NitWit Studios NitWit Studios - 1 month ago 14
iOS Question

SpriteKit: Using SKTextureAtlas causes incorrect Physics Body

I am building my first sprite kit game, and recently read an article that said to use SKTextureAtlas to improve performance.

So I migrated all my sprite images into organized

.atlas
folders and updated the code accordingly.
However, now my sprites have weird physics bodies (mainly over-enlarged).

For instance, my player sprite has three states: Flying (flat), Flying Up and Flying Down. The three images are pretty similar, with slight variations (arms pointing flat, up, down).

Before migrating to texture atlases, the physics body was pretty spot on with the image. Now however, it is much larger than the image and slightly stretched on the y-axis.

Here is my player.atlas file structure:

player-flying-iphone.atlas:


  • player-flying-down@2x.png

  • player-flying-down@3x.png

  • player-flying-up@2x.png

  • player-flying-up@3x.png

  • player-flying@2x.png

  • player-flying@3x.png



Here is an example of my code:
(Player is a subclassed SKSpriteNode)

let textureAtlas = SKTextureAtlas(named: "player-iphone")
let playerFlyingTexture = textureAtlas.textureNamed("player-flying")
let player = Player(texture: playerFlyingTexture, color: UIColor.clearColor(), size: CGSizeMake(100.0, 100.0))

let physicsBody = SKPhysicsBody(texture: playerFlyingTexture, size: CGSizeMake(100.0, 100.0))
physicsBody.dynamic = true
physicsBody.affectedByGravity = true
physicsBody.usesPreciseCollisionDetection = false
physicsBody.categoryBitMask = CollisionCategories.Player
physicsBody.contactTestBitMask = CollisionCategories.Enemy
physicsBody.collisionBitMask = CollisionCategories.EdgeBody
physicsBody.allowsRotation = false
player.physicsBody = physicsBody

Answer

(credit goes to @Whirlwind for the help)

So for me, the fix ended up being that I need to change this:

let physicsBody = SKPhysicsBody(texture: self.texture, size: self.size)

..to this..

let physicsBody = SKPhysicsBody(texture: self.texture, alphaThreshold: 0.3, size: self.size)

Apparently, the SKTextureAtlas cropping method used when generating a physics body has issues figuring out exactly what to crop, so setting the alphaThreshold explicitly seemed to fix it.

Obviously, that value will rely on your own image and what kind of transparency you have. But if you have hard edges like I did, you should be able to set that to some arbitrary value and be good to go.