Estoy haciendo un juego de clones Doodle Jump en Swift y el problema es que cuando el jugador salta, golpea su cabeza en la parte inferior de la plataforma y no pasa. ¿Cómo puedo hacer que el jugador pase por encima de la plataforma y salte a través de ellos? Tengo mi código aquí:

import SpriteKit

class GameScene: SKScene, SKPhysicsContactDelegate {

    var hero = SKSpriteNode(imageNamed: "hero");
    var stepSizeTest = SKSpriteNode(imageNamed: "step");
    var start = false;
    var jumpSpeed = CGFloat(0);
    var gravity = CGFloat(0);
    var stepPositionDivision:CGFloat = 0 //allows the step to spawn on specific places on y axes
    var setpPositionHeightIncrease:CGFloat = 0;

    var positionX:CGFloat = 0;
    var timeInterval:NSTimeInterval = 0;
    var CurTime:NSTimeInterval = 0;
    var TimeWhenThePreviousStepSpawned:NSTimeInterval = 0;
    var standart:Bool = false;
    var move:Bool = false;
    var oneJumpOnly:Bool = false;
    var cracked:Bool = false;
    var jumped:Bool = false;

    struct PhysicsCategory{
        static var None: UInt32 = 0;
        static var step: UInt32 = 0b1;
        static var hero: UInt32 = 0b10;
        static var All: UInt32 = UInt32.max;
    }

    func didBeginContact(contact: SKPhysicsContact) {
        var contactBody1: SKPhysicsBody;
        var contactBody2: SKPhysicsBody;
        if (contact.bodyA.categoryBitMask < contact.bodyB.categoryBitMask){
            contactBody1 = contact.bodyA;
            contactBody2 = contact.bodyB;
        }
        else{
            contactBody1 = contact.bodyB;
            contactBody2 = contact.bodyA;
        }

        if (contactBody1.categoryBitMask == PhysicsCategory.step && contactBody2.categoryBitMask == PhysicsCategory.hero){
            jumpSpeed = CGFloat(self.frame.size.height*0.01320422535);

        }
//        if (contactBody1.categoryBitMask == PhysicsCategory.ground && contactBody2.categoryBitMask == PhysicsCategory.hero){
//
//        }
    }

    override func didMoveToView(view: SKView) {
        /* Setup your scene here */
        stepPositionDivision = self.frame.size.height/23;

        jumpSpeed = CGFloat(self.frame.size.height*0.01320422535);
        gravity = CGFloat(self.frame.size.height*(-0.0003521126761));

        self.physicsWorld.contactDelegate = self;

        self.physicsWorld.gravity = CGVectorMake(0,0);

        let sceneBody = SKPhysicsBody(edgeLoopFromRect: self.frame);
        sceneBody.friction = 0;
        self.physicsBody = sceneBody;


        self.hero.anchorPoint = CGPoint(x: 0.5, y: 0.5);
        self.hero.position = CGPoint(x:self.frame.size.width/2, y:self.hero.size.height/2);
        self.hero.physicsBody = SKPhysicsBody(rectangleOfSize: self.hero.size)
        self.hero.physicsBody?.restitution = 1;
        self.hero.physicsBody?.affectedByGravity = false;
        self.hero.physicsBody?.friction = 0;
        self.hero.physicsBody?.categoryBitMask = PhysicsCategory.hero;
        self.hero.physicsBody?.collisionBitMask = PhysicsCategory.step;
        self.hero.physicsBody?.contactTestBitMask = PhysicsCategory.step;

        self.addChild(self.hero);
    }

    func ranPositionX() -> CGFloat{
        let stepSise = UInt32(self.stepSizeTest.size.width)
        let posX = arc4random_uniform(UInt32(self.frame.size.width) - stepSise) + stepSise
        return CGFloat(posX)
    }

    func stepSpawn(){
        if (setpPositionHeightIncrease < self.frame.size.height){
            let step = SKSpriteNode(imageNamed: "step");
            let posX = ranPositionX()

            step.anchorPoint = CGPoint(x: 0.5, y: 0.5);
            step.position = CGPoint(x:posX, y:setpPositionHeightIncrease);
            step.physicsBody = SKPhysicsBody(rectangleOfSize: step.size);
            step.physicsBody?.affectedByGravity = false;
            step.physicsBody?.dynamic = true;
            step.physicsBody?.friction = 0;
            step.physicsBody?.categoryBitMask = PhysicsCategory.step;
            step.physicsBody?.collisionBitMask = PhysicsCategory.None;
            step.physicsBody?.contactTestBitMask = PhysicsCategory.hero;

            self.addChild(step);
            setpPositionHeightIncrease += stepPositionDivision
        }
    }

    override func touchesBegan(touches: Set<UITouch>, withEvent event: UIEvent?) {
       /* Called when a touch begins */
        for touch in touches {

        }
    }

    override func update(currentTime: CFTimeInterval) {
        /* Called before each frame is rendered */

        self.hero.position.y += jumpSpeed
        jumpSpeed += gravity

        if (start == false){
            //self.hero.physicsBody?.applyImpulse(CGVector(dx: 0, dy: 40))
            start = true;
        }
        stepSpawn()
    }
}