Use PDF for Assets in Xcode 7 – SpriteKit – Photoshop

Hello,

I wanted to use different images for each screen resolutions (iPhone & iPad), and I discovered that I can use PDF files for images in assets, so I did it. Here the configuration of my asset :

I wan’t if I use the correct way or if I can do it easier. Per exemple, I have a button positioned at the center of the screen (In this case, the code does not interest us) :

let buttonWidth = self.size.width * 0.5
let buttonHeight = buttonWidth * 0.1

button = UIButton(type: UIButtonType.Custom) as UIButton
button.frame = CGRectMake(0, 0, buttonWidth, buttonHeight)
button.setBackgroundImage(UIImage(named: "buttonImageInitial"), forState: UIControlState.Normal)
self.view?.addSubview(button)

To know the button image size, I use self.size.width as the width of iPhone 4S screen (640) and I get the button width for the iPhone 4S : 640 * 0.5 = 320, and the height : 320 * 0.1 = 32. So the size of my button is : width: 320, height: 32. In Photoshop I create an image with these dimensions (in Points) and I export it in PDF. I add to the asset catalog as the iPhone button image (I think Xcode convert Points into Pixels, because in the Attribute Inspector, the size of the image is “320 x 32 pixels”).
For the iPap dimensions, I do the same, but I don’t use self.size.width as the width of the iPhone 4S screen, but as the width of iPad screen (1536), and I get 768 * 76,8 for the size of the button.

My questions are : Did I use the right way, can I use Universal in Devices to make only one image for the button, and how to choose the only correct size for each screen resolution ?

My Xcode version : Xcode 7.0 beta 5

Thanks !