Get position of Fixtures

Question

I'm constructing a Body (a box2d body using AndEngine) using multiple Fixtures and later I need to get the world position of these Fixtures.

Currently I am trying something like this;

// Declarations
Body body;
Fixture centerFixture;
Fixture leftFixture;
Vector2 centerPos = new Vector2(0, 0);
Vector2 leftPos = new Vector2(-1.5f, -1.5f);

void setup() {
  BodyDef bodyDef = new BodyDef();
  bodyDef.type = BodyType.DynamicBody;
  bodyDef.position.set(new Vector2(10, 10));

  body = physicsWorld.createBody(bodyDef);

  PolygonShape centerShape = new PolygonShape();        
  centerShape.setAsBox(1, 1, centerPos, 0);
  FixtureDef centerDef = new FixtureDef();
  centerDef.density=1;
  centerDef.shape=centerShape;
  centerFixture = body.createFixture(centerDef);

  PolygonShape leftShape = new PolygonShape();      
  leftShape.setAsBox(0.5f, 0.5f, leftPos, 0);
  FixtureDef leftDef = new FixtureDef();
  leftDef.density=1;
  leftDef.shape=centerShape;
  leftFixture = body.createFixtureleftDef);
}

void someFunction() {

  Transform transform = body.getTransform();
  Vector2 temp = new Vector2(leftPos);

  // These two seems to not be the center of the fixture 
  Vector2 worldPosition = transform.mul(temp);  
  Vector2 pixelPosition = new Vector2(worldPosition.x * PhysicsConstants.PIXELS_PER_METER, worldPosition.y * PhysicsConstants.PIXELS_PER_METER);
}

But is seems that as the body rotates the worldPosition is off.

How do I get the world position of the Fixture?

Answer 1

As I understood your question, you want to get the coordinates of the points of your body relative to the world, in other words, following the transformations applied to the body.

I did this function not long ago, I hope this will help you and guide you towards an answer:

public Vec2[] getPoints() {
    Vec2[] v = new Vec2[shape.getVertexCount()];
    for (int i = 0; i < shape.getVertexCount(); i++) {
        v[i] = body.getWorldPoint(shape.getVertex(i));
        v[i].x *= world.getMeter();
        v[i].y *= world.getMeter();
    }
    return v;
}

Here I used a PolygonShape for my body, but you can replace this by the vertices of your ficture, I think. What's important is that function:

body.getWorldPoint(shape.getVertex(i));

This will return a point of the body (Ex: (0, 0)) and return the same point with the transformations applied (Ex: (1, 0) if you rotated 90° clock-wise).