Sprite Kit bodyAtPoint and bodyInRect return incor

Question

I created a very simple sample code, just one scene, one sprite node 20x20px, at point 0.0 on screen. When I call scene.physicsWorld bodyAtPoint it returns me this node even at point eg: 34x34. But at point 35x35 it returns null. So basically all points from 0px to 34px in both axis returns this node, starting from 35px, it doesn't return it anymore. Any idea what could be the reason, if sprite visibly ends at 20px 20px? The same behaviour is seen in bodyInRect.

Here is the sample code:

-(id)initWithSize:(CGSize)size {    
if (self = [super initWithSize:size]) {
    /* Setup your scene here */

    self.backgroundColor = [SKColor colorWithRed:0.15 green:0.15 blue:0.3 alpha:1.0];
    self.physicsWorld.gravity = CGVectorMake(0, 0);

    SKSpriteNode *node = [[SKSpriteNode alloc] initWithColor:[UIColor whiteColor] size:CGSizeMake(20, 20)];
    node.position = CGPointMake(10, 10);

    node.physicsBody = [SKPhysicsBody bodyWithRectangleOfSize:CGSizeMake(20, 20)];
    [self addChild:node];

}
return self;
}

-(void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {

for (UITouch *touch in touches) {
    CGPoint location = [touch locationInNode:self];
    if([self.physicsWorld bodyAtPoint:location])
    {
        NSLog(@"Detected at point: %f %f", location.x, location.y);
    }else{
        NSLog(@"No node at point: %f %f", location.x, location.y);
    }
}
}

And here's the console log:

2013-11-09 15:05:52.822 SKTest[43143:70b] Detected at point: 2.000000 1.000000
2013-11-09 15:05:56.274 SKTest[43143:70b] Detected at point: 3.000000 7.000000
2013-11-09 15:05:57.006 SKTest[43143:70b] Detected at point: 3.000000 11.000000
2013-11-09 15:05:58.199 SKTest[43143:70b] Detected at point: 3.000000 12.000000
2013-11-09 15:05:58.918 SKTest[43143:70b] Detected at point: 3.000000 14.000000
2013-11-09 15:05:59.785 SKTest[43143:70b] Detected at point: 3.000000 17.000000
2013-11-09 15:06:00.685 SKTest[43143:70b] Detected at point: 3.000000 20.000000
2013-11-09 15:06:01.565 SKTest[43143:70b] Detected at point: 3.000000 22.000000
2013-11-09 15:06:02.915 SKTest[43143:70b] Detected at point: 3.000000 25.000000
2013-11-09 15:06:04.285 SKTest[43143:70b] Detected at point: 3.000000 30.000000
2013-11-09 15:06:05.387 SKTest[43143:70b] Detected at point: 3.000000 34.000000
2013-11-09 15:06:08.492 SKTest[43143:70b] No node at point: 4.000000 38.000000
2013-11-09 15:06:12.499 SKTest[43143:70b] Detected at point: 4.000000 5.000000
2013-11-09 15:06:13.240 SKTest[43143:70b] Detected at point: 6.000000 5.000000
2013-11-09 15:06:13.881 SKTest[43143:70b] Detected at point: 10.000000 5.000000
2013-11-09 15:06:15.064 SKTest[43143:70b] Detected at point: 12.000000 5.000000
2013-11-09 15:06:16.120 SKTest[43143:70b] Detected at point: 14.000000 5.000000
2013-11-09 15:06:16.873 SKTest[43143:70b] Detected at point: 16.000000 5.000000
2013-11-09 15:06:17.582 SKTest[43143:70b] Detected at point: 18.000000 5.000000
2013-11-09 15:06:18.066 SKTest[43143:70b] Detected at point: 21.000000 5.000000
2013-11-09 15:06:18.966 SKTest[43143:70b] Detected at point: 24.000000 5.000000
2013-11-09 15:06:19.585 SKTest[43143:70b] Detected at point: 31.000000 5.000000
2013-11-09 15:06:20.531 SKTest[43143:70b] Detected at point: 35.000000 5.000000
2013-11-09 15:06:22.581 SKTest[43143:70b] No node at point: 36.000000 4.000000
2013-11-09 15:06:26.560 SKTest[43143:70b] Detected at point: 19.000000 16.000000
2013-11-09 15:06:27.933 SKTest[43143:70b] Detected at point: 20.000000 17.000000
2013-11-09 15:06:29.856 SKTest[43143:70b] Detected at point: 25.000000 19.000000
2013-11-09 15:06:31.487 SKTest[43143:70b] Detected at point: 26.000000 22.000000
2013-11-09 15:06:33.850 SKTest[43143:70b] Detected at point: 29.000000 27.000000
2013-11-09 15:06:35.492 SKTest[43143:70b] Detected at point: 31.000000 29.000000
2013-11-09 15:06:36.854 SKTest[43143:70b] Detected at point: 35.000000 32.000000
2013-11-09 15:06:40.004 SKTest[43143:70b] No node at point: 40.000000 34.000000

Is that possible that it would be an Apple bug, in both nodeAtPoint and nodeInRect? It's hard to believe. But the code is so short that I can't see where any mistake could have been made here. Any help will be greatly appreciated.

Answer 1

I experimented a little with this. What I noticed when I enlarged the sprite is that the body area seemed to be larger than the sprite. I had to divide the sprite size by 1.15 to get the body rect to approximately match up with the displayed sprite on all 4 sides. No idea why, though. I never had an issue with gameplay, so perhaps nodeAtPoint has a threshold range?

SKSpriteNode *node = [SKSpriteNode spriteNodetWithColor:[SKColor whiteColor]
                                                   size:CGSizeMake(200, 200)];
node.position = CGPointMake(200, 200);

CGRect bodySize = CGSizeMake(node.size.width / 1.15, node.size.height / 1.15);
node.physicsBody = [SKPhysicsBody bodyWithRectangleOfSize:bodySize];
[self addChild:node];

I tested this on the Retina 4" and non-Retina iPad Simulator.