CIFilter with Metal shader

BLKTransparent.swift

/**
 A CIFilter that uses a Metal function that converts a black pixel (or almost black) to a transparent pixel.
 */
class BLKTransparent: CIFilter {
    
    private var kernel: CIColorKernel
    
    var inputImage: CIImage?
    var threshold: Float?
    
    override init() {
        
        let url = Bundle.main.url(forResource: "default", withExtension: "metallib")!
        guard let data = try? Data(contentsOf: url) else { fatalError() }
        guard let kern = try? CIColorKernel(functionName: "makeBlackTransparent", fromMetalLibraryData: data) else { fatalError() }
        self.kernel = kern
        
        super.init()
    }
    
    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
    
    func outputImage() -> CIImage? {
        guard let inputImage = inputImage else {return nil}
        return kernel.apply(extent: inputImage.extent, arguments: [inputImage, threshold ?? 0.15])
    }
}

kernel.metal

#include <metal_stdlib>
using namespace metal;
#include <CoreImage/CoreImage.h>

extern "C" { namespace coreimage {
  float4 makeBlackTransparent(sample_t sample, float threshold) {
        float4 filtered = (sample.r < threshold && sample.g < threshold && sample.b < threshold) == true ? float4(0):float4(sample.r, sample.g, sample.b, sample.a);
        return filtered;
    }
}}

ViewModel.swift

class ViewModel:ObservableObject {
    
  @Published var openingImage:NSImage
  
  init(image:NSImage?) {
        // Init with an image or one will be created
        if let image = image {
            self.openingImage = image
        } else {
            self.openingImage = NSImage(named:"Example")! // Make sure there is "Example.png" in your assets
        }
    }
  
  func metalBlackToTransparent() {
        
        let inputImage:NSImage = openingImage
        
        // Convert NSImage to CIImage
        guard let inputData = inputImage.tiffRepresentation,
              let bitmap = NSBitmapImageRep(data: inputData),
              let inputCIImage = CIImage(bitmapImageRep: bitmap) else {
            
            print("Missing something. Check references.")
            fatalError()
        }
        
        // Create the filter
        let context = CIContext()
        let shade = BLKTransparent()
        shade.inputImage = inputCIImage
        shade.threshold = 0.15
        
        // get a CIImage from our filter or exit if that fails
        guard let outputImage = shade.outputImage() else { return }
        
        // attempt to get a CGImage from the CIImage
        if let cgimg = context.createCGImage(outputImage, from: outputImage.extent) {
            // convert that to a NSImage
            let nsImage = NSImage(cgImage: cgimg, size:openingImage.size)
            self.openingImage = nsImage
        }
    }
}