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Asked: 2026-06-18T06:57:43+00:00

How do I create a body [A ball Body] that will bounce around the

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How do I create a body [A ball Body] that will bounce around the screen, never losing or gaining speed, regardless of what it hits in cocos2d-box2d?

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  1. Editorial Team

    Set restitution of the fixture to 1, and friction to 0.

    Box2D manual says:

    Restitution is used to make objects bounce. The restitution value is usually set to be between 0 and 1. Consider dropping a ball on a table. <…> A value of one means the ball’s velocity will be exactly reflected. This is called a perfectly elastic collision.

    A friction value of 0 turns off friction

    Without friction and with perfectly elastic collision your ball will bounce around the screen, never losing or gaining speed in a static environment. If environment is not static, then colliding with moving object will change speed of the ball.

    To solve this problem, I suggest next trick. Set contact listener, and in PostSolve method correct speed of you ball like this:

    void PostSolve(b2Contact* contact, const b2ContactImpulse* impulse)
{  
   if(contact->GetFixtureA()->GetBody() == YOUR_BALL_BODY ||
      contact->GetFixtureB()->GetBody() == YOUR_BALL_BODY)
   {
       float speed = YOUR_BALL_BODY->GetLinearVelocity().Length();
       float koef = YOUR_NEEDED_SPEED / speed;
       YOUR_BALL_BODY->SetLinearVelocity(koef * YOUR_BALL_BODY->GetLinearVelocity());
   }
}

    How to set contact listener see there.

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