Ball with the new Sprite kit

Question

I'm trying to make a simple forever bouncing ball with new sprite kit on ios 7. I set gravity:

 scene.physicsWorld.gravity=CGVectorMake(0, -9);

And for the ball body I set:

ball.physicsBody=[SKPhysicsBody bodyWithCircleOfRadius:17];
ball.physicsBody.mass=1;
ball.physicsBody.restitution=1;
ball.physicsBody.linearDamping=0;
ball.physicsBody.angularDamping=0;

Ball here is just SKSpriteNode. but i have one problem: every time it bounce back its position become a little bit higher and higher. And in a few minutes ball is almost at the top of the screen. Can anyone help with this issue. I just wanna make ball bounce back to the same position every time (for example middle of the screen). Thanks.

Try making the vector (0,9) instead of (0,-9) and tell me what happens — erdekhayser
the same problem but ball now bounce from the top of the screen (like if the ground is on the top) — Alexander
That's what I thought would happen. You are saying that every bounce, the ball bounces higher? — erdekhayser
Exactly. but i wanna make it bounces to the same height. Like infinite bouncing. — Alexander
do you have other bodies and if so, what are their friction and restitution values set to? Do you apply any forces to the ball or otherwise change velocity? — LearnCocos2D

rickster rickster · Accepted Answer · 2013-09-30T17:31:09

There are probably two issues in play here:

Two bodies are involved in the collision when the ball bounces -- the ball and the edge loop that it bounces off of. For no energy to be lost in the collision, you probably want zero friction and restitution on both bodies.
Even if you do that, though, many physics engines (including SpriteKit's) have trouble with situations like this because of floating point rounding errors. I've found that when I want a body to keep a constant speed after a collision, it's best to force it to -- use a didEndContact: or didSimulatePhysics handler to reset the moving body's velocity so it's going the same speed it was before the collision (but in the opposite direction).

If you just have a ball bouncing up and down, that velocity reset is easy: just negate the vertical component of the velocity vector:

ball.physicsBody.velocity = CGVectorMake(0, -ball.physicsBody.velocity.dy);

If you're bouncing off an angled surface, it's slightly more complicated: normalize the velocity vector (i.e. scale it so its magnitude is 1.0) to get the direction after the collision, then scale that to make it the speed you want. When I'm doing that kind of vector math, I like to convert the vectors to GLKVector2 so I can use the fast math functions from the GLKit framework:

GLKVector2 velocity = GLKVector2Make(ball.physicsBody.velocity.dx, ball.physicsBody.velocity.dy);
GLKVector2 direction = GLKVector2Normalize(velocity);
GLKVector2 newVelocity = GLKVector2MultiplyScalar(direction, newSpeed);
ball.physicsBody.velocity = CGVectorMake(newVelocity.x, newVelocity.y);

Ball with the new Sprite kit

2 Answers