Adaptive Threshold CIKernel/CIFilter iOS

Question

I have researched all over in order to find a kernel that performs adaptive thresholding on iOS. Unfortunately I do not understand the kernel language or the logic behind it. Be

Answer 1

Simon's Filter is the right approach to achieve the desired effect, however, you have to modify a couple of things.

First of all, switch the order of imageLuma and thresholdLuma, since we want black letters to remain black and not the other way around. Also, you should add a constant (I chose 0.01) to remove noise.

    var thresholdKernel =  CIColorKernel(string:
    "kernel vec4 thresholdFilter(__sample image, __sample threshold)" +
        "{" +
        "   float imageLuma = dot(image.rgb, vec3(0.2126, 0.7152, 0.0722));" +
        "   float thresholdLuma = dot(threshold.rgb, vec3(0.2126, 0.7152, 0.0722));" +
        "   return vec4(vec3(step(thresholdLuma, imageLuma+0.001)), 1);"     
    "}"

override var outputImage: CIImage! {
    guard let inputImage = inputImage,
        let thresholdKernel = thresholdKernel else {
        return nil
    }
    let blurred = inputImage.applyingFilter("CIBoxBlur", withInputParameters: [kCIInputRadiusKey: 5]) // block size
    let extent = inputImage.extent
    let arguments = [inputImage, blurred]
    return thresholdKernel.apply(withExtent: extent, arguments: arguments)
}

And this is, what you get Only using Apple's Core Image, without having to install any external libraries :)

Of course, you can play around a little with the values of constant and block size.