Xamarin iOS CIFilter / CIColorKernel Replace one color with another color (with threshold parameter to catch those stray variants)

ColorReplaceFilter.cs

public class ColorReplaceFilter : CIFilter
{
	const string filterName = "colorReplace";
	const int numArgs = 4;
	const string coreIageShaderProgram =
		@"
			kernel vec4 main(__sample s, __color o, __color r, float threshold) {
			    vec4 diff = s.rgba - o;
			    float distance = length( diff );
			    float alpha = compare( distance - threshold, 0.0, 1.0 );
				 if (alpha == 0.0)
					return r;
				 return s;
			}
		";

	NSObject[] arguments;
	CIColorKernel colorKernel;

	public ColorReplaceFilter() { Initializer(); }
	public ColorReplaceFilter(NSCoder coder) : base(coder) { Initializer(); }
	public ColorReplaceFilter(NSObjectFlag t) : base(t) { Initializer(); }
	public ColorReplaceFilter(IntPtr handle) : base(handle) { Initializer(); }

	public CIImage InputImage { get; set; }
	public CIColor MatchColor { get; set; }
	public CIColor ReplaceWithColor { get; set; }

	NSNumber _threshold;
	public nfloat Threshold
	{
		get { return _threshold.NFloatValue; }
		set { _threshold = NSNumber.FromNFloat(value); }
	}

	void Initializer()
	{
		arguments = new NSObject[numArgs];
		colorKernel = CIColorKernel.FromProgramSingle(coreIageShaderProgram);
		MatchColor = CIColor.BlackColor;
		ReplaceWithColor = CIColor.WhiteColor;
		_threshold = new NSNumber(0.0f);
	}

	public override string Name
	{
		get => filterName;
	}

	public override CIImage OutputImage
	{
		get => CreateOutputImage();
	}

	CIImage CreateOutputImage()
	{
		if (InputImage != null) // Avoid object creation to allow fast filter chaining
		{
			arguments[0] = InputImage as NSObject;
			arguments[1] = MatchColor as NSObject;
			arguments[2] = ReplaceWithColor as NSObject;
			arguments[3] = _threshold as NSObject;
			var ciImage = colorKernel.ApplyWithExtent(InputImage.Extent, arguments);
			return ciImage;
		}
		return null;
	}

	protected override void Dispose(bool disposing)
	{
		base.Dispose(disposing);
		arguments = null;
		InputImage = null;
		MatchColor = null;
		ReplaceWithColor = null;
		colorKernel.Dispose();
	}
}

Test.cs

var inputImage = inputVIew.Image; // Some UIImage as input;
var replaceFilter = new ColorReplaceFilter
{
	InputImage = new CIImage(inputImage)
};
replaceFilter.MatchColor = CIColor.FromRgba(red: 0.019f, green: 0.235f, blue: 0.458f, alpha: 1);
replaceFilter.ReplaceWithColor = CIColor.FromRgb(255, 0, 0);
thresholdSlider.ValueChanged += (sender, e) =>
{
	replaceFilter.Threshold = thresholdSlider.Value;
	var image = replaceFilter.OutputImage;
	outputView.Image = UIImage.FromImage(image);
};

https://stackoverflow.com/questions/47323887/custom-cicolorkernel-in-xamarin-ios