CombineWithOriginalImage.swift

/// Remove background of input image based on an alpha mask.
///
/// - Parameters:
///   - image: Image to mask
///   - mask: Input mask.  Reduces pixel opacity by mask alpha value. For instance
///       an alpha value of 255 will be completely opaque, 0 will be completely transparent
///       and a value of 125 will be partially transparent.
/// - Returns: Image mask with background removed.
func createMask(of image: UIImage, fromMask mask: UIImage, withBackground background: UIImage? = nil) -> UIImage? {
  guard let imageCG = image.cgImage, let maskCG = mask.cgImage else { return nil }
  let imageCI = CIImage(cgImage: imageCG)
  let maskCI = CIImage(cgImage: maskCG)

  let background = background?.cgImage != nil ? CIImage(cgImage: background!.cgImage!) : CIImage.empty()

  guard let filter = CIFilter(name: "CIBlendWithAlphaMask") else { return nil }
  filter.setValue(imageCI, forKey: "inputImage")
  filter.setValue(maskCI, forKey: "inputMaskImage")
  filter.setValue(background, forKey: "inputBackgroundImage")

  guard let maskedImage = context.createCGImage(filter.outputImage!, from: maskCI.extent) else {
    return nil
  }

  return UIImage(cgImage: maskedImage)
}

func createSticker(_ image: UIImage) {    
    /// Create the mask ...

    /// Use mask to create image of transparent sky.
    guard let skyRemoved = createMask(of: image, fromMask: mask) else { return }

    DispatchQueue.main.async {
      self.imageView.image = skyRemoved
    }
  }
}

Thank you