How to use CoreImage to create a smooth threshold filter?

Question

Are there existing filters that could be used as a smooth threshold? By this I mean that pixel values should be thresholded between two values, but the intermediate values are interpolated.

Basically, I'm trying to implement this:

https://www.filterforge.com/wiki/index.php/Thresholds#Interpolation_.28.22Blending.22.29

Answer 1

You'll need to write your own using Core Image Kernel Language and a CIColorKernel.

I would use a smoothstep and pass in two edge values along with the luminance of the current pixel. The CIKL could look something like this:

kernel vec4 color(__sample pixel, float inputEdgeO, float inputEdge1)
{
    float luma = dot(pixel.rgb, vec3(0.2126, 0.7152, 0.0722));
    float threshold = smoothstep(inputEdgeO, inputEdge1, luma);
    return vec4(threshold, threshold, threshold, 1.0);
}

You can wrap this into a CIFilter with the following code:

class SmoothThreshold: CIFilter
{
    var inputImage : CIImage?
    var inputEdgeO: CGFloat = 0.25
    var inputEdge1: CGFloat = 0.75

    var colorKernel = CIColorKernel(string:
        "kernel vec4 color(__sample pixel, float inputEdgeO, float inputEdge1)" +
        "{" +
        "    float luma = dot(pixel.rgb, vec3(0.2126, 0.7152, 0.0722));" +
        "    float threshold = smoothstep(inputEdgeO, inputEdge1, luma);" +
        "    return vec4(threshold, threshold, threshold, 1.0);" +
        "}"
    )

    override var outputImage: CIImage!
    {
        guard let inputImage = inputImage,
            colorKernel = colorKernel else
        {
            return nil
        }

        let extent = inputImage.extent
        let arguments = [inputImage,
                         inputEdgeO,
                         inputEdge1]

        return colorKernel.applyWithExtent(extent,
                                           arguments: arguments)
    }
}

Note that you'll need to ensure inputEdge0 is less than inputEdge1, or the smoothstep will go a little crazy. You may want to saturate them too, to ensure they're between 0 and 1.

Have a look at CIFilter.registerFilterName which will allow you to create an instance of SmoothThreshold with familiar syntax, although this will work fine:

let image = CIImage(image: UIImage(named: "monalisa.jpg")!)!

let filter = SmoothThreshold()
filter.inputImage = image

let final = filter.outputImage

Cheers!

Simon