Binarize Picture with Core Image on iOS - swift

Question

I was wondering if it is possible to binarize an image (convert to black and white only) with Core Image?
I made it with OpenCV and GPUImage, but would prefer it to use Apple Core Image, if that's possible

Answer 1

You can use MetalPerformanceShaders for that. And the CIImageProcessingKernel.
https://developer.apple.com/documentation/coreimage/ciimageprocessorkernel
Here is the code of the class needed.
class ThresholdImageProcessorKernel: CIImageProcessorKernel {
static let device = MTLCreateSystemDefaultDevice()
override class func process(with inputs: [CIImageProcessorInput]?, arguments: [String : Any]?, output: CIImageProcessorOutput) throws {
guard
let device = device,
let commandBuffer = output.metalCommandBuffer,
let input = inputs?.first,
let sourceTexture = input.metalTexture,
let destinationTexture = output.metalTexture,
let thresholdValue = arguments?["thresholdValue"] as? Float else {
return
}
let threshold = MPSImageThresholdBinary(
device: device,
thresholdValue: thresholdValue,
maximumValue: 1.0,
linearGrayColorTransform: nil)
threshold.encode(
commandBuffer: commandBuffer,
sourceTexture: sourceTexture,
destinationTexture: destinationTexture)
}
}
And this is how you can use it:
let context = CIContext(options: nil)
if let binaryCIImage = try? ThresholdImageProcessorKernel.apply(
withExtent: croppedCIImage.extent,
inputs: [croppedCIImage],
arguments: ["thresholdValue": Float(0.2)]) {
if let cgImage = context.createCGImage(binaryCIImage, from: binary.extent) {
DispatchQueue.main.async {
let resultingImage = UIImage(cgImage: cgImage)
if resultingImage.size.width > 100 {
print("Received an image \(resultingImage.size)")
}
}
}
}

Answer 2

Yes. You have at least two options, CIPhotoEffectMono or a small custom CIColorKernel.
CIPhotoEffectMono:
func createMonoImage(image:UIImage) -> UIImage {
let filter = CIFilter(name: "CIPhotoEffectMono")
filter!.setValue(CIImage(image: image), forKey: "inputImage")
let outputImage = filter!.outputImage
let cgimg = ciCtx.createCGImage(outputImage!, from: (outputImage?.extent)!)
return UIImage(cgImage: cgimg!)
}
Note, I'm writing this quickly, you may need to tighten up things for nil returns.
CIColorKernel:
The FadeToBW GLSL (0.0 factor full color, 1.0 factor is no color):
kernel vec4 fadeToBW(__sample s, float factor) {
vec3 lum = vec3(0.299,0.587,0.114);
vec3 bw = vec3(dot(s.rgb,lum));
vec3 pixel = s.rgb + (bw - s.rgb) * factor;
return vec4(pixel,s.a);
}
The code below opens this as a file called FadeToBW.cikernel. You can also post this as a String directly into the openKernelFile call.
The Swift code:
func createMonoImage(image:UIImage, inputColorFade:NSNumber) -> UIImage {
let ciKernel = CIColorKernel(string: openKernelFile("FadeToBW"))
let extent = image.extent
let arguments = [image, inputColorFade]
let outputImage = ciKernel.applyWithExtent(extent, arguments: arguments)
let cgimg = ciCtx.createCGImage(outputImage!, from: (outputImage?.extent)!)
return UIImage(cgImage: cgimg!)
}
Again, add some guards, etc.

Answer 3

I have had success by converting it to greyscale using CIPhotoEffectMono or equivalent, and then using CIColorControls with a ridiculously high inputContrast number (I used 10000). This effectively makes it black and white and thus binarized. Useful for those who don't want to mess with custom kernels.
Also, you can use an example like Apple's "Chroma Key" filter which uses Hue to filter, but instead of looking at Hue you just give the rules for binarizing the data (ie: when to set RGB all to 1.0 and when to set to 0.0).
https://developer.apple.com/documentation/coreimage/applying_a_chroma_key_effect

Answer 4

Found this thread from a Google search, and thought I'd mention that as of iOS 14 and OSX 11.0, CoreImage includes CIColorThreshold and CIColorThresholdOtsu filters (the latter using Otsu's method to calculate the threshold value from the image histogram)
See:
https://cifilter.io/CIColorThreshold/
https://cifilter.io/CIColorThresholdOtsu/

Answer 5

let outputImage = inputImage.applyingFilter("CIColorMonochrome",
parameters: [kCIInputColorKey: CIColor.white])
In you want to play with every out of 250 CIFilters please check this app out: https://apps.apple.com/us/app/filter-magic/id1594986951