In Box2D ball bouncing before hitting the floor - ios

Question

I implemented a basic box2d project in iOS with a bouncing ball, but the ball seems to bounce before it actually reaches the ground. On each progressive bounce it seems to get closer to the bottom and eventually rests on the ground. I took a video of what happens: http://f.cl.ly/items/1S06373Z1l2w1z243E0k/Bounce.m4v
Here is where I set everything up:
CGSize screenSize = self.view.bounds.size;
// Define the gravity vector.
b2Vec2 gravity;
gravity.Set(0.0f, -9.81f);
bool doSleep = true;
// Construct a world object, which will hold and simulate the rigid bodies.
world = new b2World(gravity);
world->SetAllowSleeping(doSleep);
world->SetContinuousPhysics(true);
b2BodyDef groundBodyDef;
groundBodyDef.position.Set(0,0);
b2Body *groundBody = world->CreateBody(&groundBodyDef);
b2EdgeShape groundEdge;
b2FixtureDef boxShapeDef;
boxShapeDef.shape = &groundEdge;
//bottom
groundEdge.Set(b2Vec2(0,0), b2Vec2(screenSize.width/PTM_RATIO, 0));
groundBody->CreateFixture(&boxShapeDef);
// left wall
groundEdge.Set(b2Vec2(0,0), b2Vec2(0,screenSize.height/PTM_RATIO));
groundBody->CreateFixture(&boxShapeDef);
// top
groundEdge.Set(b2Vec2(0, screenSize.height/PTM_RATIO),
b2Vec2(screenSize.width/PTM_RATIO, screenSize.height/PTM_RATIO));
groundBody->CreateFixture(&boxShapeDef);
// right wal
groundEdge.Set(b2Vec2(screenSize.width/PTM_RATIO, screenSize.height/PTM_RATIO),
b2Vec2(screenSize.width/PTM_RATIO, 0));
groundBody->CreateFixture(&boxShapeDef);
//ball
b2BodyDef ballBodyDef;
ballBodyDef.type = b2_dynamicBody;
ballBodyDef.position.Set(self.ball.position.x/PTM_RATIO, self.ball.position.y/PTM_RATIO);
ballBodyDef.userData = (__bridge void *)_ball;
body = world->CreateBody(&ballBodyDef);
b2CircleShape circle;
circle.m_radius = 60.0/PTM_RATIO;
b2FixtureDef ballShapeDef;
ballShapeDef.shape = &circle;
ballShapeDef.density = 0.8f;
ballShapeDef.friction = 0.0f;
ballShapeDef.restitution = 0.8f;
body->CreateFixture(&ballShapeDef);
and then this is what I run every 1/60th of a second to update the position
int32 velocityIterations = 8;
int32 positionIterations = 1;
world->Step(1.0f/60.0f, velocityIterations, positionIterations);
CGPoint newCenter = CGPointMake(body->GetPosition().x * PTM_RATIO,
self.view.bounds.size.height - body->GetPosition().y * PTM_RATIO);
self.ball.position = newCenter;

Answer 1

If you declare the ball body as dynamic
b2BodyDef ballBodyDef;
ballBodyDef.type = b2_dynamicBody;
// finish creating...
And initialize the world with some gravity value (which you've done), you shouldn't have to manually set the position of the ball body at all. Without seeing all of your code, my hunch is that you have declared the ball as dynamic already, and your manual position updates are conflicting with box2d's gravity. The video seems to show the ball being pulled down by gravity while having its position artificially shifted. I recommend removing the line assigning to self.ball.position entirely - directly setting a physics body's position like this is generally not a good idea.

Answer 2

I found out what was the issue. The ball on the screen is a CALayer and when changing its position it was using implicit animations. It now acts fine when setting the position this way
[CATransaction begin];
[CATransaction setDisableActions:YES];
CGPoint newCenter = CGPointMake(body->GetPosition().x * PTM_RATIO,
self.view.bounds.size.height - body->GetPosition().y * PTM_RATIO);
self.ball.position = newCenter;
[CATransaction commit];