CIFilter opacity

CoreImageFilters.swift

/*
  Change the opacity of a CIFilter.
*/
func changeOpacity(to opacity: CGFloat, on filter: CIFilter, original image: UIImage) -> CIImage? {
  guard let backgroundImage: CIImage = CIImage(image: image) else { fatalError() }
  
  // The CIColorMatrix filter, will contain the requested filter and control its opacity
  guard let overlayFilter: CIFilter = CIFilter(name: "CIColorMatrix") else { fatalError() }
  let overlayRgba: [CGFloat] = [0, 0, 0, opacity]
  let alphaVector: CIVector = CIVector(values: overlayRgba, count: 4)
  overlayFilter.setValue(filter.outputImage!, forKey: kCIInputImageKey)
  overlayFilter.setValue(alphaVector, forKey: "inputAVector")
  
  // Compose the background and the overlay filter
  guard let compositeFitler: CIFilter = CIFilter(name: "CISourceOVerCompositing") else { fatalError() }
  compositeFilter.setValue(backgroundImage, forKey: kCIInputBackgroundImageKey)
  compositeFilter.setValue(opacityFilter.outputImage!, forKey: kCIInputImageKey)

  return compositeFilter.outputImage
}