How-To: 2D Physics with Box2D

MickArea · by **MickArea** » Fri Mar 20, 2009 5:34 pm

hey, you did a great job !

but unfortunately we have always garbage collector ...
i can't imagine making a game now with physics :'( .

But we are in the good path !

lphpc · by **lphpc** » Tue Mar 24, 2009 3:23 am

Yes, there are a lot of garbage collector when the test is running.

Also, The tests run slowly, espicially the Pyramid test which have many objects.

I have no idea about optimization now.

Any good ideas?

lphpc · by **lphpc** » Fri Mar 27, 2009 3:53 am

Some discussion about box2d port of android. And a patch is available.

http://www.box2d.org/forum/viewtopic.ph ... 94&start=0

DarkNSF · by **DarkNSF** » Thu May 21, 2009 12:27 am

Seems to compile fine, but I get a NullPointerException (from an InvocationTargetException) when it makes a call to bodies[count].method

It adds 50 balls, then tries to access index position 50 (via 'count') when it should only be 49. That aside, if I comment out the code I still see no physics objects. Any ideas?

Syntax: [ Download ] [ Hide ]

Using java Syntax Highlighting

 
    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);
 
        }
 
    }
 
Parsed in 0.032 seconds,  using GeSHi 1.0.8.4

DarkNSF · by **DarkNSF** » Thu May 21, 2009 12:56 am

I've never really coded on hardware like this so maybe there was a reason that we were using Body[]. Using an ArrayList really cleaned up the code in my opinion..

Syntax: [ Download ] [ Hide ]

Using java Syntax Highlighting

import android.app.Activity;
 
import android.os.Bundle;
 
import android.os.Handler;
 
public class Thom extends Activity
 
{
 
    PhysicsWorld theWorld;
 
    private Handler theAndroidHandler;
 
    @Override
 
    public void onCreate(Bundle savedInstanceState)
 
    {
 
        super.onCreate(savedInstanceState);
 
        theWorld = new PhysicsWorld();
 
        theWorld.create();
 
        // Add 50 Balls
 
        for (int i=0; i<50; i++)
 
        {
 
            theWorld.addBall();
 
        }
 
        // Start Regular Update
 
        theAndroidHandler = new Handler();
 
        theAndroidHandler.post(update);
 
    }
 
    @Override
 
    protected void onPause()
 
    {
 
        super.onPause();
 
        theAndroidHandler.removeCallbacks(update);
 
    }
 
    private Runnable update = new Runnable()
 
    {
 
        public void run()
 
        {
 
            theWorld.update();
 
            theAndroidHandler.postDelayed(update, (long) (PhysicsWorld.TIME_STEP*1000));
 
        }
 
    }; 
 
}
Parsed in 0.035 seconds,  using GeSHi 1.0.8.4

Syntax: [ Download ] [ Hide ]

Using java Syntax Highlighting

import java.util.ArrayList;
 
import java.util.List;
 
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 static int TARGET_FPS = 40;
 
    /** */
 
    public static int TIME_STEP = (1000 / TARGET_FPS);
 
    /** */
 
    private int theIterations = 5;
 
    /** */
 
    private List<Body> theBodies = new ArrayList<Body>();
 
    /** */
 
    private World theWorld;
 
    /**
 
     * 
 
     */
 
    public void create()
 
    {
 
        // Step 1: Create Physics World Boundaries
 
        AABB 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);
 
        theWorld = new World(worldAabb, gravity, true);
 
        // Step 3: Create Ground Box
 
        BodyDef groundBodyDef = new BodyDef();
 
        groundBodyDef.position.set(new Vec2((float) 0.0, (float) -10.0));
 
        Body groundBody = theWorld.createBody(groundBodyDef);
 
        PolygonDef 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+theBodies.size(), (float) 24.0);
 
        Body newBody = theWorld.createBody(bodyDef);
 
        // Create Shape with Properties
 
        CircleDef circle = new CircleDef();
 
        circle.radius = (float) 1.8;
 
        circle.density = (float) 1.0;
 
        // Assign shape to Body
 
        newBody.createShape(circle);
 
        newBody.setMassFromShapes();
 
        theBodies.add(newBody);     
 
    }
 
    /**
 
     * 
 
     */
 
    public void update()
 
    {
 
        // Update Physics World
 
        theWorld.step(TIME_STEP, theIterations);
 
        // Print info of latest body
 
        if (!theBodies.isEmpty())
 
        {
 
            Vec2 position = theBodies.get(theBodies.size()-1).getPosition();
 
            float angle = theBodies.get(theBodies.size()-1).getAngle();
 
            Log.v("Physics Test", "Pos: (" + position.x + ", " + position.y + "), Angle: " + angle);
 
        }
 
    }
 
}
Parsed in 0.047 seconds,  using GeSHi 1.0.8.4

Stevos · by **Stevos** » Wed Mar 24, 2010 3:22 pm

Thanks for the interesting tutorial, I've been desperate to get a physics library running on Android, but sadly no luck with anything so far.

With the posted code on the linked site, I get a force close (as does one of the other commenters on that article.)

My android debugging skills are not good yet, but from repeatedly running and commenting out sections, I have narrowed it down to a call to

Syntax: [ Download ] [ Hide ]

Using java Syntax Highlighting

 
bodies[count] = world.createBody(bodyDef);
 
Parsed in 0.034 seconds,  using GeSHi 1.0.8.4

in the addball() method. I can't narrow it down more than that yet.

Does anyone else get anything like this?

mikezs · by **mikezs** » Thu May 06, 2010 10:57 am

Has anyone actually run this and checked the output? When I put logging on, the first (my only) ball seems to move to (6.0,1.8 ) and stays there in a single tick. Should this happen? Shouldn't it be at (6.0,11.8 ) where it collides with the groundbox?

I'm just trying to get started and the lack of info coming out is a bit rubbish!

mikezs · by **mikezs** » Fri May 07, 2010 12:45 pm

So... I solved my own problem

Code: Select all: public int timeStep = (1000 / targetFPS);

should be:

Code: Select all: public float timeStep = (1.0f / targetFPS);

Schermvlieger · by **Schermvlieger** » Wed Oct 27, 2010 2:02 pm

Hi all,

I've started playing around with this today and have a problem; the call to world.update() does not actually update the object's x,y positions. addBall() takes it's x,y positions from the onTouch event.

The parameters I use are:

Code: Select all: World.create() { worldAABB = new AABB(); // 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); 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); }

and for the object:

Code: Select all: public void addBall(float x, float y) { // Create Dynamic Body BodyDef bodyDef = new BodyDef(); bodyDef.position.set(x, WORLD_H - y); Body newBody = 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 newBody.createShape(circle); newBody.setMassFromShapes(); myBodies.add(newBody); // Increase Counter count += 1; }

Schermvlieger · by **Schermvlieger** » Wed Oct 27, 2010 4:14 pm

Okay problem solved, the world's lowerbound and upperbound values were not set.

The performance is horrible though ... I wonder how games like Angry Birds are handling their physics, must be in c code ...

How-To: 2D Physics with Box2D

Re: How-To: 2D Physics with Box2D

Re: How-To: 2D Physics with Box2D

Who is online