Missing gravity in box2d libgdx

Question

I'm trying make a sample with box2d in libgdx.

I want the ball fall with gravity in box2d world. Now it fall but without acceleration (I see the ball is falling with acceleration =0) . I tried to add Damping, density... but there are no changes. Please help me find my fault. Thank you so much.
This's the video for this issue: youtube.com/watch?v=NkbcVMsB2S0
; In this video you can see the circle is falling with acceleration=0. I tried to change some parametters, but look like acceleration is always zero. This's a simple class:

public class MyGdxGame implements ApplicationListener, InputProcessor {
World world;
Box2DDebugRenderer debugRenderer;
OrthographicCamera camera;
static final float BOX_STEP = 1 / 45f;
static final int BOX_VELOCITY_ITERATIONS = 6;
static final int BOX_POSITION_ITERATIONS = 2;
static final float WORLD_TO_BOX = 0.01f;
static final float BOX_WORLD_TO = 100f;

private int counter = 100;
private Body body;

@Override
public void create() {
    world = new World(new Vector2(0, -200f), false);
    world.setContinuousPhysics(true);
    camera = new OrthographicCamera();
    camera.viewportHeight = 320;
    camera.viewportWidth = 480;
    camera.position.set(camera.viewportWidth * .5f,
            camera.viewportHeight * .5f, 0f);
    camera.update();
    // Ground body
    BodyDef groundBodyDef = new BodyDef();
    groundBodyDef.position.set(new Vector2(0, 10));
    Body groundBody = world.createBody(groundBodyDef);
    PolygonShape groundBox = new PolygonShape();
    groundBody.createFixture(groundBox, 0.0f);
    // Dynamic Body
    BodyDef bodyDef = new BodyDef();
    bodyDef.type = BodyType.DynamicBody;
    bodyDef.position.set(camera.viewportWidth / 2, camera.viewportHeight);
    bodyDef.angularDamping = 0.01f;
    bodyDef.linearDamping = 0.0f;
    body = world.createBody(bodyDef);
    CircleShape dynamicCircle = new CircleShape();
    dynamicCircle.setRadius(10f);
    FixtureDef fixtureDef = new FixtureDef();
    fixtureDef.shape = dynamicCircle;
    fixtureDef.density = 10.0f;
    fixtureDef.friction = 0.0f;
    fixtureDef.restitution = 0f;
    body.createFixture(fixtureDef);
    MassData massData = new MassData();
    massData.mass = 50f;
    body.setMassData(massData);
    body.setGravityScale(1f);
    debugRenderer = new Box2DDebugRenderer();
    Gdx.input.setInputProcessor(this);
}

@Override
public void dispose() {
}

@Override
public void render() {
    Gdx.gl.glClear(GL10.GL_COLOR_BUFFER_BIT);

    if (counter < 10) {
        body.applyForceToCenter(0, 200000f, true);
    }
    counter++;
    debugRenderer.render(world, camera.combined);
    world.step(BOX_STEP, BOX_VELOCITY_ITERATIONS, BOX_POSITION_ITERATIONS);
}

@Override
public void resize(int width, int height) {
}

@Override
public void pause() {
}

@Override
public void resume() {
}

@Override
public boolean keyDown(int keycode) {
    // TODO Auto-generated method stub
    return false;
}

@Override
public boolean keyUp(int keycode) {
    // TODO Auto-generated method stub
    return false;
}

@Override
public boolean keyTyped(char character) {
    // TODO Auto-generated method stub
    return false;
}

@Override
public boolean touchDown(int screenX, int screenY, int pointer, int button) {
    counter = 0;
    return false;
}

@Override
public boolean touchUp(int screenX, int screenY, int pointer, int button) {
    return false;
}

@Override
public boolean touchDragged(int screenX, int screenY, int pointer) {
    // TODO Auto-generated method stub
    return false;
}

@Override
public boolean mouseMoved(int screenX, int screenY) {
    // TODO Auto-generated method stub
    return false;
}

@Override
public boolean scrolled(int amount) {
    // TODO Auto-generated method stub
    return false;
}

I don't understand your problem. Do you want it to fall or not to fall? Does it fall, or does it not? — noone
Sorry for my unclear question. I want the ball fall with gravity. Now it fall but without gravity (I see the ball is falling with acceleration =0) — Leo Nguyen
I don't get it, in this line world = new World(new Vector2(0, -200f), false); you set the gravity to an extremely high value (about 20 times the gravitational force on earth) and then you say it is falling without gravity? Why do you think it is accelarating downwards, if it is not gravity? — noone
I mean there are no acceleration. Wait me post the video for this. — Leo Nguyen
This's the video for this issue: youtube.com/watch?v=NkbcVMsB2S0<br> In this video you can see the circle is falling with acceleration=0. I tried to change some parametters, but look like acceleration is always zero.. — Leo Nguyen

noone noone · Accepted Answer · 2014-03-09T14:00:59

I think that's because the body is instantly at maximum velocity (because of the very high gravity). Box2D has a maximum velocity and I think it's 2m per step. It seems that you run it on a mobile device which are usually limited to 60FPS. That means that you have max 60 update steps per second, resulting in a maximum velocity/speed of 120m/s. By the way, your update rate is framerate dependent.

I see that you have defined the meter-to-pixel and pixel-to-meter constants, but you aren't using them anywhere in your code. That means that you are using a meter-to-pixel ratio of 1:1 and thus the maximum speed any body can have is 120px/s. That's not a lot and judging by that video it could be correct.

A common trick is to scale your camera's viewport down instead of scaling all box2d related things up. Like this:

camera.viewportHeight = Gdx.graphics.getHeight() * WORLD_TO_BOX;
camera.viewportWidth = Gdx.graphics.getWidth() * WORLD_TO_BOX;

Missing gravity in box2d libgdx

2 Answers