Carousel Layout Collection View Issue

Question

• Hello, I have followed raywenderlich video for collection view custom layout. UICollectionViewFlowLayout is subclassed as below

  import UIKit

  class CharacterFlowLayout: UICollectionViewFlowLayout {

  let standardItemAlpha: CGFloat = 0.5
  let standardItemScale: CGFloat = 0.5

  var count = 0 override func prepare() {

  super.prepare()

  }

override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {

   let attributes = super.layoutAttributesForElements(in: rect)

   var attributesCopy = [UICollectionViewLayoutAttributes]()

   for itemAttributes in attributes! {
     let itemAttributesCopy = itemAttributes.copy() as! UICollectionViewLayoutAttributes

     changeLayoutAttributes(itemAttributesCopy)

     attributesCopy.append(itemAttributesCopy)

   }

   return attributesCopy  

}

    **override open func shouldInvalidateLayout(forBoundsChange newBounds: CGRect) -> Bool** {

   return true

   }

    **func changeLayoutAttributes(_ attributes: UICollectionViewLayoutAttributes)** {


   let collectionCenter = collectionView!.frame.size.height/2

   let offset = collectionView!.contentOffset.y // bounds

   let normalizedCenter = attributes.center.y - offset

   let maxDistance = self.itemSize.height + self.minimumLineSpacing

   let distance = min(abs(collectionCenter - normalizedCenter), maxDistance)

   let ratio = (maxDistance - distance)/maxDistance

   let alpha = ratio * (1 - self.standardItemAlpha) + self.standardItemAlpha

   let scale = ratio * (1 - self.standardItemScale) + self.standardItemScale

   attributes.alpha = alpha

   attributes.transform3D =  CATransform3DScale(CATransform3DIdentity, scale, scale, 1)
      }    }

I don't understand the logic behind the calculation? Could you please explain why we are calculating distance and ratio?