Box2D LibGDX Rope issue

0
votes

I am creating a rope with a series of Box2D bodies with the following code:

public void create(float length, float ropeLength){
    Array<Body> bodies = new Array<Body>();
    bodies.add(BodyFactory.createBox(world, position.x, position.y, length, length, BodyType.StaticBody, 0, 0, 0, "RopeMain"));

    for(int i = 1; i < ropeLength; i++){
        bodies.add(BodyFactory.createBox(world, position.x, position.y - (((length/2) / Core.PPM) * i),
                length, length, BodyType.DynamicBody, 0, 0, 0, "RopeBody" + i));

        RopeJointDef rDef = new RopeJointDef();
        rDef.bodyA = bodies.get(i - 1);
        rDef.bodyB = bodies.get(i);
        rDef.collideConnected = true;
        rDef.maxLength = (length/2)/Core.PPM;
        rDef.localAnchorA.set(position.x, -((length / 2) / Core.PPM));
        rDef.localAnchorB.set(position.x, ((length / 2) / Core.PPM));
        world.createJoint(rDef);
    }
}

Allow me to share some parameters...

For BodyFactory.createBox it requires the following:

world, xPos, yPos, width, height BodyType, density, friction, restitution, fixture user data.(length is same because it uses boxes)

Core.PPM is the pixels per meter. Also note that the position is being divided by PPM in the constructor.

Question: why do the following lines shoot to the right?

enter image description here

Any info is very helpful, also how will density, friction, and restitution affect the rope? Thanks!

1
javalibgdx

1 Answers

2
votes

The joint's localAnchor is relative to the center of the body and isn't an absolute value. That means that if you want to set the joint to the center-bottom of bodyA and center-top of bodyB you need to use

    rDef.localAnchorA.set(0, -((length / 2) / Core.PPM));
    rDef.localAnchorB.set(0, ((length / 2) / Core.PPM));