Manipulating sprite movement in box2d

Question

For the past days, I've been trying to make a ping pong like game. I have 2 paddles and a ball. All dynamic sprites. Everything's been working well except for one issue I'm having. The ball tends to bounce on the same angle at some point. So there would be times when the player can simply move the paddle on a specific part and the game can go on for a while or might be forever, since the ball doesn't change its angular velocity regardless of which part of the paddle it hits. I'm using a combination of linear and angular velocity to keep the ball moving like so:

if(_isPaused == FALSE)
{
    _world->Step(dt, 10, 10);
    for(b2Body *b = _world->GetBodyList(); b; b=b->GetNext()) {
        if (b->GetUserData() != NULL) {
            CCSprite *sprite = (CCSprite *)b->GetUserData();
            if(sprite.tag == 2)
            {
                b2Vec2 dir = b->GetLinearVelocity();
                dir.Normalize();
                float currentSpeed = dir.Length();
                int maxSpeed = 60;
                float accelerate = vel;

                if(currentSpeed <= maxSpeed)
                {
                    b->SetLinearVelocity(accelerate * dir);
                }

                sprite.position = ccp(b->GetPosition().x * PTM_RATIO,
                                      b->GetPosition().y * PTM_RATIO);
                sprite.rotation = -1 * CC_RADIANS_TO_DEGREES(b->GetAngle());

                //Keep sprite from bouncing in a straight angle
                b->SetAngularVelocity(_body->GetAngle());
            }}}

So my question is, how can I manipulate the angular velocity to keep the ball bouncing on different angles everytime it collides with my paddle? I'm thinking something like getting the current angular velocity then multiplying it with some value but I'm not sure if that's the right way to approach the issue I'm having. Any suggestions or ideas would be greatly appreciated. Thanks in advanced.

Rahul Banerjee Rahul Banerjee · Accepted Answer · 2013-03-08T06:51:40

The way I see it, you have two options:

Check the location of a collision. If it's close to the top/bottom edge of the paddle, deflect the outgoing velocity by an angular amount proportional to the surface "curvature" at that point. Of course, this is cheating, but if the artwork and code are in agreement, it looks correct. And graphics is "the art of cheating without getting caught".
You could take into account the current velocity of the paddle as well as that of the ball. Eg: if the ball is moving downwards and to the right, and the paddle is moving down, then you can compute the outgoing direction using conservation of linear momentum. Just make sure you restrict the paddle's change in momentum along the horizontal axis to be zero.

Finally, you could combine the above techniques, but now you'd have to use accurate collision detection (not the hack I described in (1) above).

Hope that helps!

Manipulating sprite movement in box2d

2 Answers