SpriteKit SKShapeNode stops prematurely when colliding due to gravity - ios

Question

This is the setup
// Create a scene
scene = SKScene(size: self.view.frame.size)
let sceneHeight = scene.frame.height
let sceneWidth = scene.frame.width
// Create nodes
// Rectangle
let shapeWidth: CGFloat = 100
let shapeHeight = shapeWidth
let shapeNode = SKShapeNode(rect: CGRectMake(0, 0, shapeWidth, 100))
shapeNode.position = CGPoint(x: sceneWidth/2 - shapeWidth/2, y: 300)
shapeNode.fillColor = UIColor.redColor()
shapeNode.strokeColor = UIColor.redColor()
// Floor
let floorWidth: CGFloat = sceneWidth
let floorHeight: CGFloat = 10
let floorNode = SKShapeNode(rect: CGRectMake(0, 0, floorWidth, floorHeight))
floorNode.position = CGPoint(x: 0, y: 100)
floorNode.fillColor = UIColor.greenColor()
floorNode.strokeColor = UIColor.greenColor()
// Create the node tree
scene.addChild(floorNode)
scene.addChild(shapeNode)
Then I define and add two physics bodies to my two nodes.
// Create physics
let shapePhysicsBody = SKPhysicsBody(rectangleOfSize: CGSize(width: shapeWidth, height: shapeHeight))
shapePhysicsBody.dynamic = true
shapePhysicsBody.categoryBitMask = PhysicsCategory.Shape
shapePhysicsBody.contactTestBitMask = PhysicsCategory.Floor
shapePhysicsBody.collisionBitMask = PhysicsCategory.None
shapePhysicsBody.usesPreciseCollisionDetection = true
let floorPhysicsBody = SKPhysicsBody(rectangleOfSize: CGSize(width: floorWidth, height: floorHeight))
floorPhysicsBody.dynamic = true
floorPhysicsBody.categoryBitMask = PhysicsCategory.Floor
floorPhysicsBody.contactTestBitMask = PhysicsCategory.Shape
floorPhysicsBody.collisionBitMask = PhysicsCategory.None
floorPhysicsBody.usesPreciseCollisionDetection = true
floorPhysicsBody.affectedByGravity = false
// Add physics
shapeNode.physicsBody = shapePhysicsBody
floorNode.physicsBody = floorPhysicsBody
scene.physicsWorld.gravity = CGVector(dx: 0, dy: -1)
scene.physicsWorld.contactDelegate = self
This sort of works, that is the rectangle shape does fall from it's initial position and the collision delegate is called when a collision happens.
The contents of my didBeginContact: method.
func didBeginContact(contact: SKPhysicsContact) {
var firstBody : SKPhysicsBody
var secondBody : SKPhysicsBody
if contact.bodyA.categoryBitMask < contact.bodyB.categoryBitMask {
firstBody = contact.bodyA
secondBody = contact.bodyB
} else {
firstBody = contact.bodyB
secondBody = contact.bodyA
}
if ((firstBody.categoryBitMask & PhysicsCategory.Floor != 0) && (secondBody.categoryBitMask & PhysicsCategory.Shape != 0)) {
secondBody.dynamic = false
println("collision")
}
}
But the shape node stops way before there's a visual contact. I hope these images give an impression of how that looks.
Starts here
This is where the contact happens
Why is there such a gap between them? What am I missing? I want them to visually touch each other before the rectangle stops moving.

Answer 1

SKShapeNode(rect:) draws the SKShapeNode with the node.position as origin. For the SKPhysicsBody position corresponds to the center of the rectangle. This creates an offset between the physics body and the actual node. You can see that if you enable
scene.view?.showsPhysics = true.
You can correct the offset by creating the SKShapeNode using following code.
let shapeNode = SKShapeNode(rect: CGRectMake(-shapeWidth/2, -50, shapeWidth, 100))
// Other code
let floorNode = SKShapeNode(rect: CGRectMake(-floorWidth/2, -floorHeight/2, floorWidth, floorHeight))