body falls slowly in any gravity

问题

I've created a World with earth gravity and I place an entity in the scene (contains a sprite and a Body) and it falls down slowly like a balloon.

Here's how I set the World:

world = new World(new Vector2(0, -GRAVITY_EARTH), true);

and here's the relevant Box2D code for the Body etc:

BodyDef bodyDef = new BodyDef();
bodyDef.type = BodyDef.BodyType.DynamicBody;
bodyDef.position.set(positionX, positionY);

// Create our body in the world
body = world.createBody(bodyDef);

// Grab the first idle sprite to use as initial
Sprite sprite = idleSprites.get(0);

// Create a box shape to represent our hit box
PolygonShape box = new PolygonShape();
box.setAsBox(sprite.getWidth() / 2f, sprite.getHeight() / 2f);

// Create a fixture definition to apply our shape
FixtureDef fixtureDef = new FixtureDef();
fixtureDef.shape = box;
fixtureDef.density = 1f; // Give it full density
fixtureDef.friction = 0f; // Give it no friction
fixtureDef.restitution = 0f; // Make it not bouncy

// Create our fixture and attach it to the body
fixture = body.createFixture(fixtureDef);

// Remember to dispose of any shapes after you're done with them!
// BodyDef and FixtureDef don't need disposing, but shapes do.
box.dispose();

and how I draw the sprite:

TextureRegion keyFrame = idleAnimation.getKeyFrame(stateTimeSeconds, true);
Vector2 position = body.getPosition();
batch.draw(keyFrame, position.x - keyFrame.getRegionWidth() / 2f, position.y -   keyFrame.getRegionHeight() / 2f);

and the relevant code in the render() method:

@Override
public void render () {
    Gdx.gl.glClearColor(0, 0, 0, 1);
    Gdx.gl.glClear(GL20.GL_COLOR_BUFFER_BIT);

    final float deltaTime = Gdx.graphics.getDeltaTime();

    camera.update();
    spriteBatch.setProjectionMatrix(camera.combined);

    spriteBatch.begin();
    spriteBatch.draw(sky, 0, 0);
    tim.animate(spriteBatch, deltaTime);
    spriteBatch.draw(floor, 0, 0);
    spriteBatch.end();

    // Render physics for debug
    debugRenderer.render(world, camera.combined);

    // Run physics
    doPhysicsStep(deltaTime);
}

private void doPhysicsStep(float deltaTime) {
    // fixed time step
    // max frame time to avoid spiral of death (on slow devices)
    float frameTime = Math.min(deltaTime, 0.25f);
    accumulator += frameTime;
    while (accumulator >= TIME_STEP) {
      world.step(TIME_STEP, VELOCITY_ITERATIONS, POSITION_ITERATIONS);
      accumulator -= TIME_STEP;
    }
 }

I've tried changing the density of the fixture, and I've tried changing the gravity value, and I've tried changing the TIME_STEP and nothing is having an effect. The body just falls down slowly like a balloon.

回答1:

It looks to me like you're using pixels as your units, box2d treats every unit as a meter and so you're hitting the internal limit of 2.0 units per time step, see http://www.iforce2d.net/b2dtut/gotchas. You can get around this by setting up your camera in world units instead of pixels, you have to scale all your sprites and positions to fit into world units instead of pixels though.

Something like this may do:

float w = (float) Gdx.graphics.getWidth();
float h = (float) Gdx.graphics.getHeight();
camera = new OrthographicCamera(30, 30 * (h / w));

the way the camera is set up here allows the height of the viewport to be variable based on the screens' aspect ratio.

Then to setup the sprite change it by a set factor

sprite.setSize(sprite.getWidth / PIX2M, sprite.getHeight / PIX2M);

where PIX2M is a static field defining how many pixels are a meter in box2d

Alternatively you can set the dimensions of the sprite explicitly to a value which makes physical sense and with the aspect ratio of the original image(my personal preference) . So an image of a person which is 100 x 500 for example could be set like this.

sprite.setSize(.4f, 2f);

meaning the person is 2 meters high and .4 meters wide. Also with this method you don't need a PIX2M conversion factor and you will always know the exact size of your body. Since you set the camera to a specific number of world units, 30 in this case, the sprite will take up the same amount of room on the screen no matter the resolution of the display.

来源：https://stackoverflow.com/questions/28285234/body-falls-slowly-in-any-gravity

标签

libgdx

box2d