CoreImage Theshold filter based on custom CIColorKernel

gistfile1.txt

class ThresholdFilter: CIFilter
{
    var inputImage : CIImage?
    var threshold: CGFloat = 0.75
    
    let thresholdKernel = CIColorKernel(string:
        "kernel vec4 thresholdFilter(__sample image, float threshold)" +
        "{" +
            "   float luma = (image.r * 0.2126) + (image.g * 0.7152) + (image.b * 0.0722);" +
            
            "   return (luma > threshold) ? vec4(1.0, 1.0, 1.0, 1.0) : vec4(0.0, 0.0, 0.0, 0.0);" +
        "}"
    )
    
    
    override var outputImage : CIImage!
    {
        guard let inputImage = inputImage,
            thresholdKernel = thresholdKernel else
        {
            return nil
        }
        
        let extent = inputImage.extent
        let arguments = [inputImage, threshold]
        
        return thresholdKernel.applyWithExtent(extent, arguments: arguments)
    }
}