Box2d - shoot ball with high velocity

Question

My problem is that no matter the tuning I don't achieve a fast moving ball. It floats through the air rather than shoots through it.

In the following method I'm creating a ball with the radius of 5 units. Setting the radius to 0.01 yields no difference in the result.

After the creation I apply a LinearImpulse of ridiculous proportions yet, the ball only floats. (Also, I'm using the DebugRender. Isn't that supposed to take care of the unit conversions?)

Ball class

public Body createBody(float x, float y, float w, float h) {
    BodyDef bodyDef = new BodyDef();
    bodyDef.type = BodyDef.BodyType.DynamicBody;
    bodyDef.position.set(x,y);
    Body body = this.getWorld().createBody(bodyDef);

    CircleShape shape = new CircleShape();
    shape.setRadius(5);
    shape.setPosition(new Vector2(0, 0));  // relative to body position

    FixtureDef fixtureDef = new FixtureDef();
    fixtureDef.shape = shape;
    fixtureDef.density = 3f;
    fixtureDef.friction = 0.1f;
    fixtureDef.restitution = 0.8f;
    body.createFixture(fixtureDef);

    shape.dispose();
    // note the massive impulse vector
    body.applyLinearImpulse(new Vector2(10000, 100000), body.getWorldCenter(), true);

    return body;
}

render method in Game class

@Override
public void render(){

    Gdx.gl.glClearColor(0.0f, 0.0f, 0.0f, 1.0f);
    Gdx.gl.glClear(GL10.GL_COLOR_BUFFER_BIT);
    debugRenderer.render(world, camera.combined);

    // time step, velocityIterations, positionIterations
    world.step(1/40.0f, 6, 2);

    // scheduler takes care of creating game objects on the fly
}

It might be because Box2D max velocity is limited by the updaterate. How many FPS do you have? And maybe try something like world.step(Gdx.graphics.getDeltaTime()). — noone, Feb 24 '14 at 13:38
Changing the world.step() as suggested did indeed increase the experienced velocity a bit. But I have a feeling this isn't the solution. Since I'm using the debugrenderer, shouldn't all objects be scaled properly automatically? — droidballoon, Feb 24 '14 at 16:02

score 3 · Accepted Answer · edited May 23 '17 at 12:33

Okay, together with my answer to this question Is there an upper limit on velocity when using box2d? I'll try to explain your problem.

Box2D has a hard limit on velocity, which is 2meters per time step. That means the more timesteps you do per second, the faster your Bodies will be able to move. But since you actually want to have a fixed amount of timesteps per second, going with more timesteps is usually not an option.

What you should do instead is introducing a meter to pixel ratio which could be something in between 1:16 to 1:128. That means 1 meter in box2d should be translated to 16 or 128 pixels in screen units. That will increase the overall precision, because Box2D is more precise in smaller ranges. If you have a ratio of 1:1, it would mean that a standard screen these days would span a range of 1980m x 1080m, which is definetely too much. You can also see that with 60fps = 60 timesteps = 60*2m max distance per second, that would be just 120pixels per second at max speed. And this is what you currently experience as the floating body.

If you would translate 1m to 16px for example, your max speed would increase to 120px*16 = 7200px per second. That would already be sufficient in your case I guess.

Furthermore you should also set body.bullet to true to increase the collision precision for very fast moving bodies.

Great answer! I think the one thing that stumps me a bit here is the debug renderer then. I had assumed that the debug renderer took care of all world-to-pixel conversions. I'll go with your suggestion of implementing a meter/pixel-ratio. — droidballoon, Feb 24 '14 at 17:12
No, the meter to pixel ratio is something that you have to care about yourself. However there is a very easy solution. Make an extra camera which has its viewport scaled by the ratio. That means something like `Camera cam = new OrthographicCamera(Gdx.graphics.getWidth() * PIXEL_TO_METER, Gdx.graphics.getHeight() * PIXEL_TO_METER)` and supply that to your DebugRenderer — noone, Feb 24 '14 at 18:31

daniel · Answer 2 · 2014-02-24T15:44:42.553

2

One of the reason is World scale, you may probably have a very big size world scale it done if it is a big.

It suggested that for better accuracy use set bullet to true however it takes more calculation

body.bullet = true;

edited Feb 24 '14 at 15:44

answered Feb 24 '14 at 13:57

daniel

697
5
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Doing this shouldn't help with the velocity. It will just improve the collisions. – noone Feb 24 '14 at 13:58
Indeed, it has no effect on the actual velocity as I can see. – droidballoon Feb 24 '14 at 15:29
About the world scale, I haven't defined any scales anywhere and I'm only setting sizes using ``shape.setRadius(5)``. Say I'd set the radius to 1 and using the debugrenderer, shouldn't that be a proper value? – droidballoon Feb 24 '14 at 16:05
this is weird, notice setRadius is like skin with how much fats, so on any shape you can set it to any value you want even negative, it is just an offset, but for circle-shape it is radius. – daniel Feb 24 '14 at 17:26

Box2d - shoot ball with high velocity

2 Answers2