How can I attach a libGDX actor to a box2D body?

Question

I'm creating game like hill climb racing.

I created my car with the help of shapes and joints but I don't know how to attach graphics to it. I used actors and sprites before for the sake of learning, but I don't know how to use those with box2d bodies.

How can I?

Answer 1

I have seen two ways of achieving this, one is to let the UserData of the Box2D Body hold the Sprite.

body.setUserData(mySprite);

And in the update you retrieve that and apply position and rotation to the Sprite according to the Body's transform;

Sprite sprite = (Sprite)body.getUserData();
Vector2 position = body.getPosition();
// Center body is center sprite here
float hw = sprite.getWidth() / 2.0f;
float hh = sprite.getHeight() / 2.0f;
float a = body.getAngle() * MathUtils.radiansToDegrees;
float x = position.x - hw;
float y = position.y - hh;

sprite.setPosition(x, y);
sprite.setRotation(a);

The other approach is to keep track of an object that is the amalgamation of the Sprite and the Body;

public class MyGameObject {
  private Body body;
  private Sprite sprite;

  public void update() {
    Vector2 position = body.getPosition();
    // Center body is center sprite here
    float hw = sprite.getWidth() / 2.0f;
    float hh = sprite.getHeight() / 2.0f;
    float a = body.getAngle() * MathUtils.radiansToDegrees;
    float x = position.x - hw;
    float y = position.y - hh;

    sprite.setPosition(x, y);
    sprite.setRotation(a);
  }
}

I think it's largely a matter of preference, I prefer the UserData approach myself as I need to "manually" keep track of less things that way.

Answer 2

You can use the setUserData method to attach an Object to the box2D body (like for example a Sprite or an Actor):

body.setUserData(sprite);

Accordingly you can get your Sprite back like this:

(Sprite) body.getUserData();

Note that you need to cast to Sprite.