How to place nodes on the ground? - swift

Question

I am creating a game with SpriteKit. The background is a png image, endlessly moving (parallax scroll):
func parallaxScroll(image: String, y: CGFloat, z: CGFloat, duration: Double, needsPhysics: Bool) {
for i in 0 ... 1 {
// position the first node on the left, and position second on the right
let node = SKSpriteNode(imageNamed: image)
node.position = CGPoint(x: 1023 * CGFloat(i), y: y)
node.zPosition = z
addChild(node)
if needsPhysics {
node.physicsBody = SKPhysicsBody(texture: node.texture!, size: node.texture!.size())
node.physicsBody?.isDynamic = false
node.physicsBody?.contactTestBitMask = 1
node.name = "ground"
}
// make this node move the width of the screen by whatever duration was passed in
let move = SKAction.moveBy(x: -1024, y: 0, duration: duration)
// make it jump back to the right edge
let wrap = SKAction.moveBy(x: 1024, y: 0, duration: 0)
// make these two as a sequence that loops forever
let sequence = SKAction.sequence([move, wrap])
let forever = SKAction.repeatForever(sequence)
// run the animations
node.run(forever)
}
}
The example function below places a box at random y position:
#objc func createObstacle() {
let obstacle = SKSpriteNode(imageNamed: "rectangle")
obstacle.zPosition = -2
obstacle.position.x = 768
addChild(obstacle)
obstacle.physicsBody = SKPhysicsBody(texture: obstacle.texture!, size: obstacle.texture!.size())
obstacle.physicsBody?.isDynamic = false
obstacle.physicsBody?.contactTestBitMask = 1
obstacle.name = "obstacle"
let rand = GKRandomDistribution(lowestValue: -200, highestValue: 350)
obstacle.position.y = CGFloat(rand.nextInt())
// make it move across the screen
let action = SKAction.moveTo(x: -768, duration: 9)
obstacle.run(action)
}
func playerHit(_ node: SKNode) {
if node.name == "obstacle" {
player.removeFromParent()
}
}
func didBegin(_ contact: SKPhysicsContact) {
guard let nodeA = contact.bodyA.node else { return }
guard let nodeB = contact.bodyB.node else { return }
if nodeA == player {
playerHit(nodeB)
} else if nodeB == player {
playerHit(nodeA)
}
}
Instead of placing it at random, I would like to place it on the "ground". The following image illustrates the current placement and the desired placement:
Does anyone know how to place an obstacle node (object) on the ground which is not flat instead of random y position?

Answer 1

You could cast a ray, at the box's intended x position, from the top of the screen to the bottom, and when the ray hits the ground, place the box directly above the hitpoint. See enumerateBodies(alongRayStart:end:using:). In your case you might want to insert into createObstacle something like:
let topOfScreen = size.height / 2
let bottomOfScreen = -size.height / 2
obstacle.position.x = -768
physicsWorld.enumerateBodies(alongRayStart: CGPoint(x: obstacle.position.x, y: topOfScreen), end: CGPoint(x: obstacle.position.x, y: bottomOfScreen)) { body, point, normal, stop in
if body.node?.name == "ground" {
// body is the ground's physics body. point is the point that an object
// falling from the sky would hit the ground.
// Assuming the obstacle's anchor point is at its center, its actual position
// would be slightly above this point
let positionAboveBody = point + (obstacle.size.height / 2)
obstacle.position.y = positionAboveBody
}
}