretty good, but you forget this detail :

bodies=new Body[100];

by example

Thanks a lot! Fixed

If I've got some time, I'll make an example game with Mr. SnowFlakes GameTemplate (which I think is really nice and where I'm making some improvements i'll post about another time).

MrSnowflake · by **MrSnowflake** » Thu Mar 05, 2009 5:59 pm

metachris wrote:I'll make an example game with Mr. SnowFlakes GameTemplate (which I think is really nice and where I'm making some improvements i'll post about another time).

Something that's really nice can't be improved :p. Nice to see people like my simple template. I had some other gaming related stuff in the pipes, but they stayed there since birth

.

metachris · by **metachris** » Thu Mar 05, 2009 9:32 pm

MrSnowflake wrote:Something that's really nice can't be improved :p. Nice to see people like my simple template. I had some other gaming related stuff in the pipes, but they stayed there since birth .

Release early and often!

:-D

Seriously, I like your work and would certainly love to see some more, even unfinished code!

MrSnowflake · by **MrSnowflake** » Thu Mar 05, 2009 9:52 pm

Somewhere on google code there is a 2D lib of some sorts from me. It's very preliminary, but if you want to see some crappy code from me, go ahead and search for it

.

MickArea · by **MickArea** » Fri Mar 06, 2009 1:38 am

hi, now i've something which work, with displaying balls ...
but i'm afraid by the " fluidity " ..... can anyone improve this peace of code ?

PhysicsWorld.java

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import java.util.Random;
 
import org.jbox2d.collision.AABB;
 
import org.jbox2d.collision.CircleDef;
 
import org.jbox2d.collision.PolygonDef;
 
import org.jbox2d.common.Vec2;
 
import org.jbox2d.dynamics.Body;
 
import org.jbox2d.dynamics.BodyDef;
 
import org.jbox2d.dynamics.World;
 
import android.content.Context;
 
import android.graphics.Canvas;
 
import android.graphics.Color;
 
import android.graphics.Paint;
 
import android.graphics.Paint.Style;
 
import android.view.View;
 
public class PhysicsWorld extends View{
 
        protected static final int GUIUPDATEIDENTIFIER = 0x231;
 
    public int targetFPS = 40;
 
    public float timeStep = 10.0f / targetFPS;
 
    public int iterations = 5;
 
    private Body[] bodies;
 
    private int count = 0;
 
    private AABB worldAABB;
 
    public World world;
 
    private PolygonDef groundShapeDef;
 
    public int World_W,World_H;
 
        private Paint paint;
 
        private float radius=10;
 
        public PhysicsWorld(Context context,int W,int H) {
 
                super(context);
 
                World_W=W;
 
                World_H=H;
 
        // Step 1: Create Physics World Boundaries
 
        worldAABB = new AABB();
 
                Vec2 min = new Vec2(-50, -50);
 
                Vec2 max = new Vec2(World_W + 50, World_H + 50);
 
        worldAABB.lowerBound.set(min);
 
        worldAABB.upperBound.set(max);
 
        // Step 2: Create Physics World with Gravity
 
        Vec2 gravity = new Vec2((float) 0.0, (float) -10.0);
 
        boolean doSleep = true;
 
        world = new World(worldAABB, gravity, doSleep);
 
        // Step 3: 
 
        //Create Ground Box :
 
        BodyDef bodyDef = new BodyDef();
 
        bodyDef.position.set(new Vec2((float) 0.0, (float) -10.0));
 
        Body groundBody = world.createBody(bodyDef);
 
        groundShapeDef = new PolygonDef();
 
        groundShapeDef.setAsBox((float) World_W, (float) 10);
 
        groundBody.createShape(groundShapeDef); 
 
        // up :
 
        bodyDef = new BodyDef();
 
        bodyDef.position.set(new Vec2((float) 0.0, (float) (World_H+10.0) ));
 
        groundBody = world.createBody(bodyDef);
 
        groundShapeDef = new PolygonDef();
 
        groundShapeDef.setAsBox((float) World_W, (float) 10);
 
        groundBody.createShape(groundShapeDef); 
 
        // left :
 
        bodyDef = new BodyDef();
 
        bodyDef.position.set(new Vec2((float) -10, (float) 0.0 ));
 
        groundBody = world.createBody(bodyDef);
 
        groundShapeDef = new PolygonDef();
 
        groundShapeDef.setAsBox((float)10, (float) World_H);
 
        groundBody.createShape(groundShapeDef); 
 
        // right :
 
        bodyDef = new BodyDef();
 
        bodyDef.position.set(new Vec2((float) World_W+10, (float) 0.0 ));
 
        groundBody = world.createBody(bodyDef);
 
        groundShapeDef = new PolygonDef();
 
        groundShapeDef.setAsBox((float)10, (float) World_H);
 
        groundBody.createShape(groundShapeDef); 
 
        //
 
        // step 4: initialize 
 
        bodies=new Body[50];
 
        // 
 
        paint=new Paint();
 
                paint.setStyle(Style.FILL);
 
                paint.setColor(Color.RED);
 
    }
 
    public void addBall() {
 
        // Create Dynamic Body
 
        BodyDef bodyDef = new BodyDef();
 
        Random rnd = new Random();
 
        bodyDef.position.set((float) radius*2+rnd.nextInt( (int)(World_W-radius*4) ), (float)2*radius+ rnd.nextInt( (int)(World_H-radius*4) ));
 
        bodies[count] = world.createBody(bodyDef);
 
        // Create Shape with Properties
 
        CircleDef circle = new CircleDef();
 
        circle.radius = (float) radius;
 
        circle.density = (float) 1.0;
 
        circle.restitution=0.5f;
 
        // Assign shape to Body
 
        bodies[count].createShape(circle);
 
        bodies[count].setMassFromShapes();
 
        // Increase Counter
 
        count += 1;        
 
    }
 
    public void update() {
 
       world.step(   timeStep  , iterations);
 
        postInvalidate(); 
 
    }  
 
    @Override
 
    protected void onDraw(Canvas canvas) {
 
                // draw balls
 
                //
 
        for(int j = 0;j<count;j++) {
 
                        canvas.drawCircle(bodies[j].getPosition().x,World_H- bodies[j].getPosition().y, radius, paint);
 
                }
 
        }
 
}
 
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import android.app.Activity;
 
import android.os.Bundle;
 
import android.os.Handler;
 
import android.util.DisplayMetrics;
 
import android.view.Window;
 
import android.view.WindowManager;
 
public class Physics extends Activity   {
 
    PhysicsWorld mWorld;
 
    private Handler mHandler;
 
    @Override
 
    public void onCreate(Bundle savedInstanceState) {
 
        super.onCreate(savedInstanceState);
 
        getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN,WindowManager.LayoutParams.FLAG_FULLSCREEN);
 
        this.requestWindowFeature(Window.FEATURE_NO_TITLE);
 
                DisplayMetrics dm = new DisplayMetrics();
 
                getWindowManager().getDefaultDisplay().getMetrics(dm);
 
        mWorld = new PhysicsWorld(this,dm.widthPixels,dm.heightPixels);
 
        this.setContentView(this.mWorld);
 
        // Add 10 Balls
 
        for (int i=0; i<10; i++) {
 
            mWorld.addBall();
 
        }
 
        // Start Regular Update
 
        mHandler = new Handler();
 
        mHandler.post(update);
 
    }
 
    @Override
 
    protected void onPause() {
 
        super.onPause();
 
        mHandler.removeCallbacks(update);
 
    }
 
    private Runnable update = new Runnable() {
 
        public void run() {
 
            mWorld.update();
 
            mHandler.postDelayed(update, (long) (10/mWorld.timeStep));
 
        }
 
    };
 
    @Override
 
    protected void onResume()
 
    {
 
         super.onResume();
 
    }
 
    @Override
 
    protected void onStop()
 
    {
 
         super.onStop();
 
    } 
 
}
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metachris · by **metachris** » Fri Mar 06, 2009 2:47 pm

Yeah, i think the error lies here:

MickArea wrote:PhysicsWorld.java
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public int targetFPS = 40;

public float timeStep = 10.0f / targetFPS;

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you set timeStep to 10/targetFPS, which should be 1000/targetFPS!

you could tweak the targetFPS and iterations parameters though...

fakeSchwiz · by **fakeSchwiz** » Tue Mar 10, 2009 6:02 pm

I copied and pasted the two classes in the first post, and put them in a new android project, however I am running into problems when the update function in PhysicsWorld is called my app force closes. I am super new to java and the eclipse debugger so im not sure, but I do get an error message that says source code not available if i press jump over enough.
What should I put as the package name to get this to run, or does that even matter? Any other ideas why it keeps crashing?

thanks for your help!
schwiz

kensuke155 · by **kensuke155** » Thu Mar 12, 2009 12:49 am

Good post. I didn't know Chipmunk was ported to Java. I just did a demo with Phys2d, but it creeps to a halt with about 6 objects as it's not optimized for Java. I would suggest steering clear of Phys2d.

Pinio · by **Pinio** » Wed Mar 18, 2009 3:39 pm

Hi all,

I think that right now JBox2D isn't a good idea. It generates a lot of garbage, and GC is started almost every second. So, animation is rather poor (even for 3 objects - 1 static and 2 dynamic).

Also Phys2d is rather slow.

Do you know maybe other 2d physic engine, that can be used in Android?

Cheers,
P

MickArea · by **MickArea** » Wed Mar 18, 2009 5:58 pm

yes i'm interesting too ! so much garbage ...

03-18 17:56:57.816: DEBUG/dalvikvm(753): GC freed 36024 objects / 881392 bytes in 183ms
03-18 17:56:58.766: DEBUG/dalvikvm(753): GC freed 35988 objects / 880752 bytes in 174ms
03-18 17:56:59.746: DEBUG/dalvikvm(753): GC freed 35990 objects / 881272 bytes in 178ms
03-18 17:57:00.786: DEBUG/dalvikvm(753): GC freed 36057 objects / 883232 bytes in 182ms
03-18 17:57:01.776: DEBUG/dalvikvm(753): GC freed 36003 objects / 881200 bytes in 183ms
03-18 17:57:02.796: DEBUG/dalvikvm(753): GC freed 35992 objects / 880584 bytes in 187ms
03-18 17:57:04.556: DEBUG/dalvikvm(753): GC freed 35111 objects / 881192 bytes in 180ms
03-18 17:57:16.186: DEBUG/dalvikvm(753): GC freed 27264 objects / 882384 bytes in 169ms
03-18 17:57:37.826: DEBUG/dalvikvm(753): GC freed 19826 objects / 882504 bytes in 155ms
03-18 17:57:59.366: DEBUG/dalvikvm(753): GC freed 19794 objects / 881104 bytes in 157ms

lphpc · by **lphpc** » Fri Mar 20, 2009 5:58 am

I am porting JBox2D testbed to Android. VaryingRestitution test succeed to run now.

http://code.google.com/p/androidbox2d/

How-To: 2D Physics with Box2D

How-To: 2D Physics with Box2D

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