grayscale chromakey swift - swift

Question

I wanted to change all the white background of a grayscale line art to transparent, using swift. I implemented RGB chromakey document for swift: https://developer.apple.com/documentation/coreimage/applying_a_chroma_key_effect
The hue range was initially for green color and there were R,G,B and alpha parameters
self.ChromaKeyFilter(fromHue: 0.3, toHue: 0.4)
, but for grayscale there were only 2 according to apple documentation: white, alpha. I wanted to change the extreme white color to transparent, so I just set hue range as 1.
class ChromaKey{
//Hue range
let whiteRange: CGFloat = 1
func getHue(white: CGFloat) -> CGFloat{
let color = UIColor(white: white, alpha: 1)
var hue: CGFloat = 0
color.getHue(&hue, saturation: nil, brightness: nil, alpha: nil)
return hue
}
func chromaKeyFilter(ridHue:CGFloat) -> CIFilter? {
let size = 64
var cubeGrey = [Float]()
for z in 0 ..< size{
let white = CGFloat(z)/CGFloat(size-1)
let hue = getHue(white: white)
let alpha: CGFloat = (hue == ridHue) ? 0:1
cubeGrey.append(Float(white * alpha))
cubeGrey.append(Float(alpha))
}
let data = Data(buffer: UnsafeBufferPointer(start: &cubeGrey, count: cubeGrey.count))
let colorCubeFilter = CIFilter(name: "CIColorCube", parameters: ["inputCubeDimension": size, "inputCubeData":data])
return colorCubeFilter
}
func filterPixels(foregroundCIImage: CIImage) -> CIImage {
let chromaCIFilter = self.chromaKeyFilter(ridHue: whiteRange)
chromaCIFilter?.setValue(foregroundCIImage, forKey: kCIInputImageKey)
let sourceCIImageWithoutBackground = chromaCIFilter?.outputImage
var image = CIImage()
if let filteredImage = sourceCIImageWithoutBackground{
image = filteredImage
}
return image
}
the func for chromakey. when I apply the func, nothing shows up.