How to desaturate an image to make it black and white

Swift version: 5.1

Paul Hudson @twostraws May 28th 2019

One of the most useful filters to have in your toolbox is called CIColorMonochrome, and its job is to remove the color variance from an image then tint it however you want. If you use gray as your tint color, it produces plain black and white images, but you can also use other colors to get sepia tone and other effects.

Here’s some example code to get you started:

guard let currentCGImage = yourUIImage.cgImage else { return }
let currentCIImage = CIImage(cgImage: currentCGImage)

let filter = CIFilter(name: "CIColorMonochrome")
filter?.setValue(currentCIImage, forKey: "inputImage")

// set a gray value for the tint color
filter?.setValue(CIColor(red: 0.7, green: 0.7, blue: 0.7), forKey: "inputColor")

filter?.setValue(1.0, forKey: "inputIntensity")
guard let outputImage = filter?.outputImage else { return }

let context = CIContext()

if let cgimg = context.createCGImage(outputImage, from: outputImage.extent) {
    let processedImage = UIImage(cgImage: cgimg)
    print(processedImage.size)
}

To make that work you’ll need a UIImage called yourUIImage, then replace the print(processedImage.size) line at the end with whatever you want to do with your black and white image.