Box2D(LibGDX) - strange behaviour of ball - android

I am trying to create simple tennis game. Each side has a wall. There are also a box and a ball. My environment doesn't have gravity. The box and the ball don't have any velocity. When the box contacts the ball(I move this by mouse with different speed), it(the ball) just changes its position and doesn't continue moving and sometimes these objects don't collide with each other:
I want the box can strike the ball using different angle and strength.
How can I do this? What should I change in the properties of the ball and the box?
The code snippet is below:
public void createBall(Vector2 position, Vector2 velocity, float angle, Object userData){
// First we create a body definition
BodyDef bodyDef = new BodyDef();
// We set our body to dynamic, for something like ground which doesnt move we would set it to StaticBody
bodyDef.type = BodyType.DynamicBody;
// Set our body's starting position in the world
bodyDef.position.set(position);
// Create our body in the world using our body definition
Body body = world.createBody(bodyDef);
body.setUserData(userData);
// Create a circle shape and set its radius to 6
CircleShape circle = new CircleShape();
circle.setRadius(10f);
PolygonShape poly = new PolygonShape();
poly.setAsBox(12, 12);
// Create a fixture definition to apply our shape to
FixtureDef fixtureDef = new FixtureDef();
fixtureDef.shape = circle;
fixtureDef.density = 0.0f;
fixtureDef.friction = 0.2f;
fixtureDef.restitution = 1f; // Make it bounce a little bit
fixtureDef.isSensor=false;
// Create our fixture and attach it to the body
Fixture f = body.createFixture(fixtureDef);
f.setUserData("ball");
circle.dispose();
}
private Body createBox(World world, float width, float height, float density) {
BodyDef def = new BodyDef();
def.type = BodyType.KinematicBody;
Body box = world.createBody(def);
PolygonShape poly = new PolygonShape();
poly.setAsBox(width/2, height/2);
FixtureDef fixtureDef = new FixtureDef();
fixtureDef.shape = poly;
fixtureDef.density = 0.0f;
fixtureDef.friction = 0.3f;
fixtureDef.restitution = 0.1f; // Make it bounce a little bit
fixtureDef.isSensor=false;
// Create our fixture and attach it to the body
Fixture f = box.createFixture(fixtureDef);
f.setUserData("platform");
poly.dispose();
return box;
}
public void createWall(World world, Vector2 position, float hx, float hy){
// Create our body definition
BodyDef groundBodyDef =new BodyDef();
// Set its world position
groundBodyDef.position.set(position);
groundBodyDef.type=BodyType.StaticBody;
// Create a body from the defintion and add it to the world
Body groundBody = world.createBody(groundBodyDef);
// Create a polygon shape
PolygonShape groundBox = new PolygonShape();
// Set the polygon shape as a box which is twice the size of our view port and 20 high
// (setAsBox takes half-width and half-height as arguments)
groundBox.setAsBox(hx, hy);
// Create a fixture from our polygon shape and add it to our ground body
groundBody.createFixture(groundBox, 0.0f);
// Clean up after ourselves
groundBox.dispose();
}

You are moving your box by changing its position. But an important part of collision resolution is velocity. And your box's velocity is always zero (from box2d look point). Thus you experience strange collision resolution

I think your screen width and height are too large... if that's the case try using world width and height ... 20x12 units.. not 800x480 something .

Box2D doesn't like bodies, which fixtures has density 0. Usual, simulation performs somehow, but behaviour is not correct. Try value 0.3 for example.
Overlap problem may be solved by setting flag b2BodyDef::bullet. There description from Box2D reference:
bool b2BodyDef::bullet
Is this a fast moving body that should be prevented from tunneling through other moving bodies? Note that all bodies are prevented from tunneling through kinematic and static bodies. This setting is only considered on dynamic bodies.
Warning:
You should use this flag sparingly since it increases processing time.

Related

Libgdx - box2d polygonshape doesn't fit its sprite

I have a body with a polygonshape created using .setasbox but when I run my game the box is a bit bigger than my sprite.
I know setasbox uses half height and half width, I used my scaling constant to convert meters to pixels and I know the sprite has the origin of the axis on the bottom left as well. Despite of that I still have a box with a width a bit larger than the sprite and this gap is the same however I change the size of the box...
This is the code I use to create my box (160 is the constant to scale meters to pixels):
public Block(World w, float halfWidth, float halfHeight, Vector2 position, Texture tex){
world = w;
bodyd = new BodyDef();
bodyd.type = BodyDef.BodyType.KinematicBody;
bodyd.gravityScale = 0;
shape = new PolygonShape();
shape.setAsBox(halfWidth, halfHeight);
fixtured = new FixtureDef();
fixtured.shape = shape;
fixtured.density = DENS;
fixtured.friction = FRIC;
fixtured.restitution = REST;
bodyd.position.set(new Vector2(position.x, position.y));
body = world.createBody(bodyd);
fixture = body.createFixture(fixtured);
body.setUserData(this);
texture = tex;
sprite = new Sprite(texture);
sprite.setSize(halfWidth * 2 * 160, halfHeight*2*160);
sprite.setPosition((body.getPosition().x - halfWidth) * 160, (body.getPosition().y - halfHeight) * 160);
}
can you try using Box2DSprite? its very easy..
https://bitbucket.org/dermetfan/libgdx-utils/wiki/net.dermetfan.gdx.graphics.g2d.Box2DSprite
http://www.java-gaming.org/index.php?topic=29843.0
I don't see anything wrong with your code
did you consider that the size that you put for your sprite is the size of the full sprite not the size of the block inside your sprite
I think this is why your brick sprite is smaller than your brick physic :
unless the your brick has the full size of the sprite then may be the problem is related to something else
hope that was helpful !

Imprecise Box2d coordinates using LibGDX

I am using LibGDX and Box2d to build my first Android game. Yay!
But I am having some serious problems with Box2d.
I have a simple stage with a rectangular Box2d body at the bottom representing the ground, and two other rectangular Box2d bodies both at the left and right representing the walls.
A Screenshot
Another Screenshot
I also have a box. This box can be touched and it moves using applyLinearImpulse, like if it was kicked. It is a DynamicBody.
What happens is that in my draw() code of the Box object, the Box2d body of the Box object is giving me a wrong value for the X axis. The value for the Y axis is fine.
Those blue "dots" on the screenshots are small textures that I printed on the box edges that body.getPosition() give me. Note how in one screenshot the dots are aligned with the actual DebugRenderer rectangle and in the other they are not.
This is what is happening: when the box moves, the alignment is lost in the movement.
The collision between the box, the ground and the walls occur precisely considering the area that the DebugRenderer renders. But body.getPosition() and fixture.testPoint() considers that area inside those blue dots.
So, somehow, Box2d is "maintaining" these two areas for the same body.
I thought that this could be some kind of "loss of precision" between my conversions of pixels and meters (I am scaling by 100 times) but the Y axis uses the same technique and it's fine.
So, I thought that I might be missing something.
Edit 1
I am converting from Box coordinates to World coordinates. If you see the blue debug sprites in the screenshots, they form the box almost perfectly.
public static final float WORLD_TO_BOX = 0.01f;
public static final float BOX_TO_WORLD = 100f;
The box render code:
public void draw(Batch batch, float alpha) {
x = (body.getPosition().x - width/2) * TheBox.BOX_TO_WORLD;
y = (body.getPosition().y - height/2) * TheBox.BOX_TO_WORLD;
float xend = (body.getPosition().x + width/2) * TheBox.BOX_TO_WORLD;
float yend = (body.getPosition().y + height/2) * TheBox.BOX_TO_WORLD;
batch.draw(texture, x, y);
batch.draw(texture, x, yend);
batch.draw(texture, xend, yend);
batch.draw(texture, xend, y);
}
Edit 2
I am starting to suspect the camera. I got the DebugRenderer and a scene2d Stage. Here is the code:
My screen resolution (Nexus 5, and it's portrait):
public static final int SCREEN_WIDTH = 1080;
public static final int SCREEN_HEIGHT = 1920;
At the startup:
// ...
stage = new Stage(SCREEN_WIDTH, SCREEN_HEIGHT, true);
camera = new OrthographicCamera();
camera.setToOrtho(false, SCREEN_WIDTH, SCREEN_HEIGHT);
debugMatrix = camera.combined.cpy();
debugMatrix.scale(BOX_TO_WORLD, BOX_TO_WORLD, 1.0f);
debugRenderer = new Box2DDebugRenderer();
// ...
Now, the render() code:
public void render() {
Gdx.gl.glClearColor(0, 0, 0, 1);
Gdx.gl.glClear(GL10.GL_COLOR_BUFFER_BIT);
camera.update();
world.step(1/45f, 6, 6);
world.clearForces();
stage.act(Gdx.graphics.getDeltaTime());
stage.draw();
debugRenderer.render(world, debugMatrix);
}
Looks like the answer to that one was fairly simple:
stage.setCamera(camera);
I was not setting the OrthographicCamera to the stage, so the stage was using some kind of default camera that wasn't aligned with my stuff.
It had nothing to do with Box2d in the end. Box2d was returning healthy values, but theses values were corresponding to wrong places in my screen because of the wrong stage resolution.

Why physics body automatically moves upwards in andengine gles2 anchor center live wallpaper

I am trying to develop a live wallpaper using andengine gles2 anchor center , with some physics.But when i add a physics object it was moving upwards.instead of moving downward due to gravity
what are the mistakes i am making
please help me to sort out the issue
Here is my code
FixtureDef FIXTURE_DEF = PhysicsFactory.createFixtureDef(1, 0.5f,
0.5f);
mPhysicsWorld = new PhysicsWorld(new Vector2(0,
SensorManager.GRAVITY_EARTH), false);
final AnimatedSprite animatedSprite;
animatedSprite = new AnimatedSprite(500, 250,
this.mBoxFaceTextureRegion, this.getVertexBufferObjectManager());
body = PhysicsFactory.createBoxBody(this.mPhysicsWorld, animatedSprite,
BodyType.DynamicBody, FIXTURE_DEF);
scene.attachChild(animatedSprite);
animatedSprite.animate(200);
mPhysicsWorld.registerPhysicsConnector(new PhysicsConnector(
animatedSprite, body, true, true));
Just multiply SensorManager.GRAVITY_EARTH by -1.
Setting negative gravity did not respond properly to sensor data. By adding acceleration to the sensor data in the overridden method **onAccelerationChanged()**, could make the bject falling down with sensor flat.
public void onAccelerationChanged(final AccelerationData pAccelerationData) {
/* Add constant value for vertical gravity*/
final Vector2 gravity = Vector2Pool.obtain(pAccelerationData.getX(), pAccelerationData.getY() + 4.0);
this.mPhysicsWorld.setGravity(gravity);
Vector2Pool.recycle(gravity);
}
This will make your phy world in real gravity experience and you will get objects influenced by sensor data properly

Sprite collisions in android andengine

I'm developing a simple game by andengine.
I have 10 balls which are moving randomly on screen.i'm importing the balls as picture in sprites.if they move at the same coordinate , they pass though their own insides.but i want: if they move at the same coodirnates ,they should change their directions.so they cannot pass through their insides.how can i do that?
private Runnable mStartCircle = new Runnable() {
public void run() {
int i = circleNumber++;
Scene scene = Level1Activity.this.mEngine.getScene();
float startY = -64.0f;
float startX = randomNumber.nextFloat()*(CAMERA_WIDTH-70.0f);
float a= randomNumber.nextFloat()*(CAMERA_WIDTH-70.0f);
circles[i] = new Sprite(startX, startY, textRegCircle[i]);
circles[i].registerEntityModifier(
(IEntityModifier) new SequenceEntityModifier (
new MoveModifier(10.0f, circles[i].getX(), a,
circles[i].getY(),CAMERA_HEIGHT+64.0f)));
}
scene.getLastChild().attachChild(circles[i]);
if (circleNumber < 10){
mHandler.postDelayed(mStartCircle,1000);
}
}
};
Each object(ball) requires a bounding box, or in your case a bounding circle, which is equal to the size of your sprite.
When the game updates and any balls position changes, you have to test for collisions.
Circle to circle collision testing is the simplest type to do.
if distance between (ball1.pos + ball2.pos) is less than (ball1.radius + ball2.radius) = collision.
You then handle the collision by reversing the velocities or calculating new momentums or something. (You also need to move the objects apart so they are no longer colliding)
Just apply a physical connector between balls:
so it will collide and bounce back.
final FixtureDef boxFixtureDef = PhysicsFactory.createFixtureDef(0.1f, 0.5f, 0.5f);
final Body ballBody = PhysicsFactory.createBoxBody(this.mPhysicsWorld, circles[i],BodyType.DynamicBody, boxFixtureDef);
this.mPhysicsWorld.registerPhysicsConnector(new PhysicsConnector(circles[i], ballBody, true, true));
this.mScene.attachChild(circles[i]);

How to change the center of mass?

I am try
MassData md = mBody.getMassData();
md.center.set(2f, 0); mBody.setMassData(md);
But this is not working properly. Help me do this correctly.
I want add a bit more weight to the bottom half of my body.
Instead of trying to change mass directly, what you want to do is to add an extra fixture to the bottom of your body. You can do it e.g. like this (this is a snippet, not complete code; create your textures, scene etc).
Sprite sprite = new Sprite(CAMERA_WIDTH / 2, CAMERA_HEIGHT / 2, 32, 32, mDTextureRegion, vbom);
Body body = PhysicsFactory.createCircleBody(mPhysicsWorld, sprite, BodyType.DynamicBody, PhysicsFactory.createFixtureDef(0.5f, 0.5f, 0.5f)); // create a simple circle body from the sprite
scene.attachChild(sprite);
Debug.i("ORIG MASS Y: " + body.getMassData().center.y); // this is just generated info
FixtureDef fd = PhysicsFactory.createFixtureDef(10f, 1f, 1f, true); // high density fixture; it is a sensor, i.e. it won't affect collisions
CircleShape cs = new CircleShape();
cs.setRadius(0.1f); // make it very small
fd.shape = cs;
cs.setPosition(new Vector2(0, -0.5f)); // attach below the current centre
body.createFixture(fd); // attach to body
Debug.i("NEW MASS Y: " + mDBody.getMassData().center.y); // the generated info changed
mPhysicsWorld.registerPhysicsConnector(new PhysicsConnector(sprite, body, true, true));
This example body will behave like a spintop, it will try to reposition itself so the bottom of the sprite will touch the ground. When you rotate it, it will try to get to that position again.

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