Member Avatar for hwoarang69

Hi,
I am using Java, Libgdx, box2d

I main class I have created a body and I set player x and y position to 50.

// main class
public class main{
  ...
  private void createPlayer() {
    BodyDef bdef = new BodyDef();
    Body body;
    FixtureDef fdef = new FixtureDef();
    PolygonShape shape = new PolygonShape();

    /*** Body - Player ***/
    bdef.type = BodyType.DynamicBody;
    bdef.position.set(50 / PPM, 50 / PPM);
    bdef.linearVelocity.set(1.5f, 0);
    body = world.createBody(bdef);
        .....
  }
}

How can I set player x and y position to 100.

//player class
public class player{
       public player(Body body){
             super(body);
       }

       ....
       public void update(){
             //get player x position
             currentX = this.getBody().getPosition().x;

             //how to change player x and y position to 100?   
       }
}
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  • 3 Replies
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  • 4 Days Discussion Span
  • Latest Post Latest Post by Taywin

Dani AI

Generated

Short answer: BodyDef is only used when you call world.createBody(...). Changing the BodyDef later does not move an existing Body. Pass and keep the created Body in your Player object (not just the BodyDef), then move the Body itself. 's suggestion to "update" with a BodyDef won't work because the engine ignores BodyDef after creation. was right to point you at setTransform — that is how you teleport a body, but there are important caveats below.

Use setTransform to teleport (remember to convert pixels -> meters with your PPM), and avoid calling it while the world is locked. Queue teleports if needed:

Vector2 target = new Vector2(100f / PPM, 100f / PPM);

if (!world.isLocked()) {
    body.setTransform(target, body.getAngle());
    body.setLinearVelocity(0f, 0f);   // optional: stop any motion after teleport
    body.setAngularVelocity(0f);
} else {
    queuedTeleport = true;
    queuedPosition.set(target);
}

/* after world.step(...) */
if (queuedTeleport) {
    body.setTransform(queuedPosition, body.getAngle());
    queuedTeleport = false;
}

If you want smooth motion instead of teleporting, compute and set a velocity or apply impulses rather than changing transform:

Vector2 delta = new Vector2(targetX/PPM, targetY/PPM).sub(body.getPosition());
Vector2 vel = new Vector2(delta.x / timeToReach, delta.y / timeToReach);
body.setLinearVelocity(vel);

Extra tips: keep all conversions consistent (Box2D uses meters), avoid repeatedly teleporting dynamic bodies while they collide (can cause tunneling), and prefer setting the initial position in BodyDef before creation if the player should start somewhere else. Finally, change the Player constructor to accept the Body you created so update() can call body.setTransform(...) or body.setLinearVelocity(...).

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Member Avatar for Slavi

update takes an object of type BodyDef. Then you can set it using the setters just as in the object bdef

Member Avatar for hwoarang69

here is what i tried. but didnt worked bc CreatePlayer() method is in Main class constructor.

// main class

public class main{
  ...
  public main(){
      createPlayer();
  }

  private void createPlayer() {
    BodyDef bdef = new BodyDef();
    Body body;
    FixtureDef fdef = new FixtureDef();
    PolygonShape shape = new PolygonShape();

    /*** Body - Player ***/
    bdef.type = BodyType.DynamicBody;
    bdef.position.set(50 / PPM, 50 / PPM);
    bdef.linearVelocity.set(1.5f, 0);
    body = world.createBody(bdef);
        .....
    player = new Player(bdef);
  }

  ....

  public void update(float dt) {
      playerObj.update(dt);
      ...
  }
}

//player class

public class player{
       public player(Body body, BodyDef bdef){
             super(body);
             this.bdef = bdef;
       }

       ....
       public void update(){
             //get player x position
             currentX = this.getBody().getPosition().x;

             //how to change player x and y position to 100? 
             bdef.position.set(100, 100);
       }
}
Member Avatar for Taywin

Huh? You passed in only 1 argument in creating new Player object but require 2 arguments in the Player constructor class?

After looking Body API doc, there is a method called setTransform(Vector2, float) which should allow you to set the position but it takes the angle of rotation as well. Maybe that's the one you should use?

Edited by Taywin
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