Box2d set sprite to body that rotates around specific point

Question

I want to rotate kinematic body around a specific point (I already done this part using

shape.setAsBox(1.8f / PPM, 0.2f / PPM, new Vector2(5.2f, 0), 0); 

What I try is this:

PolygonShape shape = new PolygonShape();
shape.setAsBox(1.8f / PPM, 0.2f / PPM, new Vector2(5.2f, 0), 0); // !!!!

FixtureDef fdef = new FixtureDef();
fdef.shape = shape;
fdef.filter.categoryBits = categoryBits;
fdef.filter.maskBits = Constants.PLAYER;

bDef.position.set(((V_WIDTH / 32 - 3) / 2) / PPM, y / PPM);

body = world.createBody(bDef).createFixture(fdef).getBody();
body.setAngularVelocity(1f);
body.setUserData(new BodyUserData(categoryBits, colorArray[0]));

sprite.setOrigin(sprite.getWidth() / 2, sprite.getHeight() / 2);
sprite.setPosition(
    body.getPosition().x * 32 - sprite.getWidth() / 2, 
    body.getPosition().y * 32 - sprite.getHeight() / 2
);

and here how I draw sprite

public void draw(SpriteBatch sb){
    sprite.setRotation(body.getAngle() * MathUtils.radiansToDegrees);
    sprite.draw(sb);
}

Unfortunately my sprite position is not the same as my body. It seems that sprite is rotating around it's center, not my point. Is it possible to fix that?

Answer 1

Just setting your bodys angular velocity means that it rotates around its center. You never definded the point it should rotate around.

Sadly in Box2d you can't define a rotation origin, but you could try this:

1. Define a static body for your fixed point
2. Define your rotating body
3. Use a Joint to link these 2 bodies
4. Apply a force to your rotating body
5. Update your sprites position to the bodys position each time you draw it

---

Or you could calculate the circle of rotation by yourself and set the sprites position before each draw, like @Hamza Hasan described in his answer

---

If you want your sprite to rotate around a specific point, change its origin relative to that point:

// This sets the sprites origin to its center, letting it rotate around its center
sprite.setOrigin(sprite.getWidth() / 2, sprite.getHeight() / 2);
// If you want it to rotate around a point (e.g. 50 pixels to the left) you could try
sprite.setOrigin(-50, sprite.getHeight() / 2);

You just have to calculate the distance of your sprite to the point:

Vector2 specificPoint = new Vector2(200, 400);
Vector2 position = sprite.getPosition();
Vector2 distance = new Vector2(position.x + sprite.getWidth() / 2 - specificPoint.x, 
                              position.y + sprite.getHeight() / 2 - specificPoint.y);
sprite.setOrigin(distance);

Answer 2

I don't know libgdx, but I know something that might help you, in fact some trigonometry will help you at this point.

Normally I use parametric equation of circle for this purpose if I am unable to do it by all the other ways.

Here is the parametric equation of circle

x = r * Cos(theta) + displacementX
y = r * Sin(theta) + displacementY

where,

• r = radius
• theta = angle that will change upon time. (in gameloop)
• displacementX and displacementY would be the center point of the body in center. ( if displacementX and displacementY both are zero then this equation will rotate the body around origin )

Lets say, you have a box as centre point and a circle as the sprite or object that will rotate around the box.

pseudo-code:

r = 5; // radius or distance from the box
theta = 0; // could be any, up to you
rotationSpeed = 0.5; // Change in angle, also represents the rotation speed

// public void draw(SpriteBatch sb) in your case
Gameloop()
{
    displacementX = box.x
    displacementY = box.y

    circle.x = r * Cos(theta) + displacementX
    circle.y = r * Sin(theta) + displacementY

    theta += rotationSpeed // from this you can control the rotation speed

    if (theta >= 360)
        theta = 0;
} 

Answer 3

You need to set sprite position to body's position each render frame, not only in initialization method.