How to detect collisions between two fast moving objects? - swift

Question

Hey so I have a problem detecting collisions. I have this project I am making based on Ray Wenderlich's tutorial on How to Make a game like Mega Jump. So everything is works great at the beginning, where platforms and stars are static in one place. But as you go further in the game, stars and platforms start moving (I am using SKActions for this), tried to do this to make it a little harder. But when I get there, collision is NOT being detected at all, player just passes by through the objects like they weren't there. I've been reading everywhere online, I am using precise collision detection but still I see no difference. Any ideas on what can be wrong or what else can I do? Here is a bit of code on how I am doing this:
player = SKSpriteNode(texture: firstFrame)
player.position = (CGPoint(x: self.size.width / 2, y: 50.0))
player.physicsBody = SKPhysicsBody(circleOfRadius: player.size.width / 2)
player.physicsBody?.dynamic = true
player.physicsBody?.allowsRotation = false
player.physicsBody?.restitution = 1.0
player.physicsBody?.friction = 0.0
player.physicsBody?.angularDamping = 0.0
player.physicsBody?.linearDamping = 0.0
player.physicsBody?.usesPreciseCollisionDetection = true
player.physicsBody?.categoryBitMask = CollisionCategoryBitmask.Player
player.physicsBody?.collisionBitMask = 0
player.physicsBody?.contactTestBitMask = CollisionCategoryBitmask.Star | CollisionCategoryBitmask.Platform | CollisionCategoryBitmask.Monster
foregroundNode.addChild(player)
func createPlatformAtPosition(position: CGPoint, ofType type: PlatformType) -> PlatformNode {
let node = PlatformNode()
let thePosition = CGPoint(x: position.x * scaleFactor, y: position.y)
node.position = thePosition
node.name = "NODE_PLATFORM"
node.platformType = type
var sprite: SKSpriteNode
var spriteFrames : [SKTexture]!
if type == .Break {
let spriteAnimatedAtlas = SKTextureAtlas(named: "CloudBreak")
var cloudFrames = [SKTexture]()
spriteFrames = cloudFrames
let firstFrame = SKTexture(imageNamed: "Cloud02")
sprite = SKSpriteNode(texture: firstFrame)
let move = SKAction.moveToX(self.position.x - 160.0, duration:2.0)
let back = SKAction.moveToX(self.position.x, duration:2.0)
let sequence = SKAction.sequence([move, back, move, back])
sprite.runAction(SKAction.repeatActionForever(sequence))
} else {
let spriteAnimatedAtlas = SKTextureAtlas(named: "Cloud")
var cloudFrames = [SKTexture]()
spriteFrames = cloudFrames
let firstFrame = SKTexture(imageNamed: "Cloud")
sprite = SKSpriteNode(texture: firstFrame)
}
node.addChild(sprite)
node.physicsBody = SKPhysicsBody(rectangleOfSize: sprite.size)
node.physicsBody?.dynamic = false
node.physicsBody?.categoryBitMask = CollisionCategoryBitmask.Platform
node.physicsBody?.contactTestBitMask = CollisionCategoryBitmask.Player
node.physicsBody?.collisionBitMask = 0
return node
}
func didBeginContact(contact: SKPhysicsContact) {
var updateHUD = false
let whichNode = (contact.bodyA.node != player) ? contact.bodyA.node : contact.bodyB.node
let other = whichNode as GameObjectNode
updateHUD = other.collisionWithPlayer(player)
}
struct CollisionCategoryBitmask {
static let Player: UInt32 = 0x00
static let Star: UInt32 = 0x01
static let Platform: UInt32 = 0x02
static let Monster: UInt32 = 0x03
}

Answer 1

Slow the objects down and see if they collide. I had an issue where I was moving my nodes too fast and they were past the collision points when the new frame was rendered. If they move so fast they pass each other's physics body in the frame cycle they will not register a hit.
To detect a collision in that case, you can compare where they are in each frame, and if they have passed each other or their x/y plane values overlap, you can execute your collision code.

Answer 2

The collision is not detected because you didn't set the collisionBitMask.
player.physicsBody?.collisionBitMask = CollisionCategoryBitmask.Star | CollisionCategoryBitmask.Platform | CollisionCategoryBitmask.Monster
node.physicsBody?.collisionBitMask = CollisionCategoryBitmask.Player
Contact and Collision aren't the same thing. You can find more information online.
If it still doesn't work, please show us your CollisionCategoryBitmask to make sure you created it properly.
Edit :
struct CollisionCategoryBitmask {
static let Player: UInt32 = 0
static let Star: UInt32 = 0b1
static let Platform: UInt32 = 0b10
static let Monster: UInt32 = 0b100
}
Here is a table I've made for another question, that might help you too :
http://goo.gl/7D8EGY

Answer 3

Check out my collision physics engine, it's very simple but has support for continuous/predicting/bullet collisions. You might learn something from the code, and it's written in javascript so it should be easy to read.https://github.com/Murplyx/AAE---Axis-Aligned-Engine