How can I convert an UIImage to grayscale in Swift using CIFilter? - ios

Question

I am building a scanner component for an iOS app so far I have the result image cropped and in the correct perspective.
Now I need to turn the color image into Black and white "Scanned" document.
I tried to use - "CIPhotoEffectNoir" but it more grayscale then totally black and white. I wish to get a full contrast image with 100% black and 100% white.
How can I achieve that?
Thanks

Answer 1

You can use CIColorControls and set Contrast Key kCIInputContrastKey to increase the black/white contrast as follow:
Xcode 9 • Swift 4
extension String {
static let colorControls = "CIColorControls"
}
extension UIImage {
var coreImage: CIImage? { return CIImage(image: self) }
}
extension CIImage {
var uiImage: UIImage? { return UIImage(ciImage: self) }
func applying(contrast value: NSNumber) -> CIImage? {
return applyingFilter(.colorControls, parameters: [kCIInputContrastKey: value])
}
func renderedImage() -> UIImage? {
guard let image = uiImage else { return nil }
return UIGraphicsImageRenderer(size: image.size,
format: image.imageRendererFormat).image { _ in
image.draw(in: CGRect(origin: .zero, size: image.size))
}
}
}
let url = URL(string: "https://i.stack.imgur.com/Xs4RX.jpg")!
do {
if let coreImage = UIImage(data: try Data(contentsOf: url))?.coreImage,
let increasedContrast = coreImage.applying(contrast: 1.5) {
imageView.image = increasedContrast.uiImage
// if you need to convert your image to data (JPEG/PNG) you would need to render the ciimage using renderedImage method on CIImage
}
} catch {
print(error)
}
To convert from colors to grayscale you can set the Saturation Key kCIInputSaturationKey to zero:
extension CIImage {
func applying(saturation value: NSNumber) -> CIImage? {
return applyingFilter(.colorControls, parameters: [kCIInputSaturationKey: value])
}
var grayscale: CIImage? { return applying(saturation: 0) }
}
let url = URL(string: "https://i.stack.imgur.com/Xs4RX.jpg")!
do {
if let coreImage = UIImage(data: try Data(contentsOf: url))?.coreImage,
let grayscale = coreImage.grayscale {
// use grayscale image here
imageView.image = grayscale.uiImage
}
} catch {
print(error)
}

Answer 2

Desaturate will convert your image to grayscale
Increasing the contrast will push those grays out to the extremes, i.e. black and white.
You can CIColorControls:
let ciImage = CIImage(image: image)!
let blackAndWhiteImage = ciImage.applyingFilter("CIColorControls", withInputParameters: ["inputSaturation": 0, "inputContrast": 5])
Original:
With inputContrast = 1 (default):
With inputContrast = 5:

Answer 3

In Swift 5.1 I have written an extension method for OSX which also converts to and from NSImage. It uses saturation and input contrast to convert the image. I have abstracted a func for black and white.
extension NSImage {
func blackAndWhite () -> NSImage?
{
return applying(saturation: 0,inputContrast: 5,image: self)
}
func applying(saturation value: NSNumber, inputContrast inputContrastValue: NSNumber, image:NSImage) -> NSImage? {
let ciImage = CIImage(data: image.tiffRepresentation!)!
let blackAndWhiteImage = ciImage.applyingFilter("CIColorControls", parameters: ["inputSaturation": value, "inputContrast": inputContrastValue])
let rep: NSCIImageRep = NSCIImageRep(ciImage: blackAndWhiteImage)
let nsImage: NSImage = NSImage(size: rep.size)
nsImage.addRepresentation(rep)
return nsImage
}
}