kernel AdjustableThreshold {
    parameter float blackThreshold < 
        minValue:float(0);
        maxValue:float(1);
        defaultValue:float(0.4);
    >;

    parameter float whiteThreshold < 
        minValue:float(0);
        maxValue:float(1);
        defaultValue:float(0.5);
    >;

    void evaluatePixel(in image4 src, out pixel4 dst) {
        float4 inputColor = sampleNearest(src, outCoord());
        float brightness = (inputColor.r + inputColor.g + inputColor.b) / 3.0;
        
        dst = inputColor;
        
        float grayscale = (brightness - blackThreshold) / (whiteThreshold - blackThreshold);
        float ifAboveBlack = brightness > whiteThreshold ? 1.0 : grayscale;
        dst.r = dst.g = dst.b = brightness < blackThreshold ? 0.0 : ifAboveBlack;
    }
}